Wednesday, Sep 10, 2003 at 23:07
Eric,
Great thing, theory - in theory.
I can accept that some more recent diesels with electronic injection control, with processor clock speeds approaching 100Mhz, are capable of emitting harmonics up into the hundreds of Mhz, thus affecting UHF (in theory). I guess I will have to accept that you have experienced interference at the UHF level (see comment further below). It is a fact, by inherent design (usually in the IF), that some receivers are more susceptible to noise interference than others, even with a good noise floor.
But in regard to your mention of interference from charging batteries, the frequency components are much lower in frequency; this should be a relatively simple matter to sort out, and there is already a plethora of info written about how to reduce this.
In regard to the position on the roof being a factor in reducing interference, then it follows that the closed bonnet should be even more effective in stopping the engine interference than the shadow of the roof (in theory). In practice, there will be diffraction of the signal around the edge of the roof, just like you see the glow of city lights over the
hill.
The main advantage of a roof mount is sheer height - UHF propogation is primarily line-of-sight (with some diffration effects as mentioned), and raising the antenna to a point where obstructions are eliminated or reduced, improves the situation.
The main disadvantage of a roof mount is the likelihood of getting the antenna knocked off in the bush. If you don't travel where overhanging shrubbery is present, then this is the way to go. Otherwise, a GI elsewhere on the vehicle where it won't get knocked off is the next best compromise. The disadvantage is that the upper body of the vehicle may obstruct the path.
Just as an aside, I have been using a rented 3.0L GU Patrol (with the offending electronic injection) over the last week or so with a mid-band VHF mobile (in the 80MHz band) and two UHF mobiles fitted in the vehicle, and interference from the diesel (or the alternator, for that matter) was not evident in any of the units.
rgds
Gerry
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