Proposal for a 3cm Cambridge beacon

 

There have been quite a number of Cambridge amateurs who have operated WBFM on 3cm, but few have gone narrow-band. Part of the reason for this is the lack of a gauranteed local signal source. At a national level, the number of UK 3cm beacons is quite small, so another one would be useful.

 

In 2004/2005, the Cambridge Repeater Group approached the Cambridge and District ARC to see if a beacon collaboration was of interest, and this proposal stems from that meeting.

Site

At this time, either Madingley or Barkway are being suggested.

From a local point of view, Madingley would provide a very strong signal - strong enough for testing quite simple WBFM as well as narrow-band equipment. It would be a good site for propagation further afield also, though not as good as Barkway. Either way, it would be a worthwhile addition.

Antenna

The waveguide slot antenna used on the defunct wideband beacon, located at Coleridge Road site would be suitable. If any re-matching is necessary, G4BAO has offered to do this, and would also be prepared to produce a new antenna, if required.

Transmiiter

Links to modules:    864MHz exciter       PLL      3456 MHz multiplier     10368 MHz multiplier      Keying logic      PSU

The plan at the moment is to produce about 0.5W at the transmitter and either use this directly to feed the antenna, or if the feed loss is excessive, to drive a mast-head power amplifier at 1W output. Isolators would be fitted to the transmitter output and also the antenna mounted PA, if used.

A 3U shelf will be used, with the circuitry spread across several (removeable) modules. It is intended to run the equipment from 24v DC, with an external AC PSU provided to run from the mains, if needed. The signal will initially be generated at 6.7 MHz, and multiplied to 10 368 MHz via a succession of x2 stages (to 3456 MHz, followed by a x3 multiplier).

Stability from the unovened crystal source is expected to be about +- 5 KHz over the year, but this will be locked to a Racal 9420 ovened 5 MHz standard which should improve this to +- 100 Hz. It might be useful to provide remote fine adjustment to take account of ageing.

The final frequency will be somewhere between 10 368.800 and 10 368,950 MHz, as per the licence allocation,

Station keying will be 400 Hz FSK, and need only consist of the callsign repeated at 15 second intervals.

 

For discussion

Any useful comments are welcome (to Rob Compton, Mike Newport, or Bernie Wright - or to bernie(at)earf.co.uk.

What is the view regarding an antenna mounted PA, to save on what will otherwise be very expensive feeder cost?

What is the view on where the beacon should be sited?

15th Nov 2006