FARNHAM VHF GROUP
GB3FM - GB3FN - GB3FX
The coverage maps are taken from prediction programs. The stylised map was drawn using them as basis but modified in the light of experience.
Coverage maps are always dependent on the parameters of the mobile. A 25W mobile with a high gain antenna will usually provide better coverage than a 5W mobile with a low gain antenna. The siting of the antenna on the vehicle can cause markedly different propagation in different directions. This is more pronounced at 50MHz than at 433MHz as the groundplane on 433MHz doesn't need to be so big to be effective (by a factor of nearly ten).
Finally, a lot depends on the perception of the users. Some users will give up using the repeater as soon as its signal falls off the end stop of the signal meter, and don't like any flutter. Others will carry on to the bitter end and try and get service if it's just above squelch. The coverage maps here are closer to the latter, i.e. coverage is shown as present if it is usable but weak.
The map has been plotted using the rather excellent Radio Mobile program from VE2DBE. It shows expected mobile coverage. Yellow is a strong signal, green is a good signal and blue is marginal coverage; whether blue coloured areas are in coverage or not will depend on the mobile installation and on the preference of the operator to listen to weak signals. Portables should have coverage over most of the yellow area, but may need to pick the right spot to stand.
The map reflects the reduced ERP of the backup GB3FN.
There are two maps for GB3FX:
The 'high' coverage map gives an indication on where a high specification mobile installation (greater than 25W, quarter wave antenna, good groundplane for antenna) should be able to access the repeater, but with some limits. 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. The prediction maybe doesn't allow fully for the extent of electrical noise experienced at 50MHz, and performance may be a little worse depending on environmental conditions. It also doesn't fully account for clutter loss in urban areas, so results towards London are a little worse than this map predicts, and results to the South and South East may also be slightly worse due to some shadowing by the mast. North and West seem about right.
The 'low' coverage map was drawn up with more pessimistic parameters. This is where a high specification mobile should definitely have good access, and a lower specification mobile (less than 25W, loaded whip less than a quarter wave, maybe poorer groundplane on antenna) or maybe where a well sited portable should be able to access.
Variyng antenna efficiency and background noise on 50MHz make these maps less predictable than at higher frequencies. (Diesel engined cars without CD players do better than petrol engined cars with the CD goin - '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.
Some indications on how far the 'regulars' running up to 50W and quarter wave antennas have worked GB3FX:
M1 North - Luton
The three bands picture gives a general local coverage indication, and includes GB3SN on 2m.
|GB3FN coverage:||Coverage plot
|GB3FX coverage:||'High' coverage||'Low' coverage|
|Three bands:||Three repeaters||GB3SN alone|
Copyright © 2001-2017 The Farnham VHF Group
Updated August 2017