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
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.
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repeater has been replaced by the backup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.
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.
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
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.
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.
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.
Updated August 2018
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