RF on mic: lightweight, small choke for portable use a solution?

I’ve found that I get RF pickup on the mic on my new Discovery TX-500. I assume this is related common-mode currents on the coax. I tried an experimental choke yesterday - winding the coax round a length of offcut re-bar that I found under the hut, and that improved things considerably (I could up the mic gain to near-normal without getting ‘feedback’).

As such I’m going to try adding a choke to my setup. Currently have a 64:1 transformer feeding an EFHW and 10m of RG174 coax into ‘the shack’. Entire setup is hard-wired for reliability (no external coax connector on the transformer)

1: Positioning
I generally choose to believe those who say that a length of unchoked coax is advisable when running an EFHW with no counterpoise. So I’m guessing about the 5m point will be enough to provide some ‘coax counterpoise’ but keep the RF away from the radio (out of the hut). But that will be really annoying to wind onto the antenna winder.

2: Type
My default choke for home is 10 turns of coax through a toroid. But that will be both heavy and annoying half-way down my coax. And would be permanent as it needs the BNC removing and the coax feeding through the toroid. Any other options? Something lower profile but still effective? Anything that I can wind on ‘when needed’ but just keep ‘in the bag’ when not? So that it is not annoying me on my antenna winder or when pulling coax through the scrub?

3: Other suggestions? e.g. would I be better dealing with RF on the mic lead rather than common mode RF on the coax. And have I correctly identified the cause of the issue?

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Hi Matt,
I’d be tempted to tackle the problem at the source and give the EFHW a counterpoise wire that is not the coax sheath, or run out some radials, or belt a bit of metal into the ground and fasten that to the earth side of the UNUN.
73 Ed.

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In my experience with end feds, the coax is ALWAYS a counterpoise. If it’s not long enough to give the driven element it’s mirror image of a quarter wavelength, you get RF in the rig. Chokes help, but have you just tried using a longer feed line? If that helps, you still want a choke at the transceiver end regardless, but a choke isn’t really the best solution for an inadequate antenna system.

It’s no guarantee, but a LOT of RF problems are solved with more feed line.

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Hi Matt,

I experienced the same issue with the same radio and also with an EFHW, so I can elaborate about this issue and my solution.

When it happened the first time, I was on 17m and the antenna feed point of the EFHW was maybe only 1m away from the radio. When I disabled the audio compressor, the issue went away, but this was not the cause, it just “amplified” the issue.

I usually use 5m of RG-174 coax (again, like you I hard-wire it to the coupler).
When I experienced similar issues on other summits (always on higher bands), changing the coax (e.g. putting it more flat to the ground) and increasing the distance to the antenna, usually helped.

But I wanted to implement a robust solution and therefore I looked for a small and light common mode choke (CMC). Before building something myself, I always check if there is some commercial item available, either for inspiration or because I’m lazy :wink:.

On eBay, I found a small and light QRP CMC with a BNC plug and socket. Unfortunately, as of today, the item isn’t available anymore on eBay, but since you now have the image, you could use an image search to find another source.

Since I use this CMC, I didn’t experience any RF pickup anymore. It’s now part of my setup, also for other antennas.

73 Stephan

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My effort is 20cm long, can be easily put in line and removed if required. It weights 38g.

Like Stephan, I use 5m of coax, as recommended in his excellent paper on the EFHW. Sometimes I dispense with the coax and just use the choke. Ive used this successfully from 40m to 10m.


Thanks for the replies so far.
@DD5LP - have tried a 5m or 10m counterpoise on my EFHW in the past but for my rapid deploy, rapid tear down it’s just another thing to pfaff with, another thing to get tangled, and another connection to break. As I never saw any benefit before in terms of SWR, I ditched them. Now, I do admit, I have a motivation to revisit this - if I have to.

@W6KME - I already use 1/4 wave - 10m - of coax. Which is a lot for a lightweight portable setup - as others mention the norm would be 5m or less for portable operators. I only use 10m as I stay in backcounty huts a lot, and tend to need that much to get from whatever antenna tie-off point I’ve found in through a door/window. And coax is heavy. If I needed more radiating counterpoise, I’d probably go for a choke at the transformer and a counterpoise before adding more coax, if I had only that choice.

@MM0EFI and @HB9EAJ - I’d not considered choking at the radio. Will that work - i.e. is the issue common-mode RF coming down the cable and getting into the mic via the rig (in which case choking to stop that reaching the radio will work), or is it being radiated off the antenna cable and picked up by the mic cable (in which case it shouldn’t work)?

@HB9EAJ Point about audio compressor noted - will try disabling that next time this happens.

As an aside: what level do you run mic gain at on your TX-500? Quite hard to judge the best setting by listening to the echo of myself on an SDR!


Reducing the mic gain will also reduce the RF mic feedback, but the compressor is more powerful in this respect.

I set it to 3, default is 5. Mic gain is 5, same as the default. I reduced the lower frequencies a bit (TX equalizer).

Yes, but it’s a viable way to adjust the TX audio.

73 Stephan

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I have a Xeigu x6100 and have had the same problems with RF on the mic/rig, causing absolute havoc , when the metal case gets some common mode currents (caused a few sota fails…). I am having to choke the feedline right before the radio in all scenarios using a short stub of coax and a FT114-43 torroid.

Having a resonant antenna with counterpoise didn’t work for me despite tying allot. I am guessing my radio/mic is just very susceptible to the smallest amount… Even a w3edp random wire which i would like to feed direct from the radio requires a short bit of coax and choke.

Main problem is now my headphones which suffer if i’m too close to the antenna radiation or just on 10m…

Well the question is where is the RF being picked up?

My EFHW setups use approx 1 or 2m long RG58 patch lead to the EFHW match and antenna arranged as sloping Inv-L (sensible called inverted-7 by Andy G8CPZ!). The antenna is a trapped EFHW for 20/30/40m with traps for 30/20. Match is normally AA5TB style and there’s a link to open for 17m. There’s a 4m counterpoise wire laying on the ground. All bands can be tuned to better than 1.8:1 I sit right up close to the feed point when operating which should be in a strong part of the RF near field.

I have not noticed any RF pickup/feedback problems using FT817 or KX2 transceivers so far. I did have terrible RF pickup problems with QCXmini radios for 20 and for 30m. Very harsh audio in the headphones and they keyer would lock up during TX using a PalmPaddle. Wrapping the headphone lead and keyer lead around my fat hands helped. I reduced the problem to barely noticeable by fitting 10n SMD caps from the headphone outputs to ground on the bottom of the 3.5mm socket and likewise on the inputs on the paddle sockets. If you sit just-right you can just hear the keying tone is ever so slightly rough compared to a dummy load. The QCXmini case tends not to be grounded by default which probably doesn’t help but it seemed to be direct pickup on the headphone/paddle in this case.

I’d be looking at adding some decoupling on the mic and phones for both the Discovery and the Xiegu. It’s up to you if you feel competent to open up and add SMD components inside. But it should not be beyond the ‘ken-of-man’ to added them inside the connectors, SMD or leaded.

I may be missing something here, but I thought the whole point of using EFHW antennas was to replace the need for 5/10m of coax and drive the antenna from very near to the radio output socket. Sure your EFHW can be made multiband with acceptable match with fewer traps/links than a centre fed dipole. The other point being 10m of RG174 has a given loss and the match unit for the EFHW should be designed/made to have the same order. You save on the weight of 10m of coax plus having to wind/unwind it without damaging it and have a match unit with the same (or even less) loss instead. Having both seems to be the worst of both worlds!

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I agree with Andy that fixing it at the radio end of things is another way of tackling this issue. It is not unlikely the TX-500 mic input suffers from what K9YC refers to as a “pin-1 problem”, which means that common mode on the mic cable gets converted to differential mode somewhere before it gets to the mic amplifier. Adding caps at strategic places could be a solution, to properly kill a pin-1 problem you could check how the mic shield is wired (if it is a shielded cable at all?) and especially how the shield is terminated to the rig’s casing, as poor shield termination is a typical source of pin-1 problems.
Alternatively, you could try a small clip-on ferrite choke around the mic cable close to the radio to suppress common mode there.


Wouter Jan PE4WJ

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I do feel this is happening on my Xiegu. You can even hear the slightly out of phase amplification of your own audio and its massively distorted for the recipient.

What I do know is a choke stops it, and it needs to have the right inductance because one of mine didn’t and I had the issue on the lower bands. You make a very good suggestion on decoupling caps for the mic and phones. I’d be wiling to try it, if, i wasn’t waiting on a Kx2 which appears not to have this issue !?

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