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As the conditions on 10m are getting better I decided to put a small groundplane on my roof. Some time ago, I converted a Sirio New Tornado 5/8 wave CB antenna to a 20m groundplane. That was quite a big antenna and with heavy weather, I sweated bullets. I decided that Sirio would be a good starting point for a 10m 1/4 wave groundplane.

For the 20m conversion, I removed the compensation coil and replaced it with a aluminium shunt. As I didn’t want to use it as a 5/8 wave for 10m with a length well over 6 meters, I did not reinstall the coil but left the shunt in place.

I started with using only 3 sections tube sections giving me a radiator length of around 2.86 meters. The radials all had a length of 1.8 meters and I decided to leave it like that and first do some modelling in MMANA. The results were hopefull, with this lengths, the resonance would be around 29.595 MHz.

At 9 meters above ground level, MMANA promised me nice radiation pattern with two lobes, one at 11 degrees with 1.8 dBi (or -0.35 dBd) gain, and one at 34 degrees at 2.6 dBi (0.45 dBd) gain. I’m not familiar with the optimal angles for DX and continental work but those angles are quite usefull. Worst gain in between those angles is 0.2 dBi (or -1.95 dBd) which is not too bad as well.

Below a 3D plot of the radiation pattern.

Even though resonance was a bit high in frequency, I decided to put it up like that and see what it would do in practice:

As always, the actual performance did not match the virtual world, my RigExpert AA-54 found lowest SWR around 27.2 MHz, seems like the New Tornado wants to make a point being a CB antenna. Nevertheless, the SWR is way below 1:2 all over the 10m band.

Really happy with the results. I did try shortening the radials a bit, but for some reason this did not raise the optimal frequency, it seems that the radiator length is a dominant factor here, not sure. Nevertheless it’s fine as it as, not too big and it seems to work well with WSJTX in WSPR mode. Below is a picture of the results from less than one hour; about 4 transmissions @ 1 Watt ERP), rest of the time RX.

73 Pleun