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At The Event
Our original expectation was that the earth-path impedance would not be significantly reactive – and I think that probably was the case.
We planned to provide a fair match (by means of a tapped output transformer) to any resistance between 40Ω and 400Ω; but when my garden TPS 'laboratory' showed a constant 40Ω all through the winter, spring, and into the summer, we convinced ourselves that the operational earth paths would be pretty much about the same.
All are in the same alluvial chalk of the Wylye Valley, and are separated by a maximum of 3km.
As TPS set design matured, our thinking moved away from the matched TX putting nearly 200W into the ground, to an unturned TX putting about 15W into the ground.
The latter configuration provided perfectly readable signals over a 160m earth path at Upton Lovell; and avoided all the expense, complexity, and delay of tapped transformers and tuned filters.
The test set up showed some evidence of mains hum, but nothing bad enough to degrade reception.
At that stage, the varying whine of the TX PA provided the only form of side tone.
The operational conditions at Codford event proved to be rather more challenging.
Firstly, the earth-path resistance at CDS (Codford South) was about 140Ω, which reduced this station's output power to about 3.9W.
At CDN (Codford North) it was even worse. Here, a 200Ω path reduced TX output to about 3.5W.
Both stations continuously received very high levels of audio-frequency hash, and this all but prevented any communication.
The watering of electrodes provided no discernable improvement.
A very effective (ie loud) side tone was provided by random cross coupling of the TX signal into the rx.
We have some ideas about how to fix all those issues; but, since all fixes would involve engineering effort (and expense), we are taking a little time to reflect on lessons learnt.
More generally, we are considering whether we want to continue this as a project; or to accept that we have completed the original mission, and call it a day at that.
We certainly did provide some good support to the Codford event. The punters seemed interested in what we were doing, and the event organisers were obviously delighted.
So, we could now retire gracefully, having had some fun and learnt a few things along the way ... or we could continue to play TPS for its own sake, and see if we can develop our toy into something more like an operation system.
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