F2A - frequency modulated telegraphy

One might ask why one should use such an odd type of modulation - it uses just as much bandwidth as FM telephony, and it is much slower. Well, first of all, it is a great possibility to practise telegraphy if one either hasn't got any shortwave equipment or if one may not transmit on shortwave, and besides, telegraphy is simply great fun! Nowadays, F2A is becoming increasingly popular. For instance, telegraphy clubs in Germany, among them AGCW and DTC, even hold QSO parties in this mode!

This was reason enough for me to built a tone generator and to hook it to my handheld TRX. Here is the circuit diagram (cf. tone generator of DARC circuit service (PDF file), there you find a choice of dimensions for the components, too):

Tone generator circuit (26 K JPEG)

I used parts from the junk box with dimensions as given in the diagram and ended up with an output frequency of about 670 Hz. Not exactly the 800 Hz recommended for Morse telegraphy, and it isn't a pure sine tone, either, but after all, in the old days a telegraphy station was often recognised by its tone ;-)

3 to 4.5 V are amply sufficient to make this bird sing, the output voltage immediately after the low pass (which has a cut-off frequency of about 1150 Hz and which is probably not absolutely necessary) is about 1.2 V (at 3 V input), and it needs to be reduced to about 10 to 20 mV at the microphone socket of the transceiver by means of a potentiometer.

Tone generator and loudspeaker can be connected e. g. to a handheld transceiver Kenwood TH-75; follow the wiring schematics in the handbook for the TRX rather than those for a speaker microphone. I have redrawn the wiring scheme here (to be updated soon):

Wiring for connection to TH-75 (26 K JPEG)

This works for me, although I have to switch between TX and RX, and QSK is not possible with this setup, and some voltage from the TRX is interacting with the circuit so that the tone shifts somewhat after a few characters. I have to find a way to block that. Chances are good that it works for you, too, if you have got a Kenwood handheld TRX, but don't blame me if it doesn't. Always make sure that you do not overmodulate your transceiver, and read carefully the handbook (note that the connection of external microphone/modulator is different for nearly every type of transceiver, and some types require a different AF level for sufficient modulation).

If QSK is desired, one probably needs another type of oscillator, and the connection to the TRX will need some redesign. The problem is that handheld radios usually "expect" an electret microphone as an external one. These are powered with about 2 V DC applied to the microphone socket, and this voltage interferes with the oscillator. If one would simply exchange RX/TX switch and telegraph key in the above setup, it works somehow, but the oscillator frequency is awfully unstable! Back to the drawing board ...


From now on, I will call from time to time in F2A on an otherwise quiet frequency in the all-mode sections of the 2 m or 70 cm band.


Back to DL2LFH's WWW shack.

Jan-Martin Hertzsch