GB3FM - GB3FN - GB3FX - GB3SN
GB3FM - SpecificationGB3FM is located on a site 3 km North of Farnham, Surrey, UK at;
National Grid Reference; SU821494
The site is 187m above sea level and is shared with GB3FN and GB3FX, the group's 70cm and 6m repeaters, and the GB3FNM beacons. It operates as a combined beacon and repeater on the 23cms band. It has been operational since 13 March 1988. It runs about 5W ERP horizontally polarised from an Alford Slot. It used vertical polarisation between 1997 and 2007, but reverted to horizontal polarisation in late 2007.
Some antenna rework was carried out in 2008 and the repeater reverted to two antenna working. Because of the relatively low level of use, the transmitter was put into the top antenna to give the best possible performance as a beacon. The transmit notch filters were taken out (unnecessary with the vertical antenna spacing) which improved the ERP to approx 8W.
Following the demise of the antennas in 2022, a temporary antenna has been erected low down on the tower (see 'News') and the single antenna filter has been reinstated.
NOTE: The FSK keying sequence is occasionally out of action, as a backup oscillator is occasionally used on site while engineering has been carried out.
A brief specification of the repeater - when operating with normal antennas - follows.
Coverage MapsCoverage predictions are likely to be more approximate at 23cm than for our other repeaters.
There are two maps plotted - for normal antenna operation (sadly it's somewhat reduced at the moment):
GB3FM mobile - whereas 23cm mobiles are few and far between, this gives an idea of where the repeater might be heard. The repeater should be audible over the yellow area with a simple setup, such as a portable inside a car (perhaps on the dashboard). The green and blue areas will probably need an external antenna, such as an Alford slot on top of the car; the blue area probably audible only when there are not local obstructions near the mobile.
GB3FM fixed - this is intended to give an idea where a fixed station with an outside aerial may be able to hear the repeater. The yellow area has a good probability of success and the green area should also be achievable for a fixed station with a reasonable aerial without too much local obstruction. The blue area may still be good to receive the repeater, but is likely to need a good aerial and possibly some careful location of the aerial to avoid local obstructions.
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