Our speaker at the January meeting (of the Sutton and Cheam Radio Society - ed) was Chris Whitmarsh - GØFDZ. Chris is beacon keeper for our local 2m beacon, GB3VHF which is located at Fairseat in Kent. GB3VHF is just one of many hundreds of beacons located around the world, operating from HF right though to the microwave bands. The purpose of this network is for checking propagation, for the optimisation and testing of equipment & antennas and for various scientific experiments.
Most amateur radio beacons operate within the amateur bands although others operate at 5Mhz, 40Mhz and 60Mhz and require special licences. Each beacon is the responsibility of its own licensed beacon keeper who needs a variation on his or her amateur licence in order to operate the beacon. A fairly long a tedious approvals procedure is necessary to establish a beacon and this requires the involvement of Ofcom, various government organisations and the military. In the case of shared bands, such as 70cm, the procedure can be very lengthy indeed.
The UK’s beacon project started in 1958, which was 'International Geophysical Year'. The first amateur beacon to be established in the UK was GB3IGY, located near Orpington. In 1960, the beacon was moved to Wrotham where it remained for many years using the current callsign GB3VHF. The most recent move was in February 2010 when the beacon was set up at Fairseat, just 4km from Wrotham.
There have been a few changes of hardware and keying methods over the years. The original 1958 beacon had morse generated by paper punch tape, which proved to be unreliable and was soon changed to electromechanical transistorised optical disc keying with further moves to microprocessor controlled keying and EPROM keying.
The current beacon was designed and built by a team of 4, namely:- G4JNT, G4DDK, G0MRF and G0FDZ. 19” Rack construction has been used with all external connections on the front of the equipment for ease of servicing. The latest incarnation of GB3VHF now sends its callsign and locator using on/off A1A keying rather than FSK as in the past. The latest version also transmits information using WSJT JT68 mode and is locked to GPS to provide exact timing for the start and finish of each period of WSJT or Morse transmission. The Morse sequence start at each odd minute and lasts for 13 seconds and the WSJT sequence starts at each even minute and lasts for 48 seconds.
An added feature of GB3VHF is that the WSJT transmission carries accurate time information and can be used for 'Time of Flight' calculations. The time for the beacon's signal to reach an amateur’s station should be predicable. If it takes longer, the signal is not taking the shortest path indicating that some interesting propagation conditions could be about.
Getting back to the relocation of the beacon to Fairseat, GB3VHF had been operating for many years at Wrotham. However, the site has ‘changed hands’ and is now operated very much on a commercial basis by Arqiva. The combination of withdrawal of funding, high rent and difficult emergency access made relocation necessary.
The site at Fairseat is owned by BT and is used for research and scientific studies. 15 months of negotiation were finally successful and the new beacon was installed at the site by professional BT engineers. Chris showed us some photographs of the repeater hardware including the two Jaybeam 3 element beam antennas mounted on BT’s mast. These are connected (via a phasing harness) to the beacon transmitter, located in a building near the foot of the mast, using 100 metres of LDF-550 coaxial cable. The antennas are set at beam headings of 288 and 348 giving good coverage of most of the UK.
Many signal reports have been received from all over the UK and much of Europe. The addition of WSJT mode has also extended the coverage to many parts of the world.
Many thanks to Chris for his detailed, informative and enjoyable talk. Further information on GB3VHF, its history and a selection of photos can be found at: www.g0afh.com/GB3VHF
John - GØBWV
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