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Assume that one does not have access to a multi-million-dollar RF anechoic chamber.

When testing an amateur radio HF or VHF antenna, how would one absolutely minimize the amount of any RF radiation from the feedline, tower, and/or any other parasitic elements, such as power supply cords, AC wiring, computer cables, gutters, or any other nearby antennas?

Assume the antenna is (claimed to be) an imperfect dummy load, and you want to make sure the vast majority of any RF radiation is convincingly coming from the dummy load antenna, and not elsewhere.

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  • $\begingroup$ This could be used for testing lightbulbs, and comparing against other expensive commercial small HF antennas that are claimed to be no more than imperfect dummy loads, such as a lightbulb. $\endgroup$ – hotpaw2 Aug 15 at 16:43
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    $\begingroup$ Sidenote: Even in an anechoic chamber you also have to take care (shield) of the disturbance from all elements that you don't want to impact the measurement, like cables and so on. But there you usually have spare shielding material and a false bottom that makes it easier. - Just mentioned so that no one misunderstands the introduction of your question. $\endgroup$ – MagnusO_O Aug 15 at 17:12
  • $\begingroup$ Is the antenna under test a groundplane antenna, or something that should be balanced, or a terrible inbetweener like an end-fed vertical or off-centre-fed dipole? And are you interested only in impedance, or in radiation patterns? If you're mentioning an anechoic chamber, are you thinking of UHF and above, or is it for HF? $\endgroup$ – tomnexus Aug 15 at 17:50
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    $\begingroup$ What kind of antenna are you testing that claims to be a dummy load?! $\endgroup$ – Phil Frost - W8II Aug 15 at 18:55
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    $\begingroup$ And can't you trivially determine the radiation is coming from the antenna and not your gutters because your gutters aren't connected to a transmitter? Sure, they may parasitically interact with the antenna, but the radiation isn't "coming from" them. $\endgroup$ – Phil Frost - W8II Aug 15 at 19:00
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The proper mix of ferrite for your frequency of interest in a properly-designed common-mode current choke is your friend.

Look no further than Jim Brown's latest PDF on this subject at k9yc.com There is no better source of information anywhere else.

RFI, Ferrites, and Common Mode Chokes For Hams Most recent update April 2019. This tutorial is directed specifically to RFI in ham radio applications. It includes an extended discussion of the use of common mode chokes in antenna systems and for suppression of RFI. A chapter on audio and computer interconnections in ham stations shows how to make bulletproof connections between a computer sound card and ham rigs for SSB, RTTY, PSK31, and SO2R contesting without expensive interface boxes, using nothing more than simple cables with the right connectors on each end. There's also a chapter on grounding and bonding.

Of course, the unun or balun would be mounted right at the antenna feedpoint; right where the power cord meets the power supply, etc.

http://k9yc.com/RFI-Ham.pdf


Also see http://www.karinya.net/g3txq/chokes. Although he recently passed away, his expertise lives on.

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  • $\begingroup$ And use a non-conductive tower and mast. $\endgroup$ – Mike Waters Aug 15 at 17:41
  • $\begingroup$ The question is about testing an antenna. $\endgroup$ – Kevin Reid AG6YO Aug 15 at 17:44
  • $\begingroup$ @KevinReidAG6YO Look again at the title and the second paragraph. $\endgroup$ – Mike Waters Aug 15 at 17:52
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    $\begingroup$ Ah, I misunderstood. Myself, I'd consider the balun part of the antenna under test, but that's debatable. $\endgroup$ – Kevin Reid AG6YO Aug 15 at 18:04
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    $\begingroup$ Yes; I had taken the question as how to empirically test the balun along with the antenna. $\endgroup$ – Kevin Reid AG6YO Aug 27 at 22:17
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I'm breaking your question into the two major parts within.

Antenna system

"When testing an amateur radio HF or VHF antenna, how would one absolutely minimize the amount of any RF radiation from the feedline..."

  • I agree with Mike Waters about the judicious use of proper ferrites near the feedpoint and every 1/4 wave down the feedline to help quell any induction between feedline and antenna under test.
  • Perform some sort of EiRP test using as small a transmitter as possible to simply eliminate any feedlines. Perhaps a small HT for VHF and a Tiny Tin assembly for HF. Yeah you still need a way to receive the power, but if you compare the test antenna with a reference dipole or equiv. the results are often quite revealing.

Parasitic conductors and dielectrics

"...tower, and/or any other parasitic elements, such as power supply cords, AC wiring, computer cables, gutters, or any other nearby antennas?"

The goal here is to detune anything that might resonate at the frequency of interest. Ferrites often help with this. Sometimes the lengths are already non-resonate to mess with your goals and you might actually cause a resonance with a ferrite. Best if possible to remove these objects from the near field of the antenna or just test with them as part of your overall antenna "system."

It's what we do at the antenna lab

Eliminating the feedline is job number one when we perform testing at work. The EiRP test, such as this one testing HT Antennas, is our favorite way to really see how things work without benefit of additional conductors. We get our fair share of snake oil antennas that completely rely on feedline radiation to work. Our test snags them and quick.

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To add a partial answer to my own question, I realized I have at least one interesting test tool available, a tiny transmitter (TAPR WSPR board), controlled via a small computer (a Raspberry Pi Zero), smaller than the test antenna. Both can be run (for short periods) from a small battery, and the Pi can be either remotely controlled or run a timer-based CW beacon script.

Using a transmitter smaller than the antenna and directly connected at the antenna feed-point should eliminate feed-line radiation almost entirely. A battery supply and remote operation of the transmitter allows complete ground wire and control wire isolation as well. Perhaps a small toroidal common-mode choke between the transmitter and the antenna, even though lengthening the feed-line by a few centimeters, might help eliminate the transmitter from becoming part of the antenna. The setup would likely need be be placed maybe 2 wavelengths or more away from any metallic structures or wires to prevent re-radiation in the near field from distorting RF measurements.

Perhaps a useful idea only for "lightbulb" antenna's that are larger than a Raspberry Pi.

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I am going to assume that you are using coaxial cable. In this case the problem is that the outer braid part of the cable is acting as part of your antenna.

Here are 6 ways to do this. You can do one of them or combine them:

  1. wrap your coax into a big coil , probably at least 5 turns, very close to the antenna. This is sometimes called an "ugly balun" even though it is not a "balun".
  2. buy an inline choke. This is something you insert in your feedline path near the antenna.
  3. put ferrites on your feedline
  4. make sure the length of your feedline is longer than 1/4 wavelength at your operating frequency.
  5. Do not use Coaxial cable. Use what is called a "balanced feedline".
  6. add a wire to function as an RF Ground. This is not the same as a DC ground.

As far as other objects , if they are electrically conductive and very close then they may slightly effect your antenna. This is usually not a big deal, just tune your antenna.

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  • $\begingroup$ You may find karinya.net/g3txq/chokes interesting in respects to your statement in item 1. There is a chart there indicating that air core coax chokes have a narrow bandwidth as compared to others. :-) $\endgroup$ – Mike Waters Aug 27 at 20:07

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