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FARNHAM VHF GROUP
GB3FM - GB3FN - GB3FX - GB3SN

GB3FM - Specification

GB3FM is located on a site 3 km North of Farnham, Surrey, UK at;

      National Grid Reference;  SU821494
      QTH Locator;  IO91OF

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.

Transmitter Section
Transmitter Based on Storno CQF9662 exciter, modified to provide 3x output frequency, plus on frequency Microwave Modules PA.
RF Output 5W
Effective Radiated Power 5W
Transmit frequency 1297.050
Frequency stability Better than 100 Hz
Typically better than 30Hz
Receiver Section
Receiver LMW Electronics convertor, + CQF9774 receiver front end + Storno M900 IF
Sensitivity Better than 0.7V EMF (0.35V "PD")
Receive frequency 1291.050 MHz
Receiver AFC Up to 7 kHz
Antenna System
TX Antenna Home made Alford Slot, approx 6dBd gain, approx 20m AGL
RX Antenna Home made Alford Slot, approx 6dBd gain, approx 15m AGL
Feeder LDF5-50 (RX), part LDF5-50 and part LDF-4-50 (TX) (we ran out of 5-50 before the bottom of the tower)
Polarisation Horizontal
Duplex Filtering Four cavity notches are available, two for each leg, 3/4 wave at 23cm, from UHF Motorola base station plus some critical length cables. 
A single home made pass cavity is in the transmit leg to clean up transmit noise slightly.

Desensitisation of RX by TX: none.
Frequency reference Motorola ovened 5MHz oscillator
Note: this is not shared with 'FN or 'FX (because it is keyed to provide the FSK callsigns)
Logic Control
Logic PIC based by G4EPX
Access 1750 40Hz tone for minimum of 200msec
Reaccess By carrier.  The repeater sends its callsign every valid access and reaccess (after about a second).
Timeout 8 mins
Reply signal 2333 Hz decaying 'ping' (unless replaced by frequency or signal indication)
Frequency indication 'Ping' replaced by 'H' or 'L' sent in 1167Hz Morse for input signal more than approx 5kHz high or low of nominal input frequency respectively
Signal indication If 1750Hz tone burst is sent within 2 seconds of start of transmission (except the accessing transmission), the repeater will replace the 'ping' with a Morse numeral; 1-8 and N (for 9).  S1 ranges from squelch threshold to about 1uV EMF, successive S readings given at approximately 3dB increases of signal.  Note: this is approximate, not calibrated!
 
Note: the signal indication will not be given if an input signal is more than 5kHz off frequency, and qualifies for an 'H' or 'L' instead.
Identification Callsign appoximately every 2 minutes at 875Hz out of use, and at start of every over at 1167Hz when in use.
QTH locator (IO91OF) and location ("Farnham Surrey") sent every alternate 6 minutes out of use using FM.
FSK callsign every 20 seconds, except when the FM identification is sent, and except when in talkthrough.
The FSK callsign sequence can be occasionally out of operation due to engineering work.
Tone suppression 40dB in notch filter switched in after 200 msec
Subaudible tone 100Hz transmitted at 500Hz deviation during talkthrough. No receiver CTCSS access.
Note that this stays with the 'original' 23cm plan for CTCSS, and does not follow the regional CTCSS frequency plan.
It features as an 'S' ('special') on RMG repeater lists because of this.  No one has complained about lack of a regional tone yet...


Coverage Maps

Coverage 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.

GB3FM Coverage Map - Mobile                         GB3FM Coverage Map - Fixed


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Updated 2nd November 2022

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