GB3FM - GB3FN - GB3FX - GB3SN
GB3FX - SpecificationGB3FX 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 GB3FM, the group's 70cm and 23cm repeaters. It has been operational since 10 October 1997.
A brief specification of the repeater follows.
Some philisophy behind the design: the coverage from well sited 6m repeaters is tremendous, although the band is still quiet. 6m is quite sensitive to electrical interference, for example ignition noise from older petrol engines, which is why the receiver has an AFC loop to try and minimise the effect of this. The logic has been designed to try and be helpful to users without annoying them too much. That's why the repeater has the simple 1 sec hang on initial access - if someone really wants to try to get in without giving his or her callsign, they can get brief confirmation without subjecting other listeners to lots of morse code! The signal strength indicator allows a station to judge their signal if there is no one else around. The overdeviation indicator is fairly unobtrusive - but as it's set quite high, stations really should adjust their deviation so as not to hit it, if they are going to avoid distortion and squelch closure on deviation peaks. We also recognised that the CTCSS-only access on 6m would cause some stations problems as they adjusted or found the right tone, which is why stations without the correct CTCSS tone can get a few seconds of low deviation talkthrough - hopefully enough for others to help them set up correctly.
The repeater runs a single dipole about 15m above ground.
We originally used a duplexer made out of Heliax from the WB5WPA design. It worked very well, cost virtually nothing, but because we only had LDF5-50 Heliax, was a bit lossy especially on transmit. Thanks to the extreme generosity of Rupert, G4XRV, who donated both his time and the materials, we now have a custom made duplexer made with real helical filters, which has been running since January 2003. And a good couple of dB less loss. This has helped increase the ERP slightly.
The repeater has had a couple of improvements over the years as well, especially in the receiver section where the IF and front-end have been modified further to improve the sensitivity. We are now limited by the level of intermodulation products that come from another site across the road, which doesn't seem to have the greatest filtering in the world on its transmitters. This also prevents us putting out more TX power, although the box itself is capable of it.
From 2009, like GB3FN, the repeater has been modified to transmit CTCSS tone at all times, including when sending its callsign when out of use. This allows mobile operators to drive around using a tone squelch to cut out the high levels of noise and interference on 6m, whilst still hearing periodic callsigns as a confidence and coverage check. This is not to everyone's taste, but the majority of those surveyed prefer this. Note that the CTCSS tone is not sent during callsigns sent after a timeout - this is to avoid any latch up situations involving local intermodulation sources close to the site.
The repeater is reciprocal with a mobile running 12W and a receiver with 0.5µV EMF (0.25µV "PD") sensitivity.
Coverage MapsAs with GB3FN, the map has been plotted using the rather excellent Radio Mobile program from VE2DBE. It shows expected mobile coverage, with some dependency on local conditions and the abilities of the mobile installation.
Yellow shows a strong signal, and this area should be
workable for most mobiles.
Varying antenna efficiency and background noise on 50MHz make the map a little less predictable than at higher frequencies. (Diesel engined cars without CD players do better than petrol engined cars with the CD going - 'FX's output seems to be close to a harmonic of a crystal used universally in CD players!) Many mobiles will achieve somewhere between the two - depending on which direction they are pointing in and so which direction their antenna works best in.
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