Spark-Gap Transmitter

Now this isn't really for QRP (in fact, it's not at ALL for QRP), but it was in the projects list, and since the original poster included it (as a joke), I thought I'd make it available too.


The two tubes form a push-pull power oscillator, which makes alternating current, but at a radio frequency. This is done by the use of L1, L2, L3 and the two little tubes (type 833 - alternatively F-24-a). The actual frequency is not really important, as the object is to generate maximum bandwidth .. the goal of any phone station. C1 is used to obtain maximum power by resonating L1 to L2's natural frequency.

This RF voltage is stepped up by a factor of 30 to 100,000 volts by L2 & sent on to the spark-gap where the actual RF-to-be-sent is generated. An RFC accross the antenna is reccomended to keep the antenna circuit at DC ground, yet will not short out the RF ... which is sent from the spark-gap to the antenna.

The antenna wants to be one wire, and end fed. No co-ax required .. just "hook er up and duck". The length of the antenna determines the transmit frequency. the formula to use to get that length is:

FEET = (468/(((F*F)-(F*F)))*35.5

F = frequency in MHz (rounded off to the nearest 100KHz)

Voice modulation is done by use of a special Carbon Microphone, as used in the 1920's ... which can handle several amperes. Here, rather than sending R.F. through the MIC, as was done back then, only DC is passed through. That way, the operator (or inflictor) won't get an RF burn on his lips if he speaks too close to the MIC. That modulates the power to the tubes, which varies the intensity of the spark which then puts quite a suitable signal on the air. The intellegibility will match the content of some of the QSO's now on the air.


1. Place mouth near Carbon Microphone
2. Close PTT switch and adjust C1 for a dip in anode current - QUICKLY!
3. Make OINKING and MOOING noises to indicate your presence to everyone
4. Open the PTT switch when you get around to it

Have fun, de HARRY, Lunda, Sweden.

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