Steve's Place

Photography and Astrophotography

New antennas at G4KIV/G1G

I had a “play” a few weeks ago in the RSGB DX contest. I only used morse code and I didn’t bother with 80m but still came 4th in the UK (2nd place in CW). This was encouraging enough to make me consider the antennas I currently have and I “think” I can obtain better results with different antennas.

So the Prosistel PST1524TV Vee Dipole has been replaced with a 5 band Cobwebb (with thanks to my good friend “Nash” for the loan). This is the original Cobwebb antenna by the late G3TPW.

In addition, I have also added 40m and 80m traps onto the inverted L. The inverted L is still about 4m “too long” but it does tune up on 160/80/40/30 and a few other bands using the CG3000 atu so I am leaving as is at the moment.

Using a Rigexpert AA55 analyser (oh wow this is a superb piece of kit), I managed to adjust each of the 5 bands to achieve resonance in the cw/data part of the band and obtained the following resonant frequencies / swr results … one thing that I could not find online is “how much to trim” on a Cobweb antenna to raise/lower the resonant frequency so I have included some measurements that might be helpful for others tuning a Cobweb antenna

28.125MHz @ 1.22:1 – adjusting 3/4″ each side results in a 260kHz in resonant frequency shift

24.990MHz @ 1.2:1 – adjusting 1″ each side results in a 220kHz in resonant frequency shift

21.130MHz @ 1.2:1

18.135MHz @ 1.10:1 – adjusting 1″ each side results in a 100kHz in resonant frequency shift

14.080MHz @ 1.37:1

This was very pleasing – however, knowing how much to extend or reduce (fold back) was something that I could not find anywhere online.

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