[ARC5] Fw: Receiver current drain (AF output)

[J. Forster]

> I don't believe ignition noise was the primary interference source,
> John.

Agreed.

> The spark plug wires were normally shielded anyway, at least in
> most of the Erection and Maintenance manuals for specific aircraft that
> I have perused over the years.  It gave the line crews fits (even today,
> where most of the active warbirds dispense with it as an unnecessary
> bother), but install it they did.

> The greatest RF noise sources appear
> to have been primary power (14 and 28 volt lines with generator hash and
> load noise (dynamotor brush arcing, etc.), and similar RF radiation from
> the power lines carrying 400Hz (and 800Hz in earlier Navy aircraft).

My point is that such hash is much more likely to be magnetically coupled,
not radiated. If you are worried about coupling from power lines, etc, 
use twisted pairs. A shield, unless it is electrically continuous and
several skin depths thick, will not help much.

> Line filters were installed all over the place, of course,

The presence of line filters argues for conducted EMI, rather than radiated.

> but the
> operational EMI picture reflected a constant search for grounding and
> bonding issues throughout the airframe, this according to the
> maintenance newsletters of the time.  The move from all that pretty
> braided conduit to open wire bundles didn't help any, of course.  It
> seems to me that there was also a reactionary evolution in the
> interphone infrastructure design criteria that followed the growing
> complexity of the avionics (and power for it) in a given airframe
> between 1941 and 1945.    Not all the airframes used shielded wire for
> any of the interphone circuits, for example.  When it began in about
> 1943, the practice actually included *only* the microphone wiring, based
> on the installation manuals I have here - the amplifier output wiring
> evidently considered high enough level as to not be of concern at the
> time.  The airframe wiring pretty much finished up the war that way, to
> which I can attest after working on a bunch of it in the Enola Gay.
>
> 73,
> Mike KC4TOS

I really see no reason to have used shielded wire for anything, other than
mike lines. RADAR signals (SYNC, VIDEO) were usually coax

Best,

-John

================


>
> On 1/7/2011 12:15 PM, J. Forster wrote:
>> Given that a lot of wiring was shielded, I'm inclined to doubt your
>> theory. The radio wires usually went nowhere near the spark plug wires
>> also.
>>> But the lower impedance
>>> audio lines also decreased noise pickup problems (from relatively long
>>> runs in aircraft and AFV's).
>
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