11/27/2014 23:54

SPY Transmitter


The 'bug' of transmitting from anywhere with simple radios, to stay in touch with my friends, lead me to arrange a solid state, crystal controlled, transmitter, wide band, that covers from 4 to 8 MHz.

A lot of literature is available about those tactical and clandestine radios of the Cold War, from which I took a lot inspiration. Many hours have been spent studying the manuals and the schematics of those wireless sets, and I got a LOT of technical suggestions on apparently simple technical solutions, that are effective anyway today !

Soon I had been punctured by the 'spy bug', and I became attracted by simple, almost trivial, radios and solutions for communicating on the Shortwaves.

Here my first 'spy' wireless set, and my first 'spy mission', when a commercial, relatively low cost, Shortwave digital receiver had been used  for listening, with great results.

That transmitter was meant for AM/CW and my first QSO was around 4.5 MHz, on a September afternoon, with only 2W carrier on AM. Despite the very low power, the antenna, a 90 feet wire, allowed to have a very good contact with a station at 300 miles QRB.

The AM modulation is achieved by putting in series to the RF amplifying mosfet, of industrial and cheap type, another same one, driven by a carbon mike through a voltage elevator transformer. No audio preamplifier is used: a carbon mike driving industrial mosfets greatly helps to keep everything at the simplest level, which feature is essential when operating outodoor in adverse conditions ! :-)

The transmitter is used in conjunction with an 'L' impedance matching network and a simple RF meter to read the peak RF output at the antenna feeding line connection. Proper setting of the antenna tuner allows the TX PA stage to work as frequency doubler and some watts can be squeezed also on the crystal frequency harmonics. The antenna tuner connections are BNC: depending on the frequency and the antenna impedance, it can be reversed to achieve the best match.

The schematic will be published on a near future. Stay tuned ...


Radioman, Nov. 2014





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