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

[Mike Hanz]

I don't believe ignition noise was the primary interference source, 
John.  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).  
Line filters were installed all over the place, of course, 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

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).