GB3SN - Specification

GB3SN is located on the Boyneswood water tower at Four Marks, near Alton, Hampshire at:

      National Grid Reference:  SU673361
      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.

A brief specification of the repeater follows.

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 deviation peaks. 

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 overdeviation indicator. 

You can actually 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 different stations.

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.


Transmitter Section
Transmitter Storno CQF9114
RF Output power 15W
Power into feeder 8W
Effective Radiated Power 15W
Frequency 145.725 MHz
Frequency accuracy Better than 500Hz
Typically better than 100Hz
Receiver Section
Receiver Storno CQF9114 with pre-amplifier front end
Frequency 145.125 MHz
Sensitivity 0.3V EMF (0.15V "PD") for 12 dB SINAD
Sensitivity into filter 0.4V EMF (0.2V "PD") for 12 dB SINAD
Antenna system
Transmit Antenna 4.5dB commercial colinear
Receive Antenna 6dB commercial colinear
Height 30m above ground
Separation 6m horizontal, at different positions on top of water tower
Feeder LDF4-50 (both antennas)
Filter system Transmit leg: single pass cavity, three cavity notch filter
Receive leg: single pass cavity and four cavity notch filter
The filters allow single antenna operation, but are left in place with twin antenna working
Desensitisation of RX by TX Less than 1dB
Logic operation
Access 1750Hz tone +/-40Hz for minimum of 250 msec and min 800Hz deviation, or:
71.9 Hz CTCSS tone (RSGB tone B)
Reaccess Requires 1 second break in transmission.
In case of interference, a 1750Hz tone will also reset the timer.
Timeout 2.5 minutes
Identification Callsign sent:
Every 5 minutes as beacon (wth morse identification of CTCSS tone B)
On shutdown - after a QSO
On timeout (every 20 secs)
Reply signal Ringing K tone (normally)
"B" if operating on standby batteries
Keying frequencies 1750 Hz (beacon callsign out of use)
875 Hz (beacon callsign in use and shutdown callsign)
437.5 Hz (timeout callsign)
Other characteristics
  The receiver has limited range (+/-500Hz) AFC operation.
The receiver also has a levelling circuit acting on input deviation.
Shutdown callsigns is suppressed after short access or no QSO.
1750Hz tones are attenuated by 40dB in a bandstop filter within 250msec.
Battery lifetime In excess of 24 hours of mains interruption given normal use patterns.
Battery type Dryfit 12V
The repeater is reciprocal with a mobile running 10W and a receiver with 0.4 EMF (0.2 "pd") sensitivity.

Coverage Map

The map has been plotted using the rather excellent Radio Mobile program from VE2DBE. It shows expected mobile coverage. Yellow is a strong signal, green is a good signal and blue is marginal coverage; whether blue coloured areas are in coverage or not will depend on the mobile installation and on the preference of the operator to listen to weak signals. Portables should be able to use the repeater over most of the yellow area, but may need to pick the right spot to stand.
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).

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

Mobile coverage map                    Comparison map

GB3SN Home Page

Contact SN group 

Farnham VHF Group Home Page Web site Feedback

Copyright 2021 The UK FM Southern Repeater Holding Group