Wednesday, 19 October 2016

The Clansman GSA - More thoughts

I have been in contact with Martin G8JNJ, regarding the GSA antenna, he has provided some very interesting insight into the construction of the base unit, which im hoping can be expanded on to find a way to modify the antenna for best performance on the amateur bands.

Below I have copied Martins comments -

The transformer is a variation of a standard UNUN. The winding ratio is approx 3 which gives an impedance transformation ratio of 3 squared = 9.

Its actually slightly less than this as one winding has fewer turns.

The input impedance of the transformer will reduce to a very low value of impedance at the lower end of the frequency range. So the input resistor and parallel capacitor are intended to add extra series resistance which will increase in value as the input frequency is decreased. The intention is to try and make the input match more like 50R across the whole operating range. However this does mean that the efficiency on the low frequencies will be quite poor, as the resistor will be dissipating a large proportion of the applied power.
 As expected the power resistor is intended to fool the radio into thinking it has a good match and driving maximum power. Now, if the antenna was designed for just one small frequency band, say 70-71MHz, then this lossy resistor would not be required. Whether an arrangement of element lengths can give a suitable match without the resistor but also without any serious re-engineering of the UNUN, I dont know yet, but its worth considering.

Below, reproduced by Martins kind permission, is his better diagram of the internals of the GSA - 

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