Last weekend, just before the IOTA contest I decided to try and tune my OCF/Windom so that it resonated on at least some of the ham bands.
The OCF started life at 10.5m long, I then extended it to 21m so that I would get better coverage of 40 and 30m. However I discovered that it was not resonating where I wanted it (6300kHz gave the best SWR).
The way I tuned it was not scientific, and I don't have access to an antenna analyser so I used the SWR meter that is built in to the IC756pro. Probably not the most accurate but good enough for my needs.
Anyway I started trimming the OCF, 6 inches at a time. After each cut I came into the shack and tested the SWR on some spot frequencies. Slowly the resonant frequency rose. In the end I chopped off about 3 feet/1metre off the long side of the OCF, making it about 20m in length - a half wave on 40m. Lowest SWR was on the low end of 40m, which was perfect.
Now comes the part I didn't expect! Being a half wave dipole I was expecting a reasonable SWR on 21MHz as that is the 3rd harmonic - the SWR was off the scale! BUT the SWR was low on 7, 14 and 28MHz - EVEN harmonics.
One other frequency has a low SWR and that is 18MHz, I presume that the 10 feet of coax that acts as a top fed vertical radiator is having some effect there. The SWR on 50MHz is about 3:1 so operation is possible on that band with the ATU.
So we have low/lowish SWR on 40, 20, 17, 12, 10 and 6m, and very high SWR on 30 and 15m.
There are only two bands I cannot use the antenna on and they are 160m (obviously!) and 60m (I don't have an NOV so that is not an issue). However my Wellbrook loop works well on those bands so I can receive on them.
For a 20m long antenna that I have fitted into a 7x6m garden I am very pleased with it. There are full details and pics on my website .
Tuning an antenna just before a contest isn't really the best idea because any manner of things could go wrong! Luckily Murphy was away annoying someone else that day and it all went smoothly. I have noticed that resonance changes according to the weather. I guess this is due to the close proximity of wood to the antenna (it is mounted on a 10m dead tree and all the supports are wood).
Once it has been up for a couple of months I will do some further resonance checks and compare them to the original to see if there are any noticeable changes. It is useful to do this from time to time as it enables you to spot possible problems, such as a poor connection, water in the feeder etc. If there is a significant change in readings it is worth investigating as the sooner it is found, the easier it will be to fix (in theory at least!).
Watch this space for more reports on the skywire.