Hartley / MOPA TX
In search for a funny, simple, instructive way to communicate among us in Radiotelegraphy on the Shortwaves, one of the gang ended to assemble a HARTLEY OSCILLATOR - POWER AMPLIFIER transmitter, made with valves.
This wireless set comprises two sections: the oscillator/driver stage and a power amplifier ('MOPA', for those who don't know, means Master Oscillator Power Amplifier). It can deliver more than 20 W output. The oscillator is a variable frequency type and the voltages are not stabilized.
One day we decided to test the driver unit alone, on the air, without power amplifier. It is made by an ancestor of the 6SN7 tube. The receiver was a USB dongle SDR: very cheap and very easy to use, with adequate performances: to say that we are not necessarily stucked to old fashioned technological solutions, even if we prefer them for they are very fun to use.
Well, with great surprise we were able to exchange information, reliably, from 2.7 up to 11.0 MHz, on a distance of roughly 200 miles, on the lower frequencies during night and on the higher frequencies during day time. We know that with such low power much more is possible, but repeatable contacts on 11.0 MHz at such small QRB were a bit unexpected. The 2.7/11.0 MHz range is covered by a full rotation of the variable capacitor. No coil change needed. The unit is keyed in the 'B+' line (i.e., in the anode voltage supply !). The driver unit puts out from 3 to 4 Watts, this output level is constant on all the frequency range and its CW tone quality is superb. The frequency drift amounts to a few tens of Hertz during a half an hour QSO.
The coupling between a pre-WW II transmitter with a SDR receiver was a winning approach for reliable, easy and cheap way to exchange information on the Shortwaves.
We all know that a modern transceiver could have been taken off the shelf in a shop. We all know that commercial power amplifiers are available and modern digital modulation systems too. But ... where's the fun, then ? Where's the lesson to learn ? You will be pretty soon bothered to use a modern device because it allows a limited set of alternatives for experimenting, being those determined by a fixed number of menus or parameters. Try, instead, to assemble something like this and try to make it working: you cannot immagine the satisfaction you will get. Mostly because, in the end, it is made by only a bunch of components. And the difference is made, anyway, by the ability of the Operator. Like in the good old days.
Radioman, Nov. 2014
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