6 Oct 2010 – GB3EDN 23 cm Beacon by Dr. Brian Flynn GM8BJF – GM3HAM – Lothians Radio Society

There was an excellent turnout of 17 for Brian’s talk on the history and update of the GB3EDN 23 cm beacon. He described the development of the current GB3EDN beacon which has been in almost continuous operation for 32 years since its installation in 1978 at the Faraday Building at the Kings Buildings campus of the University of Edinburgh. A simple design, with an under-run varactor tripler (5-watts on 1296.990 MHz) and plenty of heat-sinking, has proven to be very reliable with only two short outages during its history.

The beacon was originally licenced for use with two corner-reflector antennas beaming NE and NW, giving good coverage of central and north Scotland and the North Sea. A new antenna consisting of a vertical waveguide with multiple slot-feeds was installed in 1986, providing an omnidirectional horizontal radiation pattern. Brian showed a map of reception reports from a number of Scandinavian countries (see below).

The motivations for the new design included improved frequency accuracy and stability, reduced power consumption and improved keying with QRA locator added. A number of drawbacks precluded the use of conventional direct digital synthesis (DDS) and so Reverse DDS (RDDS) was chosen instead. This uses a DDS chip as the divider in a conventional phase-locked loop (PLL). Conveniently, the UK Microwave Group provides a suitable printed circuit board to implement RDDS circuitry. This was designed by Sam Jewell G4DDK who writes the RSGB RadCom “Microwaves” column. A multi-section interdigital bandpass filter will be added to ensure spectral purity. The new beacon is not yet complete. Brian hopes to have it operational on the air by next spring. (Alan Masson GM3PSP)