The Bromley and District Amateur Radio Society


by Chris Whitmarsh G0FDZ

GB3VHF - A new replacement beacon designed for the 21st century
After several decades of excellent service, the 144.430 MHz beacon located at Wrotham in Kent (JO01DH), was replaced on Sunday 26th February 2006 with a new state of the art beacon incorporating several new RF and digital features.
The beacon which is the result of a year of design and construction by a team of four radio amateurs, provides facilities some of which have never been used in amateur radio beacons before.
At the heart of the beacon is a direct digital synthesiser designed, constructed and programmed by Andy Talbot G4JNT. This generates a signal directly at 72MHz and is L/C and crystal filtered and doubled to 144MHz. before passing to the PA.
The DDS clock source will be locked to GPS with a short time-constant phase-locked loop, and will normally maintain an accuracy to within a few parts in 10-9 over a period of a few tens of seconds, and better than 10-12 long term.
The frequency of the CW carrier (mark) being exactly on 144.430000MHz.
The most important feature of the new beacon is that by using the DDS, the beacon can be programmed to transmit new modes. As before, the beacon sends its callsign and locator in Morse, but using A1A (on /off) keying rather than FSK
To enable the beacon to be monitored at extreme ranges, the beacon additionally transmits it's callsign and locator using WSJT JT65B mode.
The GPS also provides for the precise timing of the keying sequence, such that the JT65B sequence will start at every even minute past the hour for 48 seconds duration.
The Morse sequence will commence at the start of each odd minute past the hour and last for 13 seconds.
At the start of each odd minute 30 seconds past the hour, at a precisely timed point, 140 microseconds after the UTC one-second reference as signalled by the GPS receiver, the phase of the carrier is reversed, 28 times in total to fill up the 30 second time slot. The result is a 1 bit/second pattern of 101010….. The BPSK mode has been incorporated to allow users to become familiar with using precise timing methods to assist in experimentation with coherent signal recovery, to measure time of flight information and propagation testing.
In the event of GPS lock being lost, the beacon will suspend the JT65B sequence and replace it with the Morse sequence, until such time as GPS lock is re-established
The RF section which was designed by Sam Jewell G4DDK, relies on a Mitsubishi RA30H1317 power amplifier module for the PA. The use of extensive RF L/C and crystal filtering throughout, ensures that the beacon complies with the most stringent CEPT regulations for transmitters operating in the VHF frequency range. The beacon has undergone exhaustive tests in this respect.
The RF power output of the beacon is 30 Watts
The power supplies delivering both 12 & 24 volts for the entire beacon, have been designed and built by David Bowman G0MRF who has ensured that generous component under rating and spare current capacity will help to provide the high reliability needed. Over-sized heat sinks mean that no fan cooling is required, whatever the ambient temperature.
Chris Whitmarsh G0FDZ, the beacon keeper, integrated the various modules and units, and produced the metalwork and housing facilities for the beacon. Ease of maintenance was very much in mind when designing the beacon, so as to ensure minimal outage times when any faults occur
This new beacon hardware is only the fourth to be employed, since the beacon first became operational in 1959 at the end of International Geo-physical Year (IGY).
The beacon antennas which comprise of two 3 element yagis beaming 288 and 348 degrees and located approximately 48 metres AGL are unchanged, and have been employed since the mid 1980's when the current mast replaced the original. The base of the mast is approximately 213 metres ASL.
Software to decode the JT65B is easily obtainable on the Internet at the WSJT website http://pulsar.princeton.edu/~joe/K1JT/ .
To decode the JT65B, tune the carrier to obtain a tone of 1500Hz with the receiver set to USB, and the dial frequency reading 144.4285. The JT65 will then be tuned in correctly.
The group would particularly like to hear of reception reports of the beacon from all corners of the British Isles and from mainland Europe.
 This beacon has been financed by the four participants only, but the team gratefully acknowledge the donation of some parts for building the beacon from Russ G4PBP, Derek G3GRO, Dave at Quartslab Ltd and Simon G3LQR.
For details of the beacon DDS please see Andy Talbot's website at www.scrbg.org/g4jnt/ ,and for more information and photos of the beacon please see the beacon website at www.g0afh.com/gb3vhf/
Chris Whitmarsh G0FDZ (Beacon Keeper GB3VHF)
Andy Talbot G4JNT
Sam Jewell G4DDK
David Bowman G0MRF
Reception reports are most welcome. Please follow this link and use the feedback form to send your report. Include details of your call sign, locator or location, and any comments.