FARNHAM VHF GROUP
GB3FM - GB3FN - GB3FX - GB3SN
GB3SN - Specification
GB3SN is located on the Boyneswood water tower at Four Marks, near
Alton, Hampshire at:
National Grid Reference;
QTH Locator; IO91LC
The site is 215m above sea level and the antennas are 30m above
ground level. The repeater has been operational since
1975. A Mark 2 unit went into service in 1984 with improved
performance, and the present Mark 3 unit was put on air to follow
the move the 12.5 kHz spacing in January 2000. The current
antenna system was put in place in November 2015.
The repeater logic has been designed to be simple to use, whilst
reflecting the variety of signals it may be subject to.
The current repeater was put into operation because of the general
move of the 2m band to 12.5kHz spacing. It was recognised
that the repeater would have to cope with a variety of signals,
some with the correct 2.5kHz deviation, but most in the early days
with the older 5kHz deviation specification. For this
reason, great care was taken in the design of the IF and audio
recovery stages of the repeater. There is an AFC loop to
centre received signals in the IF passband - overdeviating signals
have no margin of error if the squelch is to remain open on
The discriminator is followed by a deviation levelling
circuit. The purpose of this is to make all input signals
sound about the same loudness through the repeater. However,
the levelling gain has been carefully limited, and the time
constants of the levelling circuit carefully selected - too much
gain with low audio input, or too quick changes in gain would
bring up background noise on mobile transmissions between
syllables of speech, and the result would be harder to listen to,
especially with hands free microphones.
Obviously, as we expected a lot of overdeviation, there is no
You can hear how successful the audio and IF design is on the main
repeater - the backup repeater (actually the Mark 2 unit, modified
to 12.5 kHz) has none of these extra design features (like most
other 2m repeaters) which make it more vulnerable to squelch
closing on deviation peaks and also to different audio levels from
The logic is designed to be simple to use, but not annoy listeners
too much epecially if subject to thoughtless operaton or
abuse. The 1 sec hang time on initial access allows someone
determined to use their tone burst without giving their callsign
to get some feedback that they are getting in, but without
subjecting listeners to extraneous callsigns and so on. The
audio detector stops squelch breaks or blank carriers from causing
lots of ringing. Compared with the Mark 2 unit, there
is no 'T' after timeout as this seemed to confuse some users.
There is a beacon mode which can be activated for test purposes:
in this mode the repeater will transmit continuously, with
callsigns sent every 25 seconds.
A brief specification of the repeater follows.
|Power into feeder
|Effective Radiated Power
||145.725 MHz (Channel RV58/R7)
||Better than ± 500 Hz
Typically better than ± 100Hz
||Storno CQF9114 with pre-amplifier front end
||0.3µV EMF (0.15µV "PD") for 12 dB SINAD
|Sensitivity into filter
||0.4µV EMF (0.2µV "PD") for 12 dB SINAD
||Skymasts 6dBd glassfibre colinear
There is a backup 4.5dB colinear which can be switched in
as an alternative
||30m above ground at bottom of antenna
||Transmit leg: single pass cavity, three cavity notch
Receive leg: single pass cavity and four cavity notch
|Desensitisation of RX by TX
||Less than 1dB
|1750Hz tone +/-40Hz for
minimum of 250 msec and min 800Hz deviation, or:
71.9 Hz CTCSS tone (RSGB tone B)
|Requires 1 second break in
In case of interference, a 1750Hz tone will also reset the
Every 5 minutes as beacon (wth morse identification of
CTCSS tone B)
On shutdown - after a QSO
On timeout (every 20 secs)
|Ringing K tone (normally)
"B" if operating on standby batteries
|1750 Hz (beacon callsign
out of use)
875 Hz (beacon callsign in use and shutdown callsign)
437.5 Hz (timeout callsign)
|The receiver has limited
range (+/-500Hz) AFC operation.
The receiver also has a levelling circuit acting on input
Shutdown callsigns is suppressed after short access or no
1750Hz tones are attenuated by 40dB in a bandstop filter
|In excess of 24 hours of
mains interruption given normal use patterns.
The repeater is reciprocal with a mobile running 10W and a receiver
with 0.4µV EMF (0.2µV "PD") sensitivity.
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, is a little less sensitive, and a
slightly different logic arrangement.
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
There is also a comparison map, which compares with the 'local' 70cm
repeater GB3FN and 6m repeater GB3FX (although these are sited more
to the North East).
For more info, see coverage.
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