Circuit description
This transmitter was built using mosly components from the junk box, so the values shown are not mandatory. The oscillator is based on a 14000 kHz XTAL which by chance resonates on about 14002 kHz, then divided by-2 with a JK flip-flop to obtain the signal on 40m.
The power amplifier stage consists of 4 line drivers in parallel (4 out of 8 available on the 74HC240), always keyed. Frequency shift is obtained with the 1N4007 diode in reverse polarization in a sort of "on-off" fashion provided by the NE555 and the extra buffer stage.
You may achieve CW keying by controlling pin 1 of 74HC240, that is the /OE (active on low level) control.
You may remove the squarewave generator at all to obtain a single frequency signal.
You may keep 1N4007 and 220pF cap and substitute 100kOhm+1kOhm resistors with a 100kOhm trimpot to control transmitter's frequency to some extent.
Operating conditions were:
Frequency: 7001.19 kHz
Power: approx. 100mW
Antenna: 2.8m long, center loaded vertical (see picture below), estimated efficiency: 25%.
Modulation: "FM squarewave", 7Hz shift
Location: JN35TC, Torino, Italy
The beacon was battery powered (12V 7Ah gel cell) and located on the balcony where the antenna was hanging from the ceiling. A short length of coax was connecting the transmitter to the antenna. Air temperature was varying between 0 and 15C, but no significant frequency drift was observed. Moreover frequency stability on the short term was extremely satisfactory.
The beacon was enclosed in a plastic box as a protection from birds and moisture.
Positive reception reports with screenshots were received from (in chronological order):
2004-11-19, 0930z, ON6RR in JO20FX
2004-11-19, 1300z, ON5SL in JO10TT
2004-11-19, 1533z, ON6RR in JO20FX
2004-11-19, unknown, DL6JAN in JO60KU
2004-11-19, 1720z-1743z, PA1SDB
2004-11-20, 0830z, ON6RR in JO20FX
Reception reports measured the frequency being 7001.19 kHz. A sample screenshot:
Since I am not an expert of 40m propagation, these reception reports teach me that it is possible to cover a 600-700 km hop during sunlight and after sunset for a couple of hours. 100mW TX power probably mean 25 milliWatt ERP in my antenna setup.
Eccetto dove diversamente specificato, i contenuti di questo sito sono rilasciati sotto Licenza Creative Commons.
The material on this page is licensed under a Creative Commons License, unless otherwise noted.
