Sunday, 16 October 2016

The Clansman Ground Spike Antenna Laid Bare

Regular readers may have noticed, that I like to see whats inside things, and how they work. Now, most of us who play with vintage radio, military radio, or ham radio in general, will understand this, and have probably had either a sneeky peek inside your rigs, or been in them from necessity of repair and alignment. But, there are some bits of kit which always seem to remain a 'black box'. The Clansman GSA base unit is one such item.
GSA Base Unit

But, thanks to Mick over in Barnsley, I have two of these, so tonight the curiosity was too much, and I decided to find out what makes this unit tick!

Well, one thing that is very obvious when you open one of these, is the coax wound balun

All well and good you might say, lots of antennas have a balun. But, this is military kit - its designed to work in the worst of conditions, regardless of the abuse it receives, and to be used by non-specialist squaddies. It has to work whatever frequency its put to, without any adjustment or alignment, and without a proper, efficient groundplane.

How it achieves this, and just why it often seems not to perform as well as might be expected, becomes apparent as we look under the balun -

Yes, thats an 18 ohm power resistor! The GSA is a resistor terminated antenna.

The result is that resistive loss factors dominate over inductive or capacitive, smoothing out the  impedance curve, and resulting in a flatter VSWR across the design range. The result is that although the antenna itself might not be always as efficient as it could be if properly matched, the load the radio sees remains reasonable at all frequencies and so maximum power transfer is maintained.

A rough schematic of the antenna is shown below. Note that im not very good with balun design, so please dont try to rewind one based on my diagram!

At first, this might seem a get out, too much of a compromise, perhaps even a poor design to save costs? But no, of course not. The build quality shows that. Its a simple fact - in the field, getting the message across is paramount, and the kit has to work regardless of the abuse it gets. As amateurs we are often dismissive of the ranges given in the Clansman manuals, as we get much greater ranges - but these are naturally conservative, worst case figures. Its the old adage in military radio - make the rear echelon station take the brunt of the link effort!

I also now have a respirator microphone! It will likely never be used for actual comms, but Halloween is coming up soon...


mick said...

Hi martin,
i for one enjoy these lets take a look inside posts, i believe
the ranges quoted in the manuals are the ones that there
is at least a 90 percent or above chance that the message
will be recieved .
i now have a mental image of one of these scary clown
idiots being cased down the road by someone in full NBC
kit (complete with comms) this halloween.
best regards

Zak The Rabbit said...

I bloody hate clowns!