If you would like to get involved in designing and building some 80 meter CW transmitters, I have an interesting project. I’ve been involved in Amateur Radio Direction Finding (ARDF) for many years. ARDF is the on-foot athletic competition type of radio direction finding that is internationally sanctioned under a very specific set of rules. I’m a member of the ARRL’s ARDF rules subcommittee and am involved with their other ARDF subcommittees. Lack of affordable and readily available equipment has been identified as a leading obstacle to growing the sport of ARDF in the United States. I’m proposing this project as a personal project, not sanctioned by the ARRL or any specific ham radio club.
A company in China has brought out an affordable 80 meter ARDF receiver. While not great, it’s good enough to get beginners started. But, affordable 80 meter transmitters that fit specific ARDF requirements are needed. The Chinese ARDF transmitters aren’t readily available and lack important features. European transmitters are expensive and have features that don’t match up with FCC regulations.
Now for the tough part of the transmitter’s RF design. The antennas used in competition are a vertical wire 8 to 12 feet long supported by a fishing pole, with no counterpoise other than a ground rod. No ground plane wires are allowed because the competitors would trip on them or pull them loose. This means the transmitter is working into a load of 8 to 10 ohms at best. An antenna tuner would work to correct this but the transmitters need to be able to be built with an enclosure, battery and micro-controller for $50 or less. Conventional variable capacitors would eat up a lot of that budget and tuning them would complicate setting to transmitters.
If you’ve read this far, Thank You. I will be happy to answer specific questions off the reflector but here’s a short list of requirements. The transmitters need to put out around 2 to 5 watts of CW near 3.579 MHz, but with a few twists. ARDF competitions use 2 frequencies. 5 transmitters (Foxes) cycle on/off on one frequency while the Finish Line beacon on an adjacent frequency operates continuously. 2 frequencies programmable via DDS or crystal controlled with easily switchable crystals for something like 3.579 MHz and 3.585 MHz are needed. The Foxes cycle On for 1 minute sending a certain code sequence with the callsign at the end of the minute and off for 4 minutes. Timing must be just right so transmitter 1 identifies and goes off just as transmitter 2 comes on, etc. Timing needs to hold to within 1 second for several hours of operation. It’s best if the transmitters can be placed in the woods while turned off and have them all come on automatically at a certain time. This should be easy to accomplish with an Arduino or PIC as long as the callsign can be changed easily, as the transmitters will likely be passed around from club to club.
If you are interested in tackling this project please contact me.