I think it would take a very brave man to state that the GPS doesn't emit any
frequencies
that would affect a VHF comm set. I've worked with EMC for quite a
while and you'd be surprise how some innocuous devices make great little
transmitters!
There is a nice big CPU inside the GPS (a 68000 derivative if memory serves me
correctly) - this will be clocked at about 25-50Mhz but if the designers haven't
done their job well, there will be harmonics at 25MHz spacing for quite a long
way up.
Next time I'm getting a problem (if the wx ever clears up), I'll have a go at
shutting
the GPS off and seeing if that does affect things (don't think I'm brave
enough to try same with the engine!).
>
> From: "Fred Fillinger" <n3eu@comcast.net>
> Date: 2005/07/27 Wed AM 04:43:44 BST
> To: <europa-list@matronics.com>
> Subject: Re: Europa-List: Radio reception problems
>
>
>
> > ...sometimes we can hear / be heard
> > from a good distance away
> > but on occasions, we can't hear Rochester until we
> > are within about 5 miles of them (122.25Mhz).
>
> If at all times off the nose, or same azimuth point elsewhere, it
> could be a simple matter of which of their transmitters they're using.
> They need not have the same xmit power, but if an installation is
> performing poorly, above can happen. When you have squares and logs
> in the math, seemingly small matters can then cause extreme results!
>
> Like get this. At only 100AGL over really flat terrain, theory says
> at 5 miles you should hear ATC if they were using a 100milliwatt
> transmitter! Something's not working well at all.
>
> > 1) the aerials gain isn't over the required range -
>
> It's the loss, not the gain. Only a directional antenna can have gain
> of practical use. There is a small, omnidirectional gain of 2.2db due
> to pure physics. If there's greater than 2.2 in some directions,
> other points around the circle _must_ have a loss. But if a loss in
> one direction, there may not be a gain anywhere else, depending upon
> what a nearby metal element is doing (like changing the signal's
> polarization).
>
> > this could easily be fixed by
> > a more conventional aviation aerial
>
> The radiating pattern of a whip antenna pointed down is somewhat
> better than a dipole. However, more significant is where it's located
> within a fuselage. External to the fuselage, and any problems go
> away.
>
> > 2) The squelch is either over enthusiastic or being
> > triggered by noise...from the engine, GPS etc
>
> GPS can't do anything to VHF, but engine or strobe etc. noise is
> nothing but audio carrier modulation which makes no sense as spoken
> words. Same as ATC with talent as a voice comic mimicing an
> alternator whine into the mic. So squelch may break, and we hear
> noise. It won't decrease ability to receive carrier, but rather as a
> combined signal makes carrier stronger -- in a cheap circuit, squelch
> can be broken. So Narco in its 810 uses a fancy circuit which
> attempts to determine if words are being received. They call it
> "squelch comparator and carrier override." IOW, ain't voice, so let's
> ignore the stronger, noisy carrier and nothing is heard in the audio.
> Works great!
>
> Reg,
> Fred F.
>
>
>
>
>
>
>
-----------------------------------------
Email sent from http://www.ntlworld.com/
Virus-checked using McAfee(R) Software
Visit www.ntlworld.com/security for more information
|