FARNHAM VHF GROUP
GB3FM - GB3FN - GB3FX


GB3FN - Specification

GB3FN is located on a site 3 km North of Farnham, Surrey, UK at;

      National Grid Reference;  SU821494
      QTH Locator;  IO91OF

The site is 187m above sea level and is  shared with GB3FM and GB3FX, the group's 23cm and 6m repeaters, plus the GB3FNM beacons. It has been operational since 10 February 1983.

A programme of re-engineering commenced in August 2017 which will eventually replace GB3FN with a Mk2 unit.  As a first step in the process, the main repeater has been replaced by the backup unit, which has been somewhat re-engineered to provide servce until the Mk2 unit is ready.  Following a period off air in 2018, further modifications were implemented in the backup unit.  The group have been experimenting with different power levels and different antennas and more will take place before settling on the final configuration for the Mk2 unit.  This will continue.

The description here refers to the backup unit.

Transmitter and receiver

The transmitter and receiver use a Storno CQF662D-10 base station.  This now operates from a battery backed up 12V power supply, and so the repeater continues to operate during mains failure.

Transmitter and receiver frequency stability are provided from internal ovened crystal oscillators, not the external frequency reference.

Transmitter ERP is approximately 5W.  This is similar to the ERP when the repeater was first made active in February 1983.  The receiver provides reciprocal coverage with a 10W mobile with a 0.5uV EMF/0.25uV pd receiver sensitivity.  The transmitter uses a lower power PA than the main repeater unit, and is limited by the capabilities of its mains power supply.

The transmitter and receiver operate into a single antenna via a commercial 6 cavity duplexer.  Desensitisation of the receiver by the transmitter is negligible.  The antenna is either a side mounted 5dBd gain colinear which seems to have lost its gain over its long life and acts like a dipole as far as performance is concerned, or is the top mounted 6dBd gain colinear.  Both are used over different periods while coverage evaluation is ongoing.

Logic

The logic is simplified compared to the main repeater.  The repeater requires a transmission with a 1750Hz tone or 82.5Hz CTCSS for access.  If followed with a modulated transmission, it will send a 'K' one second after the input has cleared.  If the initial accessing transmission is not modulated, the repeater will close down one second after the end of the accessing transmission.  Further transmissions once accessed only require carrier access, but require modulation in order for the repeater to send a 'K' following the transmission, and the repeater may close down following an unmodulated transmission.

The timeout is 3 minutes, following which the repeater will identify and close down.  There is no beacon callsign during timeout.  A stronger station may reaccess using a 1750Hz tone to restore the timed out station (but note that CTCSS will not have any effect).  Note that there is no indication if the input clears during timeout, i.e. no 'T' or 'K' when the input transmission finishes.

A periodic callsign is sent at intervals of 5 minutes whether in use or not.  The callsign is followed by the regional CTCSS identifier ('D' for an 82.5Hz tone) when the repeater is not in use.




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Updated August 2018
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