The Farnham V.H.F. Group - GB3FM Specifications

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 only shared with GB3FN and GB3FX, the group's 70cm and 6m repeaters. It operates as a combined beacon and repeater on the 23cms band. It currently uses vertical polarisation. It has been operational since 13 March 1988.

A brief specification of the repeater follows.

The repeater used to operate with horizontal polarisation. Unfortunately, one of the Alford slots and also one of the feeders suffered from water ingress. As the top antenna on the tower is multiband, we reconfigured together with some fairly complex filters to run the repeater into this antenna, and so it now runs vertical polarisation. Unfortunately (that word again) after abour nine years of service the multiband antenna went faulty. It is still in use on GB3FM as the performance does not seem to have dropped off that much on 23cm, but is no longer in use as the GB3FN receive antenna. A single Alford slot will be reinstated and the repeater will revert to horizontal polarisation in 2006.

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!

Transmitter Section
Transmitter	Based on Storno CQF9662 exciter, modified to provide 3x output frequency, plus on frequency Microwave Modules PA.
RF Output	3W
RF Output after filter	1.5W
Effective Radiated Power	3W (depending on the antenna!)
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 0.7µV EMF (0.35µV "PD")
Receive frequency	1291.050 MHz
Receiver AFC	Up to ± 7 kHz
Antenna System:
Antenna	TSB3603 colinear, approx 6dBd gain (originally), used for RX and TX
Height	20m AGL
Feeder	LDF4-50
Polarisation	Vertical
Duplex Filtering	Four cavity notches, two in each leg, 3/4 wave at 23cm, from UHF Motorola base station plus some critical length cables. Also homebrew pass cavity in transmitter leg to remove noise at 70cm. Desensitisation is well under 1dB.
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
Main Logic	Home made, very simple using EPROM and CMOS
Access	1750 ± 40Hz tone for minimum of 200msec
Reaccess	By carrier
Timeout	8 mins 32 secs
Reply signal	2048 Hz pip
Identification	Callsign every 1.5 minutes at 1024 Hz out of use, at least every 5 minutes during incoming transmissions in use. QTH locator (IO91OF) and location ("Farnham Surrey") sent every alternate 4.5 minutes out of use. Callsign sent in FSK sequence approx every 22.5 secs out of use with 500 Hz shift; "Farnham" and "IO91OF" each sent once every 4.5 minutes.
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.