Here is a description of a simple Lowfer beacon transmitter. The transmitter crystal in my prototype is a third overtone 52.5 MHz unit that is run in fundamental mode. The CPU clock can be taken from the transmitter frequency divider if you want the keying to be synchronized to the carrier. Another option is to clock the divider from the CPU clock output pin instead. The output signal is a square wave with very fast rise time which means there are a lot of odd harmonics. The narrow bandwidth of a typical LF antenna will probably reduce radiation of unwanted harmonics to a sufficiently low level. However, the lower part of the antenna system, below the loading coil, may radiate too much harmonics. In this case, a lowpass filter must be used.
The transmitter board shown above was used in the first SM / SM QSO on the 136 kHz band. With a supply voltage of around 27 volts I got about 4 watts of output power and managed to get a CW QSO with SM6PXJ who is located 120 km from my QTH. Not bad for two cheap TO-92 transistors!
Another small longwave transmitter can be found on K0LR's web page
If you are looking for a BIG one, visit G3YXM's page
This transmitter was designed to be used by Lowfers in the U.S.A. where unattended beaconing is allowed in the 1750 meter band. In most other countries, beaconing is probably FORBIDDEN unless you have a special licence. You may wish to omit the PIC processor and associated components and use the board as a simple CW transmitter. Another option is to program the PIC to work as a CW or BPSK generator. Then, RS-232 data from a computer can be fed to the serial port in real time.
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