Wednesday, December 20, 2017

Does the Tesla Powerwall 2 produce RFI (Radio Frequency Interference)?

Figure 1:
A typical Powerwall 2 installation.
Left to right:  Utility meter/original load center fed from an underground
power feed, the"new" load center to which the household circuits now
connect, the Powerwall "Gateway" (with two 4G antennas on top), AC
disconnect for the Powerwalls, sub-panel for the Powerwalls (containing
two circuit breakers), and finally the two Powerwalls.
This type of system is typically installed outside, near the utility's
connection to the house.
Click on the image for a larger version.
Now that I have an installed and operating Tesla Powerwall 2 system I've had the opportunity to answer a question that I've not seen answered elsewhere:  Does the Powerwall 2 cause radio interference?

Why I care:

Being an amateur radio operator that uses a wide range of frequencies across the electromagnetic spectrum (from below 137 kHz to at least 24 GHz) and often "listens" over wider ranges than that I'm always on the look-out for devices that unintentionally produce radio frequency energy which will be manifest as radio interference, reducing my ability to receive signals.

This sort of interference is increasingly commonplace, the incidence having accelerated with the prevalence of "switching" type "wall-warts" (a.k.a. "power cubes") that ubiquitously power nearly anything that is plugged into the wall.  As part of their power conversion, these small devices contain powerful oscillators - typically operating in the 20-100 kHz range - that have the potential to cause radio interference at frequencies far removed from that.

What this means is that the inclusion of even more of these devices in my household - including a Tesla Powerwall 2, which is a really big switching power converter - have the potential of adding to this sea of noise.

What is a Powerwall?

A Powerwall is the Tesla-specific name for what amounts to a "whole house UPS" (Uninterruptible Power System).  There are other manufacturers of similar systems and they have their own nomenclature, generically called an "AC Battery" because they internally perform the AC to DC conversion for charging and DC to AC inverting to provide external AC power.

As the name implies, if the mains power disappears this system can provide electricity to the entire house (or a portion of it) during the power outage.  As you might expect, very large, high duty-cycle loads such as whole-house air conditioning, electric water heaters, electric clothes dryers and electric furnaces are typically not backed up by a system like this as they would draw down the battery very quickly.

When integrated with a PV (solar electric) system it can be charged from solar energy and if the grid remains unavailable the house can run indefinitely, provided that the short-to-medium-term power budget is positive - that is, more solar power is produced than is being used and the battery is not discharged so much between charges (e.g. overnight, on cloudy days) that it reaches the point of cut-off.  My system has two Powerwall units which, working in tandem can provide at least 10kW of power with a storage capacity of a bit more than 26kWh - enough for about a day (without any solar input) with normal usage or several days (without solar input) if serious power conservation measures are taken.

In areas where there are significant electric rate (tariff) differences between "peak" and "off-peak" hours, this type of system can be used to "zero out" (or reduce) utility usage during peak hours and charge  during off-peak hours from the grid and/or with solar.  In my area, this is not relevant as the power rates remain constant throughout the day and it is configured to charge only from solar.

Having one of these systems is a bit like having a back-up generator - except that if the sun is shining, the "gas tank" can be refilled.  Practically speaking this system is unlikely to save me any money in the same way that a back-up generator probably wouldn't so I would consider it to be a sort of extravagance - like owning an RV, boat or some 4 wheelers - a bit like a somewhat expensive hobby, but more utilitarian.  Being an amateur radio operator I'm also interested in having back-up power in case there is some sort of event that causes the loss of the grid for a period of time, hence the concern about possible radio interference.

How it's connected:

Figure 2, below, shows how a typical "AC Battery" might be wired into a household power system and integrated with a PV inverter.
Figure 2:
A generic block diagram of an "AC Battery" type of back-up power system.
In "Tesla speak", the "Gateway" comprises the functions depicted in the box labeled "Supervisory Control" while the
Powerwall(s) themselves are depicted by the boxes labeled as "Battery-backed inverter/charger system(s)".  Both circuit
a manual disconnect and circuit breakers are required to give first responders an easy way to kill power to the entire house
should it be necessary - such as in the event of a fire or other disaster - as simply killing the mains circuit alone would
not do this!
Click on the image for a larger version.
Comment:  As the time of writing there are some parts of the world - mostly in Europe - where, due to regulations, "whole house back-up" during a grid failure is not available.  The radio interference potentials described below still apply in these cases.

As can be seen, in normal operation the AC battery system is in parallel with the house's power and the power grid.  When "charging" from the solar, it simply monitors the output power of the PV system and the power to/from the grid and adjusts its charge rate to match.  Likewise, in the "Self-Powered" mode (described below) when there is a grid connection it will charge/discharge at a rate that precisely matches the house's usage, effectively zeroing-out the power going to/from the grid or export power to the grid once the battery has been charged in the same way as a typical "net meter" installation.

If the mains power fails the "Grid Isolation Relay" opens, disconnecting the house from the grid, allowing power to the backed-up loads to be maintained without back-feeding the utility.  The process of detecting a power failure, disconnection from the grid and full restoration of the power seems to take between 200 and 750 milliseconds but the return to a grid connection after the power has returned and stabilized for a few minutes is nearly instantaneous.

Does the Tesla Powerwall 2 cause radio frequency interference:

Yes and no.

The "no" part:

On the HF bands I have determined that in my particular case (and prior to mitigation techniques described later) the interference potential on the HF bands to be minimal or negligible.  When the unit is operating (either charging or discharging) and I am using my normal HF antenna system I cannot detect any interference from it on the HF amateur radio bands of 80 through 10 meters (e.g. 3.5-30 MHz).  Additionally, I cannot detect any interference from the Powerwall 2 system on any VHF or UHF band, either.

If I walk up to the Powerwall 2 system with a portable shortwave radio while it is operating I can hear a bit of noise when I am within a foot or so (less than a meter) that is likely due to influence from short-range magnetic fields, but this noise energy doesn't seem to be being coupled to the connecting wires outside the unit.

The "Yes" parts:

160 meters:

Prior to noise mitigation techniques on the highest MF band, 160 meters (1.8-2.0 MHz), the story is a bit different:  When the unit is operating, I can just detect a bit of noise from the unit in the far background, just below the local noise floor - but whether or not I can hear this at all depends on which antenna I'm using for receive.  For example, on an active E-field whip I can just hear this noise, but it is not at all audible when using a wire antenna.

On lower frequencies:

Going down lower in frequency - into and below the AM Broadcast band (e.g. below 1.7 MHz) - the RF noise being produced by the Powerwall 2 (again, when it is charging or discharging) gradually increases, being fairly obvious by the time one gets to the bottom of the AM broadcast band (e.g. 530 kHz).  Below the AM broadcast band are two more bands - relatively recent additions to amateur radio in the U.S. - and both of these are bands on which I operate:  The 630 meter band (472-479 kHz) and the 2200 meter band (135.7-137.8 kHz).

At these frequencies the interference from the Powerwall 2 (when it is operating) ranges from "significant" at 630 meters to "considerable" at 2200 meters - but this is not surprising.  It would appear that the main power converter(s) inside the Powerwall(s) operate at 32 kHz - and the 2200 meter band is at only about 4 times this frequency.  Because the 2200 meter band's frequencies are comparatively close to the operating frequency of the inverter and its 4th harmonic at 128 kHz - and because RF interference filtering works better as frequency is increased while the harmonics of these converters (and their significant mains-frequency modulated sidebands!) also decrease in amplitude - the amount of energy at 2200 and 630 meters will naturally be higher than it would be on the HF bands.

In short:  If you do not plan to operate on the 160, 630 or 2200 meter bands, you will likely not experience any interference at all, even if no mitigation techniques are used.  I can only speak from experience with my system:  Other systems may be better or worse in terms of interference, depending on the situation.

An interference source that can be controlled:

If the RF interference from the Powerwall 2 were to be of great concern it's worth noting that the user has pretty good control of when this might happen as interference from the Powerwall 2 seems to occur only in two possible states:  When it is charging, or when it is discharging.  What this means is that even if you use the MF (160 or 630 meters) or LF (2200 meters) bands it will not cause interference when it is "idle."

A typical Powerwall 2 owner would operate it in one of two modes, selectable from a phone app:
  • Backup-only.  In this mode the Powerwall 2 operates only as a "whole house UPS" - that is, it is not producing power except when the utility mains is offline (e.g. a power failure or the user has disconnected it from the grid).  In this configuration and in a typical installation, charging of the Powerwall 2's battery is done only with energy from the PV system (solar + inverter) when it needs to do so - and this usually occurs only if the battery has been discharged below 95% or so.
  • Self-powered.  In this mode the Powerwall 2 monitors the net inflow and outflow of power from the house.  In this configuration the Powerwall will either output enough power to "zero out" the usage of the house so that there is, on average, no power going to/from the utility and/or it will take excess power from the PV system to charge its battery which will also "zero out" the power to/from the utility.  If the battery is fully-charged, excess power from the PV system will be fed back into the Grid, just as is done in a normal "Net Metering" situation.
Note:  At the time of this writing there is expected to be a "load leveling" mode offered in the near future where the Powerwall may be configured to charge/discharge at specific times to take advantages of time-based tariffs (e.g. lower-cost power during "off" hours).  This does not apply to me and such operation is beyond the scope of this article.

In the "Backup-only" mode the Powerwall 2 system is not usually operating (charging/discharging) and will thus not typically produce any noise on any amateur band - but in the "Self Powered" mode, the only time that interference would not be being produced would be when the Powerwall 2's battery is fully-charged and the excess PV power is being exported to the utility grid.

What this means is that if there is the possibility of interference, one would typically operate in the "Backup-only" mode where it is fairly rare for the unit to operate at all.  In my case, the charging portion of the inverter will operate only for a few hours in the morning as soon as the PV system starts to produce power, one or two days a week when it "tops off" the battery.

If, for some reason one wanted to completely eliminate the possibility of the unit going active - say, during some sort of contest - the Powerwalls could simply be turned off, but this would be done at the risk of losing the power back-up capability in the event of a grid failure

Mitigating interference from the Powerwall 2:

If we were dealing with a normal switching power supply the mitigation of interference would be quite straightforward:  Apply "brute force" L/C filters to all of the AC connections in and out of the device - a topic that has previously been discussed in great detail at this web site (see the links to related articles at the end of this blog posting.)

Applying filtering to a plug-in device that is capable of up to a kilowatt or two is one thing, but mitigating interference issues on a device that is permanently wired in to the house's electrical system and capable of many kilowatts is an entirely different matter!  For example, my Powerwall 2 system consists of a two battery/inverter modules that, together, are rated for 14 kW for short periods, or over 10 kW continuously, representing over 58 and 41 amps at 240 volts, respectively.

To afford a wide safety margin any added inductive filtering would need to be capable of handling at least 100 amps with any capacitors being conservatively rated for the voltage.  Finding and installing a commercially-available AC mains filter with such ratings could be difficult, expensive and awkward, probably requiring a separate enclosure - not to mention appropriate sign-off by inspectors.  What's more is the fact that on a battery-inverter system like this, two such filters would be required:  One on the AC mains feed-in from the utility to the Powerwall and another on the AC mains from it.

A more practical solution - and one that works effectively for 160 meters - is to install snap-on ferrite sleeves on these six conductors (e.g. the two "hot" phases and the neutral for each of the lines.)  It so-happens that readily-available devices that will fit over RG-8 coaxial cable will also fit nicely over power cable that is appropriately sized for 125 amp circuits.  (The dimensions of these devices is approximately 1.55" [39.4mm] long, 1.22" [31mm] diameter and are made to accommodate cables up to about 0.514" [13.05mm] - but could be modified to go over cables that are nearly 0.6" [15.24mm] diameter).

For exclusively HF, the so-called "Mix 31" ferrite material a reasonable choice, each device providing equivalent resistance as follows:
  • 1 MHz:  25Ω
  • 5 MHz:  71Ω
  • 10 MHz:  100Ω
  • 25 MHz:  156Ω
  • 100 MHz:  260Ω
  • 250 MHz:  260Ω
I used two of these devices on each of the leads (for a total of 12) which, at 160 meters, would provide an equivalent of about 60Ω of resistance.  Considering that there are 3 leads per feed, this parallel resistance is roughly equivalent to 20Ω per feed, so for 160 meters a bit more "help" may be required, so I also used some "Mix 75" ferrite devices of the same size.

Intended for lower-frequencies, the equivalent resistance of each of these devices is:
  • 200 kHz:  20Ω
  • 500 kHz:  58Ω
  • 1 MHz:  102Ω
  • 2 MHz:  70Ω
  • 5 MHz:  50Ω
Figure 3:
Beneath many of the boxes is a raceway/channel that contains some of the
conductors, including data lines and, as depicted above, the wires coming
from the utility mains, connecting to the Powerwall's gateway.  In
my installation there are no exposed conductors in this raceway and there
is plenty of room for the installation of the ferrites.  The marked ferrite
devices are the "Mix 75" while the unmarked are the "Mix 31."  While
it probably doesn't make a difference, I placed the Mix 75 ferrites on the
end of the leads closest to the Powerwall in the unlikely event that low-level
harmonics are generated in the Mix 75 ferrites that need to be attenuated
by the Mix 31 ferrites.  Placing large ferrites over all three conductors
at once for common-mode filtering would be preferred, but doing so
is not always practical as discussed below.
Click on the image for a larger version.
As can be seen, for covering 160 meters and higher frequencies a combination of both types of devices is suggested.  At 1.8 MHz, it is estimated that total equivalent resistance on each lead of the four devices (two Mix 31 and two Mix 75) will be on the order of 220Ω, or about 73Ω for each of the three sets of wires in parallel.  As can be seen from the pictures, I used two of these "Mix 75" devices on each of the leads.


At this point, there are a few "weasel words" that I must include:
  • While it is possible to put these ferrite devices (or anything at all!) inside the Tesla Powerwall's gateway box, doing so would probably require the "official" permission of Tesla's engineering department to avoid the possibility of voiding a warranty/service agreement.  Because of this, it is better to mount them on the conductors outside the gateway.  Filtering could also be installed at the disconnect and/or circuit breaker between the Gateway and the Powerwalls, but this, too, may require appropriate approval and sign-off by Tesla engineering to avoid warranty issues.
  • Placing any ferrite devices as described here outside the Gateway box will not affect its operation and would be less intrusive than, say, installing a whole-house surge protector as no physical connections are being made.  Because of the wide difference between the mains frequencies (50/60 Hz) and the lowest RF frequencies of interest (136 kHz-1.8 MHz) for which these devices are designed, these ferrites will have no measurable effect at mains frequencies.
  • The installation described below involves the exposure of high voltage, high-current circuits inside a breaker panel.  DO NOT even think of opening such a panel when it is "live", let alone installing any such devices inside it.
  • DO NOT even think of installing such devices in a panel - even if it is powered down - unless you have experience working with electrical circuits.  If you do not have such experience, refer to a licensed electrician to install such devices.
  • Where I live it is permitted for the homeowner to make modifications to the home's electrical system, but it is up to YOU to determine the safety and legality of any sort of modification of your electrical system and determine if you are competent to work with it.  Do not presume some/any of the described modifications to be legal or in compliance of safety regulations in your (or any) jurisdiction!
  • I cannot be responsible for injury or damage related to anything described on this page.  You have been warned!

First off, note that all of the units (the two Powerwalls, breaker panels, etc.) in my installation are connected together with metallic conduit and if properly installed this conduit will quite effectively bond all of the various boxes together electrically which means that it is likely to be quite effective in both preventing direct radiation of RF energy from the contained conductors as well as minimizing differential RF currents between the various boxes.  What this de-facto shielding will not do is stop RF from being conducted on the wires that leave this system - notably those that go into the house or to the power utility.  In my case, utility power is fed from underground which means that the most likely source of interference from the Powerwall is likely to be conducted into it from the main breaker panel and onto the house wiring.

Visible in Figure 1 (above) is a channel that runs underneath several of the boxes and in this channel are the conductors that, in my installation, go from the utility mains panel to the Powerwall's Gateway - and I installed one set of the ferrites (a total of 12 devices) in it as depicted in Figure 3.  Because there are no exposed electrical connections in this channel, these devices can be safely installed without turning off power.

Vibration prevention:

These ferrite devices are, by their nature, quite magnetic and as such the magnetic field associated with the AC current flowing through the wires over which they are slipped will cause mechanical movement.  When I installed the first of these devices I could hear them buzzing slightly, the apparent result of the two halves of the ferrite moving with respect to each other.

Figure 4:
 This is a view inside my main house's breaker panel with the "dead front"
cover removed.  In the upper-right corner is a 125 amp circuit breaker that is
the main feed-in from the Powerwall Gateway (the partially-visible box to the
right) which can carry the power from the utility and/or from the Powerwall.
The space for these ferrite devices is a bit crowded, but they do fit.
As noted in the warning, this panel has exposed, live connections and you
should not even think about working in it unless you have experience
in working on electrical systems and the power is turned completely off!
Click on the image for a larger version.
To prevent this movement - and the possible damage of these devices due to this constant motion over time - I spread an extremely thin layer of clear RTV (silicone) adhesive across the mating surfaces of the two halves to bond them gently together.  These devices have two mirrored halves of the ferrite that, when assembled, touch each other and are polished smooth, so one need only barely "wet" their surfaces with the slightest film - only the tiniest fraction of what would be used normally, an amount so small that it looks somewhat like an oil slick is sufficient for the polished surfaces.  Alternatively, a small drop of cyanoacrylate (e.g. "Super Glue") could be used, but unlike RTV, this would make removal difficult were it required in the future!  Adding anything between the two, polished halves of the ferrites will reduce their effectiveness somewhat so it is important that the two surfaces be as close to each other as is possible by using the smallest amount of RTV.

Installation in the main breaker panel:

In my installation there was another location at which these ferrites were to be installed:  On the power feed from the Powerwall to the household circuits where the majority of RF noise is likely to be conducted - but instead of being in a raceway where there are no "live", exposed connections, the only place that this wiring appears is in the main circuit-breaker panel.

Figure 4, above, shows the installation of the ferrites on the conductors within the breaker panel.  As can be seen, there are "live" exposed connections that pose a shock hazard which means that these devices can be safely installed only if the power is turned completely off.  As was done with the other devices, an extremely thin layer of RTV was put on the mating surfaces of the ferrites' halves to prevent their buzzing.

It would be preferable to be able to wind several turns of the large power cables through non-split ferrite cores to achieve much higher effective resistance at the frequencies of interest, but this is simply not possible in the available space with the existing wiring - particularly in the preferred common-mode fashion (e.g. all conductors going through the same core(s)).  Because the conductors were already in place and routed, it was deemed to be too awkward to disconnect one end of the (heavy!) cable to allow ferrite devices to be slid over it, so "split" devices were used instead.
If one is starting from "scratch" - or has the ability to add it later with some rewiring - enough extra cable length added to allow the winding of multi-turn chokes through large ferrite (toroidal) "non-split" cores inside a dedicated, metal junction box would be desirable.  Doing this can greatly increase the series inductance and provide a commensurate reduction of conducted RFI.
It would also be preferable to pass all of the power cables through the center of a single ferrite (of ferrites) as a single bundle to provide a "common mode" impedance path, but this is difficult to do as I have not found a source for split ferrites of 31 or 75 mix that would accommodate three cables that are about 0.5 inch (approx. 1cm) diameter.  The obvious alternative would be to pass the conductors through a stack of adequately large ferrite beads/cylinders or toroidal cores, but doing this would require that the conductors be disconnected from one end and temporarily pulled back.  If this were done at the time of the original installation, it would be the preference - particularly if several turns could be passed through some large cores - but this is much harder to do after the fact, particularly with the limited length of wire in an already-installed system.
Finally, while there is plenty of room in the raceway to accommodate the bulk of a number of these cores, there is much less available space within the cramped confines of the breaker panel to accommodate a large stack of ferrite rings/sleeves, particularly if one were to wind several turns of wires through them.  If you are contemplating a brand new installation, or if you are willing to pull wire out and do mechanical re-work, by all means put several turns of the three wires (both "hot" and the neutral leads) through common cores to maximize common-mode impedance.
Other RF interference paths:

In addition to the power connections to/from the Powerwalls, there are two other possible egress paths for radio frequency interference:
Figure 5:
Also contained in the raceway is the CAT 5/6 cable for the Ethernet
cable that provides the Powerwall with internet connectivity.  In my
installation there is also another data cable that goes to current/voltage
monitoring equipment where the PV (solar) equipment feeds into its
sub-panel.  Multiple turns and conductors of wire were fed through several
ferrite devices to choke any RF that might egress.  The upper device
consists of three square snap-on ferrite cores while the bottom device is the
ferrite core from the yoke of a scrapped CRT computer monitor.
Note shown are additional multi-turn chokes wound on ferrites at
the "other" end of these same cables.
Click on the image for a larger version.
  • The Ethernet connection from the Gateway.  It is common to "hard wire" a CAT5/6 cable from the Powerwall's Gateway to an Ethernet switch (behind a firewall) to provide internet connectivity.  While an Ethernet interface is, by its nature, galvanically isolated from its support circuitry, it does have some capacitive coupling.  It is possible to wirelessly (via either WiFi or via a 4G cellular network) connect the Powerwall to the Internet - which would avoid such cabling - so one would have to determine the nature of the specific installation.
  • Serial power cable to voltage/current monitoring.  A typical Powerwall 2 installation uses devices made by Neurio to monitor the voltage and current at both the connection to the power mains and at the PV (solar) electrical connection.  While a wireless connection between some of these devices is possible, there may be a 2-wire (half-duplex, RS-485 serial) connection between some of these devices and RF egress could occur on this cabling as well.
In my case I have both an Ethernet cable going to my firewall and a wired RS-485 connection to the Neurio monitoring the PV system.  To reduce the possibility of either of these lines conducting RF energy into a circuit that might radiate, the two cables were put together and wound through several ferrite devices as shown in Figure 5.  The upper devices are square, snap-on ferrite chokes while the lower device is the mass of ferrite from the CRT yoke of a discarded computer monitor.  The use of multiple devices and multiple turns greatly increases the effective inductance of this coil and its effectiveness overall.

While using ferrite devices on CAT5/6 cable will not normally affect the high-speed Ethernet signals within, CAT5/6 cable should not be coiled extremely tightly as doing so will distort the geometry of the twisted pairs and the integrity of the signals.  While this is unlikely to have much of an effect on 10 or 100 Megabit connections unless the cable is very tightly wound, it can degrade a "Gig-E" (1 gigabit) Ethernet connection (the Powerwall only uses a 100 Mbps connection) if the coil is smaller than 3-5 inches (about 8-12cm) in diameter or if the outer jacket of the Ethernet cable is "kinked".

The results:

While it may be a bit of overkill, the addition of the two types of snap-on ferrites (e.g. two of each type on each conductor for a total of 24 snap-on devices) has reduced the interference on 160 meters to the point of inaudibility and greatly reduced it on 630 meters.

On 2200 meters the interference is reduced, but still significant:  To completely quash interference at this frequency it would probably be necessary to, at the very least, install pulse-rated bypass capacitors (perhaps 1uF or greater) between each of the three conductors (ground, L1 and L2).  If I do this I'll do so using a low-current (15 amp) circuit breaker to provide the connection between L1 and L2 and the ground as a safe and simple way to make the connection.

If adding such capacitors were found to be insufficient to reduce the interference to inaudibility and working "around" the operation of the Powerwall were not practical (e.g. when it was not active) the next step would be a rather awkward and potentially expensive one:  The addition of the aforementioned extra junction box and re-running of the cables to allow the installation of multi-turn common-mode chokes.

What about RF interference to the Powerall? 

The Powerwall itself is a computer-based system with a number of analog monitoring points and as such, it is theoretically possible for external RF to cause it to malfunction if that energy somehow "glitches" one of its computers and/or causes one of its many sensors to read incorrectly.  To provide protection, the Powerwall is designed very conservatively and in the event of a serious discrepancy or fault, it will shut itself down.

The question should be asked:  Is it possible for external RF to cause such a shut-down?

The answer is:  Maybe.

About a week after my Powerwall was installed I happened to tune up on 40 meters using my 1.5kW amplifier.  While I was doing this, the power to my entire house "blinked" several times and went off, with the Powerwalls indicating some sort of error condition.  Unfortunately, the isolation relay had tripped and my house was disconnected from the mains and the Powerwalls did not reset themselves even after turning them "off" for over 15 minutes.  After a bit of hassle, I was able to get the Powerwalls reset - but the question remained:  What happened?  I opened a ticket with Tesla support and they came out to investigate a few days later.

It was determined that a possible cause of this "loss of power" event was arcing at one or more connecting clamps on the mains side of the isolation relay in the gateway that had not been properly tightened when it was installed.  The extra 2+ kW of load on the AC mains from the RF amplifier may have been enough to cause arcing in that loose connection and the Powerwall, detecting this as a potentially dangerous fault (as arcs can be!) killed all of the power for reasons of safety.

Since the clamps were tightened I have never been able to recreate this event, but being "gun shy" I immediately started installing the various ferrite devices on the power and data communications cables - not only to keep RF interference from the Powerwall from radiating, but also to prevent RF from getting in!

Parts sources: 

There are several sources of snap-on ferrite devices described on this page, including:
  • KF7P Metalwerx - link - Supplier of a variety of Ferrite devices and many other things.  At the present time he stocks the "Mix 31" devices, but does not stock "Mix 75" snap-on cores.
  • Mouser Electronics - link - The "Mix 31" snap-on cores - P/N:  623-0444164181  (Fair-Rite P/N:  0444164181);  "Mix 75" snap-on cores - Mouser P/N:  623-0475164181  (Fair-Rite P/N: 0475164181).  Mouser Electronics has other sizes and mixes of these various devices. 

Other solar power related posts at this site:
Other articles related to the mitigation of interference from switching power supplies:
Some of the above articles contain additional links to other web pages on related topics.


This page stolen from

Wednesday, December 6, 2017

KA7OEI now QRV on 630 and 2200 meters

Figure 1:
The LF/MF transmit station, configured for 630 meter operation.
At the time this picture was taken I had not yet completed the gear and
put the various pieces in their boxes, hence the mess of clip-leaded-
together modules sitting on my workbench.  Not visible is
the low-pass filter in the power amplifier box or the thermocouple-type
RF ammeter.  The pictured 630 meter variometer has been supplanted
with a "new" one wound with 660/42 Litz wire for lower loss (see Figure 6).
Click on the image for a larger version.
It so-happened that I had a few days off around Thanksgiving and I took this time to throw together a fairly simple transmit converter for the "new" amateur LF and MF bands - notably 2200 and 630 meters.  Having had already obtained my approval from the UTC to operate on both of these bands, I was "good to go".

It took only two evenings to put together the transmit converter and power amplifier as I had the parts on hand - and none of them were particularly exotic.  While the transmit converter will be described in greater detail in a future post, the signal path for the transmitter is approximately thus:

(See the block diagram in Figure 2, below.)
  • A 5 MHz IF is used, allowing a "broadbanded" FT-817 (with TCXO) to serve as the exciter.
  • The 5+ MHz signal (about 5137 kHz for 2200 meters, 5475 for 630 meters) is mixed (using a 74HC4066) with a 5 MHz local oscillator (a 10 MHz OCXO divided-by-two) to yield frequency-stable LF/MF signals.
  • A low-power post-mixer amplifier boosts this signal to a level capable of driving the power amplifier.
  • A single-ended MOSFET-based broadband power amplifier, running on 12-30 volts, provides between 10 and 50 watts of RF at either 630 or 2200 meters.  Because the transmit converter is broadband, it is agnostic to the operating frequency meaning that one needs only use the appropriate low-pass filter to change bands.  (The 630 meter low-pass filter is always in line - another filter is added for 2200 meter operation.)  This power amplifier is designed to be driven by either the transmit converter or another device, such as a QRP Labs Ultimate 3S beacon transmitter configured for these bands.
  • The 50 ohm output of the power amplifier goes to a tapped autotransformer wound on what is probably an FT-240-61 toroidal ferrite core and is used to match the transmitter's output to input resistance of the loading coil.
  • Also in the drawing is a relay the disconnects the loading coil from the autotransformer when not transmitting.  This was necessary to prevent the transmit antenna from "sucking out" some of the receive signal being intercepted by my E-field whip and also to prevent the transmit antenna from conducting "house noise" from the transmitter onto the transmit antenna which gets coupled into the receive antenna, reducing ultimate sensitivity. 
  • The loading coil, placed in series, cancels out the capacitive reactance of the antenna system.  For 630 meters my antenna requires about 230uH while about 2.5mH is needed to resonate the same antenna at 2200 meters.
Figure 2:
Block diagram of the 630 and 2200 meter transmit chain.  The transmit converter is broadband, capable of operating
from below 100 kHz to at least 500 kHz which means that one need only provide appropriate matching and low-pass filters to operate on either band.
Click on the image for a larger version.

When I made my first-ever transmission I had not yet constructed the variometer, but I fished around in my "box-o-inductors" and found several Litz-wound ferrite inductors that were probably rescued from some scrapped TVs or computer monitors and wiring enough of these in series I was able to achieve  resonance with about 750mA of antenna current.  On the very first WSPR transmission I managed to be "heard" by several stations (See Figure 3, below.)
Figure 3:
A screen shot (from of the very first 630 meter WSPR transmission that I made with the badly-kludged loading coil.
Not too bad for a temporary lash-up!
On the next night, after observing a few stations engaging in JT-9 QSOs, I answered a CQ by VE7SL and he replied, giving me a signal report of -22dB while I gave him -19dB.  This was quickly followed by two other QSOs as both W7IUV and NC0B noticed the "new guy" on the band!

Over the next several days I got around to constructing the "new" variometer depicted in Figure 4 and this boosted my antenna current to about 1.25 amps - a theoretical improvement of about 4.4dB with more QSOs to follow - including 2.5 (one "partial") CW contacts on the band.  After operating for a while it became apparent that, for the most part, I could work anyone that I could "hear".

A few days later I constructed yet another variometer for 630 meters - this time using some 660/42 (e.g. 660 strands of 42 AWG) Litz wire which reduced the skin-effect losses by a significant amount and this, along with minor improvements of the ground system, decreased losses and resulted in a further increase of antenna current to a bit over 2 amps - a theoretical ERP improvement of more than 8.5dB as compared to my original configuration. The measured resistance at the input of the 630 meter Litz coil is about 13.5 ohms, implying an overall antenna system efficiency roughly 1% - but still enough to work quite a few stations with a few 10s of watts of RF.

Figure 4:
 The "Mark 1 version of the 630 meter variometer.  This device is wound
on "4 inch" ABS triple-wall sewer pipe using 22 AWG insulated hookup
wire.  Inside is "3/4-inch" ABS waste pipe (actual O.D. about 1-1/8")
that forms the rotatable portion of the variometer.  This unit has an
adjustment range of approximately 175-235 uH.
Click on the image for a larger version.
The "Q" of the antenna system+Litz wire coil is now such that if I QSY from 475.75 kHz for WSPR operations down to about 475.0 kHz for JT-9 I actually see noticeable drop in antenna current until I readjust the variometer, but if I QSY from 475 kHz down to 473 kHz for CW operation the antenna current plummets to a few hundred milliamps and I absolutely must retune!

As is the custom on both the LF and MF bands, my WSPR signal reports not the transmitter power, but rather the estimated EIRP.  I've typically been reporting 0.5 watts (+27dBm) which, assuming about 25 watts of RF power, implies an antenna efficiency of about 2% which, while in the general ballpark, may still be a bit optimistic.  With the recent changes/improvements in my system (mostly improving the grounding, radials and counterpoise network) I will have to re-analyze my estimated system efficiency.

Operation on 2200 meters:
Figure 5:
Antenna and ground system of my LF/MF TX antenna system.  The
yellow line represents the outline of the "Lazy Loop" - a horizontal HF
antenna fed with 450 ohm window line with both conductors of the
feedline being tied together and fed as a tophatted vertical on LF/MF.
The total circumference of this antenna is about 215 feet (65 meters) -
dimensions mostly dictated by the locations of trees at an average
height of roughly 30 feet (9 meters).
The red lines show the extent of my ground/radial system showing
extra wires, including sections of chain-link fences with electrically-
bonded sections and wires buried in the ground, including an
abandoned CATV line.  The roofs of both the house and garage are
metal which are ultimately tied into the ground/radial network.  There
are several ground rods near the feedpoint of the antenna to which
all of the grounds/radials are connected.
Click on the image for a larger version.

I have since wound yet another variometer (visible in Figure 6, below) - also on 4" ABS pipe - for 2200 meters.  This coil, adjustable from about 1.7-2.0mH, uses the same 22 AWG hook-up wire as my original 630 meter loading coil.  As it turned out this coil, by itself, doesn't have quite enough inductance to resonate my antenna at 137 kHz so I place the other two 630 meter coils in series with it.  As compared to the 630 meter loading coils, it is somewhat lossy, but I am able to obtain about 900mA of antenna current:  Not surprisingly, this coil runs slightly warm in operation due to the losses - but these are, no doubt, minor in comparison with the ground losses.

Update - 12 December, 2017:  After improving the ground system my antenna current is now around 1.1 amps on 2200 meters, implying an improvement of at least 1.7dB from current alone.  The actual far-field improvement, based on readings seen from monitoring stations on WSPR, appears to be in the area of 2-3dB.

The measured resistance at the input of this loading coil is about 43 ohms implying an overall antenna system efficiency of well under 0.1%.   Based on estimated antenna efficiency, I've configured WSPR to report my ERP as 50mW, which assuming a transmitter output power of about 25 watts implies an actual antenna efficiency of about 0.2% which is probably very optimistic!

Not surprisingly, operation on 2200 meters - even at this power level - can be a bit hazardous.  With the rather low antenna capacitance the voltages on the feed are quite high - an estimated 5000-8000 peak volts!  What this means is that the feed wire has to be kept well clear of other conductors or else corona will occur, sapping transmit power, filling the room with ozone and becoming a potential fire hazard.  Fortunately, at this modest power level - and with the current-regulated power supply that I'm using - almost any sort of fault will detune the antenna system to the point that the high voltage will all but disappear and/or the power supply will go into current limiting and effectively shut down the transmitter.

Figure 6:
Left to right:  The original 630 meter variometer (seen in figure 4
wound with 22 AWG stranded wire , the new 630 meter wound with
660/42 Litz wire and the 2200 meter variometer, wound with the
same 22 AWG stranded wire and insulated with PET tape to allow it
to withstand the high voltages.  In the lower right corner is the
autotransformer wound on an FT-240 ferrite core.  With my current
antenna I must put all three of these variometers in series to
resonate the system at 2200 meters.
Click on the image for a larger version.

Despite this simple arrangement I've managed to be "heard" by at least seven other stations in the western U.S. and Canada using WSPR to date, but I've not yet made any 2-way contacts.  The relative scarcity of stations that listen or transmit on 2200 meters - coupled with my rather weak signal - means that a contact will probably have to be arranged and conducted using a weak signal mode like JT-9 or QRSS.


There are plenty of improvements to be made, most notably getting the feed of my antenna a bit higher, laying out a few additional ground wires to further-reduce losses and improving the variometer for 2200 meters - but there are only so many things that I can do on my relatively small city lot.  This entire arrangement has so far been precariously sitting on my workbench meaning that the high RF voltages are also also nearby, just waiting to leap out at me when I reach over to tweak a variometer.

At some point I'll "remote" the matching network outside, but I need to get/build a few other items first, namely some stepper motors, control circuity, more vacuum relays and a means of remotely monitoring the antenna current.

Comment:  Despite having the feedpoint in my shack, I've not had any problems at all with transmit RF getting into computer speakers or other devices in my house.

* * * * * * * *

My recent operation, as of the date of this post, seems to be the only actively transmitting station on either 630 or 2200 meters in Utah.  I have been running WSPR on 2200 meters most of the time, occasionally switching to 630 meters in the local evenings when the activity level on that band is highest.

If you are QRV on 2200 or 630 meters and would like to arrange a CW, JT-9 or QRSS contact with me, or if you are interested in just "hearing" my signal (via your ears or with a computer+sound card) drop me a line using my callsign at arrl dot net.

Other entries on related topics found at this site:
Other web sites that have information on 630 and 2200 meters: 

This list is by no means comprehensive.  Peruse the "links" sections on the sites below for even more information.
  • NJD Technologies - link  - This web page has a wealth of information related to 630 meter operation, propagation and reports of activity, plus lists of known-active operators on both 630 and 2200 meters.  This web site also has many links to others that have credible information on LF and MF band topics.
  • W1TAG's web site - link  - John, W1TAG, has long been an experimenter and operator on the MF and LF bands.  This site has details on equipment both for operating and measuring performance at these frequencies.
  • W1VD's web site - link - Jay, W1VD, has long been an experimenter on the LF/MF bands and this page offers a lot of information on equipment for transmitting and receiving on these bands.
  • Antennas by N6LF - link - The callsign gives  you the clue that this guy likes LF/MF operation.  This page includes detailed information on LF/MF antennas and how to characterize/improve them.


This post stolen from

Friday, December 1, 2017

Containing RF noise from a sine wave UPS

An amateur radio friend of mine (WA7X) has a cabin in the mountains.  It is not a particularly "rustic" cabin as it is festooned with radios, antennas, propagation beacons, computers and cameras and has an internet connection, but because it is in a remote location it occasionally has 3-6 hour power outages and thus it also has a UPS (Uninterruptible Power System) to keep many of these things online in the interim.

This "sine wave" UPS is a 1.5kVa unit that was cast off from by someone for the same reason most UPSs are cast off:  Its internal battery went bad.  Rather than simply replacing the battery, its previous owner simply got another UPS and asked the question "Do you want this?"

Instead of replacing the internal battery, the DC connections for the batteries were brought out and a bank of six 12 volt lead-acid batteries was wired up (two sets of three parallel batteries connected in series amounting to a nominal 200-ish amp-hours at 24 volts) to provide the needed 24 volts and a DC circuit breaker was added for safety, this battery capacity allowing the unit to run for far longer than it could have on the original battery.

While this UPS was more efficient than the previous unit and produces a fairly nice sine wave rather than the typical, ugly "modified sine wave" there was a price to pay:  RF Interference (e.g. RFI) that was present whether the unit was active or on standby.

But first, a few weasel words:
  • This project involves high voltages and/or currents:  Do not attempt to construct a similar device unless you are thoroughly familiar with electrical safety and the wiring of such devices.
  • If you use an external battery bank with a UPS it is imperative that you include some sort of current liming, such as a fuse or circuit breaker rated for both the expected current and battery voltage.  Such devices are available from auto-parts stores.
  • If you use an external battery bank with a UPS you must determine if this battery bank is "mains referenced" internally by the UPS or not.  If it is mains referenced (e.g. connected directly or indirectly to the mains AC voltage) then the low-voltage DC terminals will pose a line voltage safety hazard and care must be taken - the least of which is enclosing the battery and any exposed DC terminals to prevent accidental contact.  This UPS's battery terminals were isolated from the mains and given that the room in which the UPS has restricted access, the low-voltage battery connections themselves were deemed to be "adequately safe" left exposed.
  • YOU are responsible for the safety and any liability if you choose to do something similar to what is described on this page.  You have been warned!
    Figure 1:
    The completed AC/DC filter.  This
    box contains a "brute" force L/C filter
    for the battery (DC) leads as well as
    separate filters for the AC mains in/out
    power connections.
    Click on the image for a larger version.


The radio frequency "grunge" from the UPS manifested itself on the HF ("High Frequency" or shortwave) bands in several ways.  Most obvious was a loud "buzz" every 40-50 kHz on the lower (80 and 40 meter) bands, but there was also more subtle interference that pervaded these and higher bands:  A background "hiss" that might initially escape the notice of the casual listener until one realizes that this noise masked signals that should have still been perfectly audible.  If one switched the receiver to AM it would be observed that this "hiss" was subtly modulated at twice the mains frequency, 120 Hz.

To determine the extent of this sort of interference the typical procedure is to start turning things in the house off one-at-a-time (or, more reliably, the reverse:  Turn everything off at the breaker panel and then turn on one thing at a time) until the culprit is found.  This was done with the UPS and the magnitude of its "radio interfering" nature was determined.  Clearly, this "grunge" was being conducted from the power leads going in and out of the UPS.

Further experimentation revealed the true extent of the noise:  Even with everything disconnected from the UPS - that's to say, with it running on its battery, unplugged from the mains and the load disconnected - there was still detectable noise getting to the antenna and a quick check with a portable shortwave receiver proved that the remainder of this noise seemed to be being radiated by the physically-large battery bank and the wires that connected it.

While the proper application of a "brute force" AC line filter would likely quash the noise conducted in and out of the UPS on the mains power leads, something else would be required to minimize/eliminate the noise emanating via the DC lines.

"Brute force" line filters:
Figure 2:
A typical "brute force" L/C line filter typically found on devices
to minimize conduction of extraneous RF energy on the
AC mains.  For a 1500kVa UPS the filter would need an
appropriate current rating - particularly the fuse!
In some filters, two sections are used, with the components
L1, C1, C2 and C3 repeating.  This same filter
topology is used for the described DC filter.
Click on the image for a larger version.

One of the best ways to eliminate or minimize the amount of RF energy that might be conducted out of a potentially interference-generating device is to apply a combination of inductance and capacitance to that line as depicted schematically in Figure 2.

With the noise coming from the power supply (the UPS in our case) capacitor "C4" effectively "shorts" this RF noise to both sides of the power line, leaving the AC signal (pretty much) unchanged.  Inductor L1 consists of two equal windings on a ferrite core and it is practically invisible to signals that are equal and opposite, but it acts as a series choke for signals that are "common mode" - that is to say, equal on both sides of the power supply's mains leads - such as the RF noise energy.

On the "mains" side of the filter capacitor C3 reinforces the common mode again while capacitors C1 and C2 shunt any remaining RF energy from the power supply - its impedance now made much higher by inductor L1 - to ground - which would be the metal enclosure in which everything was mounted.

As it happens, these filters are available on the surplus market, and we would need three of them:
  • One for the AC mains connection to the UPS.
  • Another for the UPS's AC output
  • A third one for the battery connection to the UPS.
With the UPS being rated for 1.5kVa, some surplus AC mains filters, rated for 16 amps, were obtained while a filter designed specifically to filter DC lines rated for at least 50 amps at 60 volts was found (at The Electronic Goldmine - item G21652 - link) to filter the power connection between the UPS and the battery:  50 amps at 24 volts is not quite 1.5kVa, but there was nowhere near this amount of power being drawn from the UPS and the added circuit breaker/disconnect would provide the required safety - and the filter should be able to handle a brief overload, anyway.

Putting the filters in a box:
Figure 3:
Inside the filter box.  Along the top edge is the main AC input.  On the bottom edge - and just below the outlet on the left side - are the short leads that conduct the "dirty" AC power to/from the UPS, each through its own, separate filter - the two black boxes.
On the far right is the high-current DC filter with stud (bolt) connections being used to make the connections between the battery and the DC input of the UPS.  Barely visible along the bottom, one of the three studs is used to connect a piece of heavy wire or braid to bond this box to the chassis of the UPS to minimize conducted/radiated RF.
In the middle is a circuit board that contains a mains transformer, a high-current mechanical relay and a small solid-state relay.  This board - added later in the design - allows one pair of the outlets to be turned on and off with a simple contact closure of an internet-connected remote power switch.  On the control cable for the relay (the thin white wire) is a ferrite device which minimizes the amount of RF energy that might possibly be conducted in or out of the box on that control lead.
Click on the image for a larger version.

Ideally, one would have put the UPS and the batteries in a large metal box and passed the power leads in and out of this box only via the filters, but this simply wasn't practical, so the next best thing had to be done:  Put the filters in a single, metal box that would be electrically bonded to the UPS chassis and make the connections in and out of the UPS using short leads.  By keeping the leads short and bonding our new filter box to the UPS, we'd do our best to limit the number and length of conductors that carried the RF interference and, most importantly, preventing RF circulating currents from finding their way on external connections.
Figure 4:
The end of the box with the mounted outlets.  As noted,
one pair of these outlets is connected to a relay to allow the
connected devices to be remotely controlled.
Click on the image for a larger version.
  To that end a power distribution box was found at a home improvement store (Lowes Depot, I think) for less than $25 and the "guts" removed (the box with "guts" was cheaper than just an empty box by itself!) and the filters mounted inside.

With the short-as-possible conductors between the UPS and the filter, they will have minimal ability to directly radiate RF while the RF conducted on these lines will be filtered by the circuitry in the box itself with the bonding of the two boxes minimizing differential RF currents.  The power "to" the UPS was made with a short length of "SO" cord with a female connector on it while the power "from" the UPS is via a short cable with a male connector, plugged into one of the UPS's outlets - and being the ONLY thing plugged into the "dirty" AC output of the UPS.


Figure 5:
Perhaps a bit cluttered, this is the UPS sitting atop the filter, installed
and working.  In the lower-left corner of the picture, above two
batteries may be seen the DC circuit breaker/disconnect that protects
the DC circuit for short circuits.
Click on the image for a larger version.
Being that this is a remote location, the filter unit was installed a few weeks after it was constructed after having been tested (as best as could be done) on the workbench:  Power flowed through the various filters and the load control relay worked properly.

When installed, the filter box was placed underneath the UPS as it had a slightly larger footprint - and to minimize the length of the "noisy" DC and AC power leads from the UPS, along with the lead used to bond the two cases together.

To connect the DC, the cable coming from the batteries was effectively cut so that there was just enough of it emerging from the UPS to connected to the "output" side of the filter:  On both sides of this cable heavy "ring" lugs were attached and these were connected  to the studs of the DC filter.  To eliminate the probability of accidental shorting, the ground stud on the "input" side of the filter was removed and near both connectors a plastic wire clamp (one may be seen in Figure 6, below) to keep the positive wire in a fixed position and rotating into and shorting to the other stud.

Figure 6:
The back side of the UPS, showing the coiled power cord and the
(green) bonding wire that connects the chassis of the UPS and filter
box firmly together.  At the bottom of the picture can just be seen the
plastic clamp the keeps the positive wire lifted and away from the
the negative wire, to prevent shorting.
The ONLY thing plugged directly into the "dirty" AC output
of the UPS is the cord going to the filter:  Plugging anything else
directly into the UPS would at least partially negate the filtering! 
Click on the image for a larger version.
The AC input and output of the UPS was simply "plugged in" to the cables and the excess cordage was neatly coiled and stowed at the back end of the UPS using plastic "zip" ties:  It was important to do this because these cables are unfiltered and are "noisy" with RF meaning that they should be kept as small and as close to the metal of the cabinet as possible and kept away from any other conductors to minimize cross-coupling which would defeat the purpose of this filter.

To the remaining ground stud of the DC filter was attached a short piece of heavy (8 AWG) green wire with a ring lug on each end, this wire being visible in Figure 6.  The other end of this wire was attached to a marked grounding screw on the UPS chassis to bond the two boxes together and minimize RF circulating currents and to prevent the UPS chassis itself from being a source of radio frequency interference by direct radiation.

The result:

Taking the 20 meter (14 MHz) amateur band as an example, the UPS caused an extra 2 "S" units or so of noise above that of the typical ionospheric noise floor when it was powered up, before the filter was installed - this, being detected on a Carolina Windom antenna lofted between two trees high above the cabin's roof.  After this filter was installed the noise from the UPS was completely undetectable on any HF band, revealing other weaker low-level noises from other devices - the quieting of some of these will be discussed in later installments.

Figure 7:
A general block diagram of how the parts are interconnected.
The external UPS battery bank is protected with a DC-rated circuit breaker/
disconnect switch.  This particular UPS operates from 24 volts, hence the two
series-connected 12 volt batteries.  If the battery is inside the UPS's metal
cabinet, the DC filtering and connections are not needed.
Note that the "output" side of both AC line filters are both connected to
the UPS:  This is done because many filters are designed such that the
"output" side is that best-suited for connecting to RF-noisy circuits.
This diagram does not include the remote relay described.
Click on the image for a larger version.
Before the installation of the filter I'd placed my FT-817 (a small, portable HF transceiver) across the small room from the UPS, receiving with a short antenna and tuned to the 20 meter band.  When the UPS was operating and connected to its loads its noise was clearly audible, causing several S-units of indication on the signal meter.

After the filter was installed and the unit was powered up again with the loads connected, the noise was barely audible in the FT-817 and from across the room, it went away completely when the green wire was connected, bonding the UPS and filter chassis together.  If the radio was moved to within a foot or two of the UPS I could start hearing the "hash", but it seemed to be emanating only from the coils of AC cables "zip"-tied to the back end of the unit.   Because of the short length of the wires - and their being close to the metal case and not near any other wires into which this noise could be coupled it is unlikely that these short conductors will radiate any detectable noise at a distance greater than a few feet.

Links to other articles about power supply noise reduction found at this site:


This page stolen from

Tuesday, November 21, 2017

Matching the 2 and 6 meter cycloid dipoles to 50 ohms

The "Cycloid" dipole - a circularly-polarized antenna capable of (more or less) omnidirectional (toward the horizon) radiation was discussed in previous posts:
Figure 1:
The 6 and 2 meter Cycloid dipoles at the WA7X beacon.
The 2 meter cycloid dipole has been in service since 2001 and while the 6 meter
cycloid was (mostly) built at about the same time, it has been in service
only since 2015.
Click on the image for a larger version.
The 2 and 6 meter antennas were installed at the site of the WA7X beacon to impart a circular polarization on the transmitted signals, making them (generally) agnostic to the antenna used by the listener - which is to say that it wouldn't matter whether the receive antenna was vertically or horizontally polarized.  The use of circular polarization also reduces the problem where the ionospheric reflection may rotate the signal to the "other" plane and cause fading at the receive location owing to cross-polarization.

While this antenna is described as being "omnidirectional", that is not true in the proper sense of the word.  Its circularity and most of its radiated power is directed toward-ish the horizon (at some elevation angle) in all directions while relatively little energy is radiated at a high angle upwards or downwards - and what is being radiated in those directions not likely to be very circular.  As with any antenna, the proximity of the coaxial cable, metallic support and feedline will, no doubt, skew the pattern in some way - and this antenna is no exception - but this is unavoidable.

While the dimensions of the "antenna" part of the Cycloid dipole are spelled out in the linked article(s) above, details related to matching of these antennas to 50 ohms is not - with only the suggestion that a "1/2 wave matching network" be used.  While this matching network is very simple, it may be unfamiliar to some, so what follows is a paraphrased response to an email on this very question.

Matching the cycloid dipole to 50 ohms:

While I carefully noted the dimensions of the dipole when I designed them, we never precisely measured the various dimensions of the matching networks of the 2 and 6 meter Cycloid Dipoles as the need for precise replication would render them as mere "starting points" and they are simply stub-tuned 1/2 wave sections - only the dimensions of the actually "antenna" portion are on the web page.

In retrospect, a full 1/2 wave section was probably an overkill as a 1/4 wave may have sufficed - but that "extra" bit of open-wire balanced transmission line (e.g. the portion of the pipe between the coax tap and the "cycloid" part of the antenna) worked out well to provide physical support, rigidity, and counterbalance, would not cause any significant loss, and it all but guaranteed that we would be able to find a good match. in almost any conceivable situation.

The details of the matching network and its tuning are thus:
  • We used about 1/2 wave length of copper tubing protruding from the "back" of the antenna, used also to support the main antenna body.  As can be seen from the pictures, it was folded upon itself, zig-zagging to reduce its overall size.  This extra weight can help to counter-balance the antenna itself.
  • There is a 1/2 wavelength coaxial balun to go from 50 ohms unbalanced to 200 ohms balanced using small 50 ohm coaxial cable.  This type of balun well-described in literature and one of several online calculators may be found here: .  For the 2 meter antenna we actually used some small, 50 ohm hardline (the RG-58-sized equivalent of "UT-141" PTFE coax) that was obtained on the surplus market, but RG-8x or even RG-58 would have been fine.
  • When the antenna was tuned, it was mounted on a nonmetallic support (a fiberglass ladder) placing it several feet/meters above the ground and an MFJ analyzer was connected to the far end of the coax (10-15 feet away) to minimize the effect of having a person too close to the antenna and affecting tuning.  For initial tuning, it should be mounted to the same type of mast as that which will be used for permanent mounting.  For the antennas at the WA7X beacon, plastic pipe has proven to be durable with the 6 meter antenna being mounted using black ABS sewer pipe.  In the case of the 2 meter antenna, it was mounted using some PVC piping that seems to be holding up despite being out in the weather for well over a decade.
  • We prepared two nonmetallic sticks - 5-6 feet long (1x1, wood dowels, bamboo, small plastic pipe, etc.) and one of these had a piece of heavy wire to use as a shorting stub and the other had the balanced (200 ohm) side of the coaxial balun, also connected to 2 wires. The wires/balun were simply taped to the end of the stick to allow contact to be made.
  • Make sure that the copper pipe from which the matching section is made is clean and free of oxide using steel wool or sanding with fine-grit paper to allow a reliable connection while sliding the connections back and forth - both for finding a match and for ease of soldering.
  • At the position farthest from the "antenna" portion, the sliding shorting bar was placed while sliding connection to the balun was placed near it, on the "antenna" side of the shorting bar.
  • With the sticks, the two pieces (coax attachment and short) were slid around to achieve 50 ohm match.  While the two sliding portions are held in place, another person marks their position with a permanent marking pen on the antenna when a match is found.  It is easier to move the connection on the balun back and forth while watching the VSWR while slowly moving the shorting bar back and forth, looking carefully for a match.  Typically, the two connections will be fairly close to each other as seen in Figure 2, below.
  • Once a preliminary match is found, the sliding shorting bar is replaced with a piece of heavy, solid wire (#10-#14 AWG) that is wrapped around the pipes at the marked position. The other sliding bar (on the balun) is then re-checked for a good match, the shorting bar's position tweaked as necessary.
  • Once the position of the shorting bar has been established, the wire on the balun section is wrapped around the pipe at the location of the best match, allowing the wooden stick to be removed.  The positions of the two connections are then tweaked by sliding the wires back and forth for best match.
  • The two connections are soldered in place, and the match re-checked.  If it is OK, the connections are sealed and the match re-checked and adjusted as necessary.
  • In both Figure 1 and Figure 2 one can see small pieces of acetal (e.g. Delrin tm) plastic on the matching network - this material being chosen for its low RF loss characteristics and its durability to UV exposure.  Note that PTFE (a.k.a. Teflon tm) would have also worked well.  Some of the pieces (those at the far left edge of the matching section) are used for mechanical support, but the others are used for fine tuning:  The position of these pieces of dielectric slightly alter the tuning.  After the antenna was fully assembled, these were moved back and forth for the best match and secured in place with blobs of RTV (e.g. Silicone tm) sealant on both sides.  (RTV does not stick to this plastic, but the blobs keep it from moving about.)
As can be seen in Figure 2 on both the 2 and 6 meter antennas, the attachment point of the balun is fairly close to the shorting bar.  The proximity of the coaxial cable balun to the match may affect tuning a bit, so it must be fixed into place before the final tuning is done.

Figure 2:
Annotated image showing the locations of the shorting bars, coax baluns, balun connections and the rain shields on
the 6 and 2 meter cycloid dipoles.  The matching network on the 6 meter antenna is longer than
necessary to allow its far end - which is electrically neutral - to be clamped to the mounting pipe and attached to
a ground wire for static discharge/lightning protection.  The connecting cables were secured with good-quality
electrical tape and black "zip" ties, which were also covered with electrical tape to protect them from UV.
Click on the image for a larger version
Note that for each antenna a "snow/rain shield" was placed over the top of the matching section to minimize the effects of moisture, but the addition of this shield did change the tuning, as did the addition of RTV sealant on some of the connections, so final tuning must be done with such hardware and sealant in place.

The entire procedure is a lot easier if there are 2 or 3 people participating as it is pretty tricky for a single to hold two wires on sticks in place and mark them. If there is only one person available, the shorting bar wire would be wrapped around the (clean!) pipes at a position correlating to about 0.4 wavelength on the pipe and the balun portion slid back and forth to see a "dip" in the VSWR, iteratively adjusting the shorting bar back and forth experimentally while sliding the connection from the balun to get the best match.

As I noted, it is possible that a 1/4 wave section would have been fine, but we just used the 1/2 section as there would be no doubt that it could be matched - and we wanted to minimize the hassle related soldering/unsoldering things as much as possible.  Importantly, this type of match - using the large pipes and "open wire" line - is very low loss compared to many other matching networks (e.g. those using small wound coils and discrete capacitors) and it contributes to the mechanical strength of the antenna itself.


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