From: David W. Tamkin <[EMAIL PROTECTED]>
> Gaz asked,
>
> B> What puzzles me about these VHF modulators is why they drift off frequency
> B> so much.
>
> Jim answered,
>
> C> The units need to be tunable so you can tune to an unused spot on the
> C> dial. They are also of low power, and as such as you drive near
> C> another radio station, interference casues the radio to drift to the
> C> stronger signal. (FM capture and intermodulation products probably)
>
> On top of that, try *finding* an open frequency in a large metropolitan area.
> I tried two models and couldn't get either to work in my driveway, let alone
> throughout a short in-town commute. Even parked I had to keep retuning the
> receiver, so some of the drift has to be in the transmitter.
What I was thinking was, why not make a model where you plug a crystal in
the back, or front or anywhere, and you choose the frequency crystal you
want, eg. in Newcastle I would go for anything between 107-108MHz as it
is unused. Since I don't have a car it's all hypothetical(?).
Admittedly in London the VHF FM band is probably full of sigs across the
whole 87.5-108MHz range, likewise in large US cities. So why not have
the unit pump out a bit more juice. A 100mW ERP transmitter in your car,
less than 4 metres from your receivers' antenna would probably drown out
a 100KW Xmtr on a hill you're driving past. The only downside of this idea
perhaps being that anyone on the road within a few hundred metres would
also hear you. And perhaps 100mW could be excessive as it may overload
your tuner's frontend. But you get the idea :-)
I think this "grabbing a strong signal" thing is rubbish, if a VHF tuner jumps
onto a sig more than 0.2MHz away, it should be de-installed from the car
and installed in a suitable bin for immediate disposal :-) My point is that
a *crystal* locked oscillator won't wander all over the VHF broadcast band,
and when you get your for instance 107.5MHz crystal (obviously thats a
harmonic but I won't go into that) and adjust your transmitters tuning of it's
frequency multiplier and possibly power amp stage, it'll be as stable as any
other pirate radio station. It may wander 10 or 20KHz but thats less than
half any tuner's minimum tuning step.
Cheers,
PrinceGaz -> prone to wandering all over the place, after too much beer
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