CONSTRUCTING A 1930’s MOPA TRANSMITTER

 

Up until the late 20’s, most amateur transmitters consisted of simple one-tube self-excited oscillators. Be they Hartleys, TNT’s or Tuned-Plate-Tuned-Grid styles, most of them suffered the ill’s of ’29. Not the least of these was frequency instability caused by antennas blowing in the wind. One needs only to listen to the AWA’s annual Bruce Kelley 1929 QSO Party to hear what the bands probably sounded like during the time when these simple rigs ruled the airwaves.

During the 1929 re-vamping of radio regulations and the call for increased control over frequency stability, a new type of transmitter began to evolve … the Master-Oscillator-Power-Amplifier or MOPA. Having an extra stage of isolation between the (usually) self-excited oscillator stage and the antenna, meant that changes in antenna capacitance as antennas swayed in the breeze, would no longer directly affect the oscillator. 
Early QST articles described one-tube amplifiers that could be lashed to the oscillator in order to realize the benefits of isolation. Later articles described complete transmitters, usually using two tubes, to keep duplication simple and to encourage builders to make the change … and to ‘go modern’ in the new era of post -’29 amateur radio.

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