Sunday, 13 April 2014

CHOTA Antenna Planning

Another post with no photos!

The time has come to start deciding on what antennas will be needed for the CHOTA activation from St Mary's, Lead.September seems a long time away but it will soon be here if im not prepared!

There are several important issues to be considered, of which not a few are essentially mutually exclusive and require compromises. But essentially it boils down to a simple trade off - complexity against frequency agility.

Of course, I could just use a terminated dipole, after all, many Naval vessels do well with them! But, unfortunately, most of my contacts will not be with stations equipped with diversity reception from numerous receive sites around the world fitted with HF log periodics! I could also of course limit my bands of operation, after all this is a one day event and if I cant get enough contacts on 40m then im not trying hard enough!

But I need to be able to demonstrate amateur radio at its best ( I want to be invited back next year!) and a SID (Sudden Ionospheric Disturbance) could knock 40m on the head, and there would I be? Or, perhaps more likely, as occurs with the Doncaster Show, I might find im up against a contest! So frequency agility is a must.

Theres also to be considered the possibility that I might have other operators, so more bands could be utilised. I at least want to have the 20m PSK station running, alongside SSB on 40m, but I'd also like to run a WSPR beacon, perhaps on 10m (IF I ever get it working!)

Two antennas then that Im considering. The first is the ZS6BKW multiband dipole. This variant of the venerable G5RV would give the 'best bang for the buck', and should work fine from the 27ft Larkspur mast. But it would still leave me a single antenna station. The next option, is a Cobweb antenna, covering 20m and up, combined with a 40m dipole (or 40m/80m fan?)

The Cobweb, design variant by G3TXQ, would give me 5 bands, omni-directional HF coverage, and using fibreglass spreaders (which I already have) should be possible to mount atop the Larkspur with a suitably designed bracket. The Clansman antenna kit hung below would give me the 40m inverted V dipole.

As said, I have the 'fibreglass' spreaders already. Mine actually say they are carbon composition but I very much doubt it, I will do an absorption test on them at some point. I just need to work out two things - 1, how to mount the whole antenna onto the relatively thin top section of the Larksput mast, and 2, how to mount the poles and wires to the bracket in a way that makes the whole antenna easy to dismantle and put together in the field.

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