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January 2023 - Remote Aerial Switching Unit
The inverted V doublet described in a previous article has performed very well - except on Top Band and really, it isn't long enough for the band as it stands. So it was time to try the solution employed by Ian G3ROO with a similar aerial which was to short the feeder and load against ground. This is known as a Marconi "T". This solution was published in September 2002 Radcom. [The G3ROO multiband Multimode Antenna]
Here are the two scenarios.
So clearly, some remote switching is desirable and that involves relays which I failed to find at rallies in a suitable form. Then Ian G3ROO discovered automotive relays. These are the kind that you have in your cars and there seems to be very few varieties. All those I found were identical in ratings and pin outs. Which was great and you could go for the best price. They all had one single changeover contact rated at 30-40 amps. I think I paid around £12 for 5 relays.
Here is an example:
This then was how I wired them:
The Construction.
Fortunately, the local Sunday boot fair provided a toolbox for 50p that was the perfect container which was screwed to the post supporting the Spiderbeam pole.
The Remote ATU is housed temporarily in a food container box above the switch box. Next summer I will be looking for another toolbox!
Both boxes have proven up to the job of keeping the rain out.
Inside the toolbox are the relays, the various connection points and the 1:1 balun for use with the doublet.
The CAT 5 cable from the shack goes first to the box containing the remote ATU supplying power. Then a separate screened twin cable goes from there to the switching box which controls the relays. There is a ground connection in the toolbox to where the screen is connected.
There is no claiming it is a thing of beauty but it works very well. Back in the shack I have a simple toggle switch that allows me to switch between the configurations and compare signals on the two aerial configurations.
The Results
So far the performance on Top Band is remarkable. For instance, in the afternoon, the OK0EV beacon in Prague is barely readable in the doublet configuration but is a solid signal in the Marconi "T" mode.
Plus, the Marconi matches easily on Top Band, whereas the Doublet does not.
On other bands there may be occasions where the vertical polarised Marconi provides a better performance than the horizontally polarised doublet. However, there is a noise penalty to be paid with vertical polarisation, even here in the countryside with fewer houses and therefore, fewer noise sources.