FARNHAM VHF GROUP
GB3FM - GB3FN - GB3FX


GB3FN - Specification

GB3FN 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 GB3FM and GB3FX, the group's 23cm and 6m repeaters, and the GB3FNM beacons at 5.7 and 24GHz. It has been operational since 10 February 1983.

A programme of work was ongoing from 2017 until January 2020 when the MkII repeater was put into service, replacing the backup unit that has been running since 2017.  In this period of works, the power, charging, battery backup and frequency standards have been completely revised and/or replaced.

The MkII unit uses a Storno CQF9662 base station, with the oscillators phase locked to an external high stability frequency reference.  The logic is based on the logic from the Mk1 repeater - it seemed a pity to throw it away.

Transmitter and receiver

The CQF9662 transmitter and receiver are combined with a pair of commercial multiple cavity filters.  The antenna is a Jaybeam 6dBd colinear mounted on top of the tower, 20m above ground, fed with LDF4-50 feeder.  More information on the radio aspects can be found in the table at the bottom of this page.

Logic operation

The repeater can be accessed by a 1750 (+/-25)Hz tone burst at least 300msec long at any time.  It can also be accessed by a transmission with an 82.5Hz CTCSS tone (RSGB tone D) provided it is not transmitting already (e.g. a periodic callsign) and is not timed out.

Timeout is 3 minutes.  The timer is reset following a 1 second break in carrier.  A 'K' is sent (at 1167Hz) after a transmission, provided that it was modulated and at least a few seconds long.

If the repeater times out, talkthrough is switched off and the repeater identifies with a high pitched callsign (1167Hz) and closes down.  If the input remains occupied, callsigns are repeated with 25 second intervals between them.  A station with a stronger signal into the repeater can send a 1750Hz toneburst over the timed out station to recover the transmission (CTCSS has no effect).  Once the input has cleared, a 'T' is sent to indicate the end of the timeout.

When there are no further transmissions, the repeater will remain on air for a few seconds, then close down.  It may send a callsign when closing down following a QSO (but not after a quick call through).  The repeater can be reaccessed during the shutdown callsign by any transmission with or without CTCSS.

If there is interference on the input, a temporary 'mode 2' condition is automatically entered following a timeout.  In this condition, the transmission timer can be reset by a 1750Hz tone at any time, so stations with tonebursts that have stronger signals into the repeater than the interfernce can continue to use the repeater without the interference causing timeouts - provided the interval between tone bursts is not greater than 3 minutes.  This condition is reset once the input has remained free of signals for at least 15 seconds.

The repeater will identify with the low pitched callsign (875Hz) every five minutes whether in use or not.  The RSGB tone designation 'D' is sent when out of use.

Other information

1750Hz tones are suppressed by a bandstop filter switched into the audio path within a couple of hundred msec.

The repeater has a battery backup supply, and will operate for several days of mains power outage. When on batteries, the power is reduced by approximately 4dB.

The 82.5Hz CTCSS tone (D) is transmitted whenever the repeater is on air.  RB15 suffers a lot from local oscillator interference from car remote control units which can make listening with a carrier squelch painful.

There is an engineering beacon mode which can be activated for test purposes.  The repeater transmits continuously, with 25 second intervals between callsigns.  It may or may not be available for use while this is in operation.  No CTCSS is sent (unless in talkthrough).

A brief specification of the repeater RF sections follows.

Transmitter Section
Transmitter Storno CQF9662
RF Output power 8W
Power into feeder 5W
Effective Radiated Power 12W
Frequency 433.375 MHz (Channel RB15)
Frequency accuracy Better than 30Hz
Typically better than 10Hz
Receiver Section
Receiver Storno CQF9662.
Frequency 434.975 MHz
Sensitivity 0.4V EMF (0.2V "PD") for 12 dB SINAD
Sensitivity into filter 0.5V EMF (0.25V "PD") for 12 dB SINAD
Antenna system
Antenna Jaybeam 6dBd glassfibre colinear
Height 20m above ground at bottom of antenna (top of tower)
Feeder LDF4-50
Filter system TX: 6 cavity notch
RX: 4 cavity notch
Desensitisation of RX by TX Well under 1dB
Frequency reference Motorola ovened 5 MHz oscillator
(Shared with GB3FX, separate from that used on GB3FM)
The repeater is reciprocal with a mobile running 10W and a receiver with 0.4V EMF (0.2V "PD") sensitivity.

Backup repeater There is a backup repeater, based on a Storno CQF600, which is occasionally put on the air when engineering is carried out on the main unit. This has a lower ERP (approx 3dB down), is a little less sensitive, and a slightly simplified logic arrangement. 

Coverage maps  Plotted using the rather excellent Radio Mobile program from VE2DBE.  Yellow is a strong signal, green is a good signal and blue is marginal coverage.  Portables should have coverage over most of the yellow area, but may need to pick the right spot to stand..   For more info, see coverage.

NB: Note that the prediction program prints the names to the right of towns' locations without showing the towns. So a town itself is just to the left of its printed name.

GB3FN coverage map


Home Page GB3FX Specification GB3FM Specification Feedback

Updated Jan 2020
Copyright 1997-2020 The Farnham VHF Group