FARNHAM VHF GROUP
GB3FN - Specification
GB3FN 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 only shared with GB3FM and GB3FX, the group's 23cm and 6m repeaters. It has been operational since 10 February 1983.
The repeater has mainly run on twin antenna working for most of its life, but following the failure of the receiver antenna in 2006, ran on a single antenna until 2008. A new receiver antenna was installed in 2008, and the repeater reverted to twin antenna working, with the RX antenna on top of the tower, and the TX antenna side mounted.
Since 2009, the repeater has been sending CTCSS tone whenever it transmits, including during beacon callsigns when out of use. RB15 suffers in particular from local oscillators in car remote control locking/alarm systems, and this allows a tone squelch to keep the receiver quiet, but still to allow the operator to hear the callsigns as he or she drives around. We appreciate that not everyone likes this change, but most members do when asked - most like to listen to the callsigns as a confidence factor when the repeater is not in use.
A brief specification of the repeater follows.
|RF Output power||18W|
|Power into feeder||10W|
|Effective Radiated Power||25W|
|Frequency||433.375 MHz (Channel RB15)|
|Frequency accuracy||Better than ± 30Hz
Typically better than ± 10Hz
|Receiver||Modified Storno CQF662D-20, with preamplifier, and helical filter and front end from Storno CQM700.|
|Sensitivity||0.4µV EMF (0.2µV "PD") for 12 dB SINAD|
|Sensitivity into filter||0.45µV EMF (0.23µV "PD") for 12 dB SINAD|
|Tx antenna||TCL 5 dBd glassfibre colinear|
|Height||17M above ground at top|
|Rx antenna||Jaybeam 6dBd glassfibre colinear|
|Height||20m above ground at bottom of antenna (top of tower)|
|Filter system||TX: Phelps Dodge 6 cavity notch
RX: Phelps Dodge 6 cavity notch plus 1/4 wave pass cavity
|Desensitisation of RX by TX||Well under 1dB|
|Frequency reference||Racal 9420 ovened 5 MHz oscillator
(Shared with GB3FX, separate from that used on GB3FM)
|Access||1750 ± 40Hz tone for minimum 200 msec or 82.5Hz CTCSS tone (RSGB tone D)|
Every 5 minutes as beacon
On access - from completely cold
On shutdown - after a QSO
On timeout (every 20 secs)
|Reply signal||"K" (normally)
"H" for signal > 2.5 kHz high of nominal
"L" for signal > 2.5 kHz low of nominal
"B" if operating on standby batteries
"T" (long) after timeout
|Keying frequencies||875 Hz (normal callsign)
1167 Hz (other signals and timeout callsign)
|Callsigns suppressed on access and
shutdown after repeated access, or no QSOs.
Valid tone bursts suppressed by 40dB in bandstop filter within 200msec.
3kHz pips sent on top of audio if operating on standby oscillators with ovened crystals only.
Hang time shortened and shutdown callsigns suppressed in battery operation.
Reaccess possible in timeout, but with timout pips over audio.
CTCSS tone "D" (82.5Hz) is implemented. The repeater may be accessed by transmitting CTCSS tone "D" and the repeater transmits this itself whenever it is in talkthrough. It is not sent during beacon callsigns when out of use.
|Battery lifetime||In excess of 72 hours of mains interruption given normal use patterns.|
|Battery type||Dryfit 24V (4 x 6V) 150Ah|
|The repeater is reciprocal with a
mobile running 10W and a receiver with 0.5µ EMF (0.25µ "pd") sensitivity.
Backup repeater There is a backup repeater which is occasionally put on the air when engineering is carried out on the main unit. This has a lower ERP (approx 10W), and a slightly simplified logic arrangement. It can be recognised by a slightly slower keying sequence and a "double pip" timeout sequence.
Coverage maps Scanned in from prediction program. The more solid central area gives 90% probability, the hatched area gives 50% coverage. The 50% coverage area should be OK for a well sited mobile, or a fixed station. Prediction seems to match actual coverage fairly well. 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.
Updated Apr 2010
Copyright © 1997-2010 The Farnham VHF Group