Monday, July 21, 2008


Well its been an interesting summer (Not that we have had much sun yet) conditions on the HF bands have been poor but its motivated me to make some improvements to the new antenna farm.

I finally constructed a trap for 80m out of RG58 and extended my inverted V for top band. This should have been simple but I noticed that the trap tended to drift during transmit. I’m not sure if the coax was warming up or what but its very annoying as my auto ATU could start to re-tune half way through a QSO. The top of the vertical was very hot & experiencing very high voltages.

I decided to replace the trap with a simple coil and capacitor. I also tuned the trap away from resonance to cool things off. Being the skin flint I am I thought that I would build the 100pf capacitor out of coax.

Well that worked well and was very stable for about 2 minutes at which point I exceeded the breakdown voltage of the coax and it burst into flames! It was in the air so apart from the coil former no damage was done.

I tried again but used a 100pf doorknob capacitor with low drift... it cost me a bit but still drifted >:-(

I had a re-think and decided to have another go building a coaxial trap. This time I used RG412 PTFE coax and a larger 2.5” coil former. RG412 coax is the same dimensions as RG58 so easy to wind into a coil but its not cheap.

Success! The trap was tuned to 3.8Mhz and there is no drift running full legal (400 watts in the UK). Now I have Top Band as well as 80m. You can find me on 1.84Mhz or 3.85Mhz RTTY


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