The Farnham V.H.F. Group - GB3FN Specifications

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 only shared with GB3FM and GB3FX, the group's 23cm and 6m repeaters. It has been operational since 10 February 1983.

A brief specification of the repeater follows.

Some philisophy behind the design: because of the coverage, GB3FN does attract a reasonable amount of use for a 70cm repeater. The logic has been designed to be helpful (with 'H' and 'L' for example) but tries to cut down the annoyance to listeners that repeated 'K's' following squelch breaks cause, or repeated access attempts without callsigns.

Transmitter Section
Transmitter	Storno CQF662D-20
RF Output power	18W
Power into feeder	10W
Effective Radiated Power	25W
Frequency	433.375 MHz (Channel RB15)
Frequency accuracy	Better than ± 30Hz Typically better than ± 10Hz
Receiver Section
Receiver	Modified Storno CQF662D-20, with preamplifier, and helical filter and front end from Storno CQM700.
Frequency	434.975 MHz
Sensitivity	0.4µV EMF (0.2µV "PD") for 12 dB SINAD
Sensitivity into filter	0.45µV EMF (0.23µV "PD") for 12 dB SINAD
Antenna system
Tx antenna	TCL 5 dBd glassfibre colinear
Height	17M above ground at top
Feeder	LDF4-50
Rx antenna	Same as TX antenna (single antenna working)
Height	Same as TX antenna (single antenna working)
Feeder	Same as TX antenna (single antenna working)
Filter system	TX: Phelps Dodge 6 cavity notch plus 3/4 wave pass cavity RX: Phelps Dodge 6 cavity notch plus 3/4 wave pass cavity Filter provides single antenna working.
Desensitisation of RX by TX	Well under 1dB
Frequency reference	Racal 9420 ovened 5 MHz oscillator (Shared with GB3FX, separate from that used on GB3FM)
Logic operation
Access	1750 ± 40Hz tone for minimum 200 msec or 82.5Hz CTCSS tone (RSGB tone D)
Reaccess	Carrier
Timeout	3 minutes
Identification	Callsign sent; Every 5 minutes as beacon On access - from completely cold On shutdown - after a QSO On timeout (every 20 secs)
Reply signal	"K" (normally) "H" for signal > 2.5 kHz high of nominal "L" for signal > 2.5 kHz low of nominal "B" if operating on standby batteries "T" (long) after timeout
Keying frequencies	875 Hz (normal callsign) 1167 Hz (other signals and timeout callsign)
Other characteristics
	Callsigns suppressed on access and shutdown after repeated access, or no QSOs. Valid tone bursts suppressed by 40dB in bandstop filter within 200msec. 3kHz pips sent on top of audio if operating on standby oscillators with ovened crystals only. Hang time shortened and shutdown callsigns suppressed in battery operation. Reaccess possible in timeout, but with timout pips over audio. CTCSS tone "D" (82.5Hz) is implemented. The repeater may be accessed by transmitting CTCSS tone "D" and the repeater transmits this itself whenever it is in talkthrough. It is not sent during beacon callsigns when out of use.
Battery lifetime	In excess of 72 hours of mains interruption given normal use patterns.
Battery type	Dryfit 24V (4 x 6V) 150Ah

The repeater is reciprocal with a mobile running 10W and a receiver with 0.5µ EMF (0.25µ "pd") sensitivity.

Backup repeater There is a backup repeater which is occasionally put on the air when engineering is carried out on the main unit. This has a lower ERP (approx 10W), and a slightly simplified logic arrangement. It can be recognised by a slightly slower keying sequence and a "double pip" timeout sequence.

Coverage maps Scanned in from prediction program. The more solid central area gives 90% probability, the hatched area gives 50% coverage. The 50% coverage area should be OK for a well sited mobile, or a fixed station. Prediction seems to match actual coverage fairly well. For more info, see coverage.

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

Mobile coverage map............. Portable coverage map