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 now 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.

Ideally, the repeater could do with more ERP, but there really don't seem to be the users out there to justify the engineering effort.  But if anyone wants to donate a 23cm PA module, we're listening!

In 2009, the 'temporary' logic (which had run for many years following the demise of the original Z80 controller) was replaced with a new board based on a PIC.  This reinstates some of the features which were originally provided, such as signal strength measurement, and 'H' and 'L' off frequency indication - and the decaying 'ping' reply signal.  The FSK beacon sequence is also slightly extended, and provides more unkeyed carrier time.

NOTE: The FSK keying sequence has been occasionally out of action, as a backup oscillator has been periodically in use while engineering has been carried out on the site and on GB3FN.

A brief specification of the repeater follows.

Transmitter Section
Transmitter Based on Storno CQF9662 exciter, modified to provide 3x output frequency, plus on frequency Microwave Modules PA.
RF Output 3W
Effective Radiated Power 8W
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, two in each leg, 3/4 wave at 23cm, from UHF Motorola base station plus some critical length cables.   Only the two RX sections are currently in use; TX noise isolation is achieved by antenna separation.
Desensitisation of RX by TX: none.
Frequency reference Racal 9420 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 has been temporarily out of operational from time to time 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.....

Home Page GB3FX Specification GB3FN Specification Feedback

Updated August 2018
Copyright 1997-2018 The Farnham VHF Group