SOTA 20m Dipole

Written on 2 July 2019

I have had a lot of success with my EFHW antennas for SOTA activations but discussion on the RSGB Technical Forum made me wonder if the matching transformer was too lossy. If it is open- or short-circuited you would hope for an infinite SWR, although in practice 10:1 would be good. I was getting around 2.5:1 when open and 6 or more when shorted. So it appears to be lossy, although it clearly works and is easy to deploy. It is inherently multiband too (a half-wave for 40m is a full-wave for 20m, for example).

For comparison I decided to make a 20m dipole. Last Saturday, on the hottest day of the year so far, I had a few hours to myself so thought it would make an ideal garden project. I used the SOTA linked dipole calculator and decided to cut 5m of wire plus 2m of nylon cord for each element. I also cut another 7m of cord so that the antenna would be self guying (i.e. the two elements plus the cord would be the guys). I wired this up with 9m of RG174 coax and hoisted it up my new 6m Decathlon fishing pole so that the centre was 5m above ground.

Unfortunately the FT817ND plus SWR meter showed that the SWR was far too high. The SWR was lower at the top of the band than the bottom. As I generally only operate CW the aerial was too short. It’s much easier to shorten a wire than lengthen it. But I thought the first thing to try was a balun in case the coax was detuning it. So I wound about 14 turns of the coax around an FT140-43 toroid and reconnected it. At this point rain stopped play so I couldn’t try it for a couple of days. When I did it hadn’t helped. Tonight I got to do some more experimenting and record the SWR readings.

At 14.005MHz I was getting 4:1 and at 14.345MHZ it was 3:1.

I have a couple of wire aerials over the lawn so I thought it was possible they were detuning the dipole. I could easily lower the 20m Zepp that I no longer use - after doing this I was getting readings of 2.8:1 and 1.5:1. I was surprised by that but then realised that in order to lower the Zepp I had moved one of the dipole elements. I put it back to its original position and got readings of 4:1 and 2:1. At the CW end of the band it was just as bad as before. Then I noticed my daughters’ swing is near the element I had moved. This big lump of metal was probably responsible so I rearranged the dipole again, away from this metal and also made it straighter. This time the readings were 2.5:1 and 1.3:1. It was looking better but still too short.

For portable operation I’d like to avoid using a balun. It’s extra weight and potential wind loading. So I removed it and found it made no difference to the SWR.

To make the elements longer I untied where they meet the nylon cord and retied them with shorted tails. This way I made each element 5cm longer. This time the readings were 1.9:1 and 1.1:1. It’s still too short but these readings are OK. My FT817ND’s own display showed very little SWR so it’s looking good. I need to make the dipole centre into a more permanent form and in doing this I can probably make the elements slightly longer which should help.

So trying to tune an antenna in a small garden is fraught with difficulty. I shall be trying it in the field soon.