VK5FQ | 630m Transmitter

630m (475kHz) low power transmitter

Building a simple 475kHz transmitter for the MF band.

Fear not! The hour is upon us ....

Australian amateurs have been granted a portion of the MF band between 472 kHz and 479 kHz and unless you have been living under a rock lately, there have been a number of publications over the last few years showing us how we can build transmitters of various designs, power levels and with varying degrees of complexity or simplicity. I've chosen the latter for this low power version. I would seriously consider more levels of complexity in a higher power design. Personal safety needs to be paramount when building any transmitter.

Aha! Been there. Done that.

I was lucky to stumble upon the internet Hans Summers' website. Hans makes and sell various kits and parts and among the list, there is a number of WSPR beacon kits. The initial one I chose was the Ultimate 2 QRSS kit. This has now been replaced by the Ultimate 3 QRSS kit - providing the builder with more "bang for buck". These little kits include a pre-built DDS module which connect to the little micro control board and a liquid crystal display. Along with two switches and a minimum of one output filter board, you get a fully programmable multimode transmitter. Albeit low power, in the order of hundred or so milliwatts, it does provide a quick on air approach in a kit form from about DC to 30 MHz quite reliably. Some soldering of some connectors and PTH components along with winding some small toroid cores for the filter makes this a fun kit build. Judging by the amount of international contacts via the WSPR website, this kit has performed really well on most of the ham bands. Follow the link below for Hans' website and further details to some cheap kits. Highly recommended.
Ok. That's the heart of my transmitter. Next. Yep. Someone has already thought about this and added a PA to the output of this module. I was quite happy to follow suit. I've included the link to the PDF file below. I've also modded the U2 kit so the output switching FET is used to switch a relay to provide better rf isolation on the RF output to the PA circuitry. This being deemed necessary (by me), as seen by a number of other users as the only real flaw to an otherwise fantastic kit. The frequency does seem to vary under load if using the default output setup, yet I have not experienced any such issues using the relay isolation method. In fact, long term stability is quite good, as noted by running as WSPR beacon often overnight. There is a calibration process for both the crystal oscillator and the DDS output frequency that are well worth pursuing. Of course, there is also the GPS input option, which would also help. See kit manual/instructions for further details.
The PA's input has a simple one transistor buffer amplifier which also acts as a dc switch for the following squarer circuit when not in transmit mode. When not in transmit mode, 5 volts is applied to the base of the transistor via a current limiting resistor to turn it hard on. The squaring circuit transistors are therefore turned off and no signal is applied to the output FET. When transmit is on, 5 volts is removed and the DC input is derived from the generator along with the RF output from the DDS module. This modulates the first transistor's base and "bangs" the squaring circuit between the power rails (less C-E drop) and swings the FET input and hence, FET output accordingly.
The PA bits were built onto pcb material which was affixed to the aluminium baseplate which also serves as a heatsink for the output FET. The whole project was placed into a recycled RF transceiver housing, extending its life a little further...
The only real modifications to date, was to change the the greencaps to siver mica variety. This improved the RF output stability and current capability, especially into not-quite matched loads, found during testing! Only one FET lost its life in the course of initial test transmissions and none, since the changes and many on-air transmissions.

Final results.

The DC power consumed by the output PA chain (alone) is around 21.6 Watts. (15 Volts x 1.44 Amps DC) The calculated RMS output power is around 18 Watts. This gives us an efficiency of more than 80%. Not bad for a small transmitter.
Now, to build the bigger version, using the Ultimate 3 kit and a bigger PA ....! Stay tuned.