On Sat, 11 Dec 2004 19:10:18 -0500, Some Guy wrote:

Getting back to the original question (poor to non-existant AM
reception), I understand the idea of aperature and long wavelenths of
AM radio and the size of airplane windows - but what about the effect
of ALL the windows on a plane? Don't they create a much larger
effective apperature when you consider all of them?

This becomes a matter of the distance between them and the phase
separation at any wavelength. What you describe is a common technique
for coupling power between waveguides (in what are called directional
couplers). However, this is not the same thing as accumulating and
enlarging an opening because such couplers will add energy in one
direction, and subtract it in the other (which makes for their
directionality).

And since the
plane isin't grounded, isin't the exterior shell of a plane
essentially transparent to all RF (ie it's just a re-radiator) because
it's not at ground potential?

Ground does not always mean "at one with the dirt and rocks." At one
time it did, when cowboys put up talking wires, and indians pulled
them down. Ground has since come to mean "common" (which when you
think of it, brings us back to dirt, metaphorically). Common means
that everything is at the same potential. If there is no potential
difference, then there is no way to measure a voltage based signal.
In other words, it's a massive short circuit, and the only way to
sense a signal is to inductively couple to the short circuit current.

This takes us to the second killer courtesy of physics. High
frequency current travels on the surface of smallest, positive radius.
AM frequency qualifies here in spades, even though it is
conventionally called not HF but MF (even VLF qualifies as High
Frequency in this context). The aircraft frame thus presents both
curvature and radius such that the current confines itself to the
outside of the shell with an inclination for the narrow wings and tail
section, rather than the elongated body.

You might be tempted to inductively tap into this frame current, but
then you are on the negative, inside radius of the current carrier
(makes the tube interior self-shielding). Whatever current is
flowing, is on the outside of the skin, not the inside - that is,
until we consider skin depth and penetration. But then it appears
that experience described here suggests that not much of that frame
current penetrates inside.

73's
Richard Clark, KB7QHC