Random Wire with 9:1 - RF back through coax

Hi all,

Im using a ‘random’ wire antenna most of the time (~8.8m or so, not that random, based on info I found online about non resonant lengths).

I;ve used it in two locations:

  1. one portable with the wire hanging from the fishing pole I have and the balun ( www.earchi.org/92011endfedfiles/Endfed6_40.pdf ) connected to the radio via a vert well made 15m coax from Richard (SOTAbeams).
  2. the second location is my main QTH in central London, the same setup with the fishing pole outside on a small ‘balcony?/recessed?’ space.

The second location is quite small so the coax is just doing loops as Im sitting a couple of meters away.

As the balun Im using uses the outer part of the coax as the countpoise, I wanted to ask if the second location is a NO NO for this setup? Im running QRP most of the time (5 PSK and 10 SSB), with max 10W PSK.

My setup is battery powered but I think a get RF back when Im using the laptop for PSK.

I just made a current choke, Common-mode chokes , 2x FT240-52 and 11turns of RG58.

Will this help at all?

Apart from that the antenna work perfect for the location I am!! (very bad location for Ham)

Thanks and 73s!

What makes you guess your laptop is getting RF through the coax line?
Since you are sitting just 2m away from the antenna, the RF electric field around the antenna must be pretty high when you run 10 watts. Also you must be sitting in an appartment or flat with your laptop and the RF from the antenna must not be having a perfect lobe and homogeneous radiation pattern.
I have a magnetic loop antenna in my balcony and even transmitting with 0.5 watts makes the movement sensor I have in my wardrove to switch lights on inside.
73 de Guru - EA2IF

Oh!!! Good! :slight_smile: I need to change my sitting arrangements :smile:

Im burning my cells!

I can think of two mechanisms that are leading to RF getting into your laptop:

  1. This antenna system does indeed use the shield of the coax cable as a counterpoise. RF could be conducting down the coax shield, through the chassis of your radio, and into the laptop via the cable that you are connecting between the radio and laptop to do PSK. The choke that you have built and inserted into the coax will (ideally) create an open circuit in that path, or at least greatly reduce the amount of RF that conducts in this way.

  2. As Guru points out, you and your laptop are very near the antenna. Any and all of the cables connected to your laptop could be serving as antennas and receiving the radiated RF from the antenna system. The RF then conducts into the laptop via those cables.

If your new balun does not solve the problem, I would try placing chokes into each of the cables that attach to your laptop (PSK cable, power supply cable if you are using one, etc.), and place them as close to the laptop-end of the cables as possible. To make the chokes, get some ferrite toroids made of Type 31 material (or Type 43 if you can’t find Type 31) and wrap as many turns of the cable as you can manage through the toroid. See this excellent document from Jim K9YC for more info:



Eric KU6J

Free SOTA Spot Monitor Software + RBNGate FAQ:

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YEs by design the 9:1 balun I use turns the outer coax into a counterpoise, but with the coax flat on the ground for 10+ meters I think it does the job well.

With me near the antenna and all the cables in a big mess I think it is indeed very bad.

The current choke (2X FT240-52) I have today seems to be killing my tuning capabilities. I have to run it via the analyser to see what is happening. I couldnt tune on any band.


By the way, the 9:1 device from EARCHI is really a unun rather than a balun, and the device you created is really just a choke rather than a balun. I didn’t want to get hyper-technical with the terminology, but I donn’t want you to read K9YC’s document and have the terminology differences be an issue. Best of luck in solving the problem, wouldn’t want your next report to be that your body parts are starting to sizzle! :smile:


Eric KU6J

Free SOTA Spot Monitor Software + RBNGate FAQ:

yes your correction for the terminology is needed.
I do get the basic differences between the terms, I just used the wrong one here.

The antenna is an end-fed and is high impedance, when resonant there is little current flowing as it’s all voltage fed. Whatever current there is will be on the outer of the coax braid but there wont be much. But this is not resonant by design so there will be much more current at some frequencies than others and that current has to go somewhere. Your current choke stops that current. If there’s no current the antenna wont work.

This design of antenna works quite well when you have enough coax to act as counterpoise. The 9:1 UnUn will tame the high impedance so the radio is happier. But you can’t have an effective random wire antenna without an effective RF earth that will work in all situations and have an acceptable match. So it’s probably not the right antenna for the balcony situation.

In any case, you need to put the choke balun between your radio and tuner.

73 Angel

My personal experience of living in a London apartment & ham radio was not great… but after some experimentation I found a magnetic loop to work ok from a ground floor flat with no garden, hung in front of the front window.

A vertical would lead to similar RF issues in the laptop, which was a problem as I was running an SDR. Same as my current flat with a multiband dipole, my Solution was to add a choke balan and this fixed the crashes for me.

Alternatively get out portable and kiss goodbye to these restrictions, I speak out of first hand experience!

73s James M0JCQ/P

True with the long coax this antenna did a very good job from another more open location and portable.

I havent test this combination. Ill try it tomorrow.

I know… its really miserable near centre London when you combine with ham radio :frowning:

I’ve got a loop too but its efficiency wasn’t as good as the long wire when compared outside/portable.
Its also a pain to hung it outside. Crazy space limitation.

My current setup (857D+SLAB) it is ‘portable’ but not really. I see many here use the KX3. Ill probably spend some time in the States later this year (early summer) so Im thinking about it… (pocket is itching…) heheh

Thank you all!

Shame you don’t have your M0 otherwise you could activate a few US summits, once again I speak out of experience… I had my M0 and went out twice to California last year with work and didn’t take any radio stuff #fail

Also wish I had purchased a K3 out there, I ordered my KX3 straight from Elecraft but got smacked with import tax by HMRC :unamused:

I can feel you…
no no no!!! Im not doing that!!!

The commodity code for importing Amateur gear is 8525600000 and there is no duty to pay. That code is for equipment that can transmit and receive. If you import it as a transmitter alone then the code is 8525500000 and the duty is 3.6%

You always have to pay VAT at 20% plus whatever handling charge there is for collecting this. (£13 at present)

If it makes you feel better James. I have just had one. I worked out the cost of importing it Vs the UK distributor and there was not much in it with respect ro duty.

At most you would save around £150 but then you have the delay and worry with the code.

My first impressions of the KX3 are not that great. Bought mine as a kit. I will see how well it performs tomorrow.


I didn’t know that! Even though +20% is already a lot!

Shame it is… Ill stay in Aspen CO, I cant imagine the summits there! I want there last year for the first time (C++Now/BoostCon, for the tech-freaks in here).
Amazing scenery…


To get past the tuning problem that began after you inserted the choke, try attaching one or more 1/4 wavelength lengths of wire to the binding post on the EARCHI device (the one with the wingnut). The wire radial(s) will serve to replace the coax shield counterpoise that you open-circuited with your choke. The EARCHI documentation for the matchbox discusses this here:

“Best performance is achieved with a coax of 16’ or longer with the coax shield providing the counterpoise function. Additional counterpoise is usually not required in this design, although the lower wing nut provides a convenient counterpoise connector if needed.”

Hat tip to Allison KB1GMX who suggested this over on the eHam forum when I mentioned your problem in a thread about a similar topic:


If you still have RFI problems, try reorienting the radial(s) and/or antenna wire, and if that doesn’t help you can try adding chokes to the computer leads as I suggested above.


Eric KU6J

Free SOTA Spot Monitor Software + RBNGate FAQ:

Im not sure i discribed the setup correctly.

Long wire - 9:1 unun - 15m coax (not stretched but most of it as a loop on the floor) - and then I installed the choke here - after the choke is the tuner (mfj971).

That setup couldnt tune at all.
A couple of post up there was a suggestion about putting the choke between the tuner and the radio (not after the tuner and before tha long coax that acts as a poise… Like i had)

I need to test that config soon.


Tasos, have a look in Mar 2015 QST, There is a design for a 40m to 6m OCF dipole which needs a 6m pole and looks to be well suited for use from balconies etc.

Another approach worth exploring is an earth lead tuner, there is a circuit of the SM6AQR design on page 32 of Backyard Antennas by Peter Dodd and MFJ market a similar unit. Use a short counterpoise and tune that ignoring the coax.