Balun for G0FTD's Indoor Antenna

I recently had a change around in the bedroom shack which meant re-routing various mains cables and feeders.

I noticed that due to the slight imbalance inherent with the loop antenna being fed by my ATU situated at skirting board level, that the new cabling arrangements suffered from more common mode currents and other undesirable effects.

This had the effect of killing my 40m performance as well as making the antenna difficult to tune because every time I touched the rig's metal casing or ATU the SWR would vary wildly. 28Mhz performance was OK, but once again the hand effect was evident.

I decided to tackle the problem by making a simple balun.

Normally the inherent imbalance is NOT a problem but under some circumstances it can be such as in the case of a self contained shack such as mine.

I constructed a dual 1:1 and 4:1 balun on a 3 inch piece of non-descript ferrite rod pinched from an old domestic radio.

A trifilar arrangement was made with approx 8 trifilar turns to ensure LF efficiency and 18SWG enamelled copper wire fitted to the rear of my ATU. I could then select either 1:1 or 4:1 ratio's just by swapping one flying lead on the balun. I use the balun in it's 1:1 configuration.

The results were conclusive - it cured the problem and restored the antenna's efficiency back to what it should be as well as as well as clearing up a lot of the LF noise and shack PC noise - brilliant !

One thing that was immediatly evident was that ATU tuning was VERY sharp. So sharp that it's easy to miss the correct points and assume the antenna can't be tuned - so be warned !

I can easily tune the antenna from 80m - 6m with no problems and my first two QSO's were with YO-land using 25 watts SSB on 7.1Mhz and with EA8TX on 28MHz SSB once again with 25 watts.

So it might pay users of this loop antenna to experiment with a simple balun which could improve performance.

One extra note about the balun - I found that the ferrite core on 80m easily saturated with as little as 30 watts so QRO user might like to try less turns on the balun. However since I mostly work QRP I decided to leave the turns alone and gain a little more efficiency at QRP levels.

Regards - Andy, G0FTD

• Read the original Indoor Antenna article