GB3SN - Specification

      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.

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 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.

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.

Backup repeater

Coverage maps

 Top of Page        Home Page