europa@pstewart.f2s.co wrote:
> Fred and John
>
> Thanks for the help.My confusion has come I now realise because I
> have an old installation manual which does ask for a twisted pair of
> wires at 2 turns per inch to the PTT switches. I will follow your
> advice and look forward to the sound of silence.
>
> Regards
>
> Paul
> G-GIDY
Paul, I hope I didn't steer you wrong, and you may have an iteration of the XCOM
which requires shielding/twisted on PTT. It's possible you have a model of
the box which is susceptible to external "noise." In older days, we switched
between receive and xmit with a mechanical relay where this low impedance of
flipping
device didn't require any special wiring to prevent external noise from
triggering xmit when we didn't intend to xmit. Modern comm boxes switch
functions
in semiconductor ways which are trivial now to prevent such problems, though
they can be "hair-trigger" if not designed right, but the biggest nut to
crack was isolating input/output (many volts vs. microvolts) from the single
antenna which we use for receive/transmit (a simple, isolating, wonderous "PIN
diode" for the technically curious).
Anyway, you mentioned your harness has shielded wire for PTT, so I would use
that
for as far she physically goes, not necessarily a ground in the shield. When
we deisre to xmit, all we're really doing is pulling a "plus voltage" to ground,
though a typically low resistance, hence noise immune. Nuthin' to worry
about 'til you might run into an anomalous problem in servce. This function is
trivial, and all you need is a ground return for your panel jack and/or PTT
switch
Somewhat related here, but on my electronics bench right now is an eBay purchase
of a Narco COM 120/20 for low price -- nominal 20W xmit on 14V. A rare device
on the used market if alleged working and she does; and I purchased it for
my other plane as a slide-in replacement for a working box. Why exactly I don't
know, but that's eBay. Not that appropriate for a typical Europa, but holy
mother of pearl!! She draws spec .5 Amps in receive, but key xmit and stuff sure
happens. Measured 10A current draw, and thence better than 20-watt RF carrier
output. The point is, you simply ground the PTT line from the back of the
box, and xmit indeed happens if all normal.
Fred F.
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http://forums.matronics.com/viewtopic.php?p=47556#47556
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