UK FM SOUTHERN REPEATER HOLDING GROUP
GB3SN - Specification
GB3SN is located on the Boyneswood water tower at Four Marks, near Alton, Hampshire at:
National Grid Reference: SU673361
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.
|RF Output power||15W|
|Power into feeder||8W|
|Effective Radiated Power||15W|
|Frequency accuracy||Better than ± 500Hz
Typically better than ± 100Hz
|Receiver||Storno CQF9114 with pre-amplifier front end|
|Sensitivity||0.3µV EMF (0.15µV "PD") for 12 dB SINAD|
|Sensitivity into filter||0.4µV EMF (0.2µV "PD") for 12 dB SINAD|
|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|
|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
In case of interference, a 1750Hz tone will also reset the timer.
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
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 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 0.4µ EMF (0.2µ "pd") sensitivity.
Coverage Map Scanned in from prediction program, which provides mobile coverage for 10W mobile and 5/8 wave whip. 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.
Copyright © 2015 The UK FM Southern Repeater Holding Group