<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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