Fred,
I looks as though the real problem lies in the low 120's frequency range, in
the receive condition. The middle 130's range is a smaller problem in that
we get a poor indistinct reception. In the lower 120's we get no reception
at all at times.
It is interesting that a number of correspondents are confirming the same
phenomena. Undoubtedly there will be others who are watching but don't like
to say anything, and there will be more who's aircraft never get exposed to
the extremes where these characteristics display themselves.
Perhaps we could focus on the antenna. What design shortcoming would cause a
weak receive in this specific frequency range. It is not there at less than
120MHz, and it is not there at more than 124MHz. Indeed! Maybe there is some
wavelength relationship between the low 120's and the above 134's that
provides the clue.
The floor is yours! We all await your thoughts with awe. You may even
indulge yourself with the odd chicken or two, or even yet, - a door lintel,
if it lubricates your creative urges. Seriously though, this is a problem
that needs to be resolved. It is very disconcerting (and probably dangerous)
to be out of touch with the control authority when negotiating their
airspace.
Best regards.
Bryan
----- Original Message -----
From: "Fred Fillinger" <n3eu@comcast.net>
Subject: Re: Europa-List: Radio reception problems
>
> <sideslip@tiscali.co.uk>
>
>> On my installation, when I swich off the GPS (Skyforce Skymap2) the
>> background static type noise heard when receiving dissapears.
>
> Just as in a (cheap) plastic notebook computer, such a device can emit
> vigorous EMI, but rendered very weak if all else is up to snuff.
> Signal-to-noise ratio (S/N) can be computed on a 4-function
> calculator, as amplified S + amplified N equals amplified S + N. This
> is why a "TV only"offer of an amplified antenna to pull in distant FM
> stations works to only a very limited degeree, due to an unrelated
> phenomenon inside the receiver. For just 3 easy payments of $19.95!
>
> A fix by switching GPS off confirms that the strength of ATC VHF
> signals rec'd at the back of your receiver are really weak. Else,
> when ATC transmits, there should be no noise (a way new S/N ratio).
> Thus, implied receiver and/or antenna problems.
>
> When I get time, I'll separately post as to how to cobble a
> "reference" antenna to go flying with as a pax to hopefully narrow
> between the two sources.
>
> But it's noteworthy that you see upper frequencies affected, but OK
> down at 118 MHz. EMI doesn't have to have harmonics within the
> aviation band to be heard, but in your case sounds like it is,
> aggravating the problem. The EMI's fundamental has found a sweet spot
> on your radio dial. Else, the EMI strength decreases rapidly with the
> cube root of the distance to the antenna. Close enough, it's still
> noise. That's why we shouldn't bundle antenna coax with noisy wires.
> That's too up close and personal, but actually if the coax line is
> balanced (minimum VSWR), the noise can still self-cancel as far as the
> receiver is concerned. It rejects common mode.
>
> Reg,
> Fred F.
>
>
>
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