Every time I visit the Wyong field day I look forward to interesting radio items. These are not necessarily functional items or other valuable parts or lab instruments, they are radio related curiosity items which I purchase just for fun. In the past, I have purchased a life boat radio with no batteries and a hand cranked generator, I 2182 kHz emergency radio monitoring receiver (which was converted to 1843 kHz AM) and many other similar items. This year, the curiosity item was a collection of radio buoys that are used by fishermen to locate their drift nets. They seem to operate in the MF part of the spectrum, from 1600 kHz to over 3 MHz. Two of them are simple TX-only beacons sending out their callsigns (non ITU, possibly self allocated) and a long dash (about two seconds) which, I am guessing, is used for the RDF system to get a good angle measurement. The other one contains a receiver which is tuned to 2331.5 kHz (AM) and it expects a specific sel call number which, if received, will activate the beacon. I have no idea how these things are licensed and I have a gut feeling they may be not.
The two TX-only radio beacons are made by the Japanese manufacturer Ryokusei and are model SV-CM3B. There are two boards, one produces the RF and is based on a rather beefy transistor (2SD1046) and the other produces the modulation.
Callsign, long dash Callsign, long dash Callsign, long dash About 3 minutes of silenceThe frequency of the transmission is determined by a HC6 crystal and it is 1800 kHz and 1810 kHz for the two beacons I purchased. They are powered by a rather large 24 V battery and they produce about 10 watts of RF. I wasn't sure about the design output impedance but I noticed that the last element before the connection to the whip was a large variable inductor made with Litz wire. I bypassed that and plugged it to a 50 ohm dummy load and saw the following waveform on the CRO.
