FARNHAM VHF GROUP
GB3FM - Specification
GB3FM 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 GB3FN and GB3FX, the group's 70cm and 6m repeaters, and now the GB3FNM beacons. It operates as a combined beacon and repeater on the 23cms band. It has been operational since 13 March 1988. It runs about 5W ERP horizontally polarised from an Alford Slot. It used vertical polarisation between 1997 and 2007, but reverted to horizontal polarisation in late 2007.
Some antenna rework was carried out in 2008 and the repeater reverted to two antenna working. Because of the relatively low level of use, the transmitter was put into the top antenna to give the best possible performance as a beacon. The transmit notch filters were taken out (unnecessary with the vertical antenna spacing) which improved the ERP to approx 8W.
Ideally, the repeater could do with more ERP, but there really don't seem to be the users out there to justify the engineering effort. But if anyone wants to donate a 23cm PA module, we're listening!
In 2009, the 'temporary' logic (which had run for many years following the demise of the original Z80 controller) was replaced with a new board based on a PIC. This reinstates some of the features which were originally provided, such as signal strength measurement, and 'H' and 'L' off frequency indication - and the decaying 'ping' reply signal. The FSK beacon sequence is also slightly extended, and provides more unkeyed carrier time.
A brief specification of the repeater follows.
|Transmitter||Based on Storno CQF9662 exciter, modified to provide 3x output frequency, plus on frequency Microwave Modules PA.|
|Effective Radiated Power||8W|
|Frequency stability||Better than ± 100 Hz
Typically better than ± 30Hz
|Receiver||LMW Electronics convertor, + CQF9774 receiver front end + Storno M900 IF|
|Sensitivity||Better than 0.7µV EMF (0.35µV "PD")|
|Receive frequency||1291.050 MHz|
|Receiver AFC||Up to ± 7 kHz|
|TX Antenna||Home made Alford Slot, approx 6dBd gain, approx 20m AGL|
|RX Antenna||Home made Alford Slot, approx 6dBd gain, approx 15m AGL|
|Feeder||LDF5-50 (RX), part LDF5-50 and part LDF-4-50 (TX) (we ran out of 5-50 before the bottom of the tower)|
|Duplex Filtering||Four cavity notches, two in each
leg, 3/4 wave at 23cm, from UHF Motorola base station plus some critical length cables.
Only the two RX sections are currently in use; TX noise isolation is achieved by
Desensitisation of RX by TX: none.
|Frequency reference||Racal 9420 Ovened 5MHz oscillator
Note: this is not shared with 'FN or 'FX (because it is keyed to provide the FSK callsigns)
|Logic||PIC based by G4EPX|
|Access||1750 ± 40Hz tone for minimum of 200msec|
|Reaccess||By carrier. The repeater sends its callsign every valid access and reaccess (after about a second).|
|Reply signal||2333 Hz decaying 'ping' (unless replaced by frequency or signal indication)|
|Frequency indication||'Ping' replaced by 'H' or 'L' sent in 1167Hz Morse for input signal more than approx 5kHz high or low of nominal input frequency respectively|
|Signal indication||If 1750Hz tone burst is sent
within 2 seconds of start of transmission (except the accessing transmission), the
repeater will replace the 'ping' with a Morse numeral; 1-8 and N (for 9). S1 ranges
from squelch threshold to about 1uV EMF, successive S readings given at approximately 3dB
increases of signal. Note: this is approximate, not calibrated!
Note: the signal indication will not be given if an input signal is more than 5kHz off frequency, and qualifies for an 'H' or 'L' instead.
|Identification||Callsign appoximately every 2
minutes at 875Hz out of use, and at start of every over at 1167Hz when in use.
QTH locator (IO91OF) and location ("Farnham Surrey") sent every alternate 6 minutes out of use using FM.
Callsign sent in FSK sequence with approximately 20 second gaps when out of use with 500 Hz shift; one of "Farnham", "Surrey" and "IO91OF" are sent in turn following each third FSK callsign - repeater moves 500Hz LF and keys back onto nominal frequency.
|Tone suppression||40dB in notch filter switched in after 200 msec|
|Subaudible tone||100Hz transmitted at 500Hz
deviation during talkthrough. No receiver CTCSS access.
Note that this stays with the 'original' 23cm plan for CTCSS, and does not follow the regional CTCSS frequency plan.
It features as an 'S' ('special') on RMG repeater lists because of this. No one has complained about lack of a regional tone yet.....
Updated Apr 2010
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