Bob Bob wrote:
. . .

A coax line will radiate (if its a transmitting circuit) and receive
signal (as a receiving cct) if it isnt terminated into the (same)
impedence at both ends of the cable that is the same as the cable
itself. In other words the shield wire isnt always an effective screen.

This is a common misconception, but it's not true -- the presence of
current on the outside of a coax line (which is the cause of radiation
and signal pickup) has nothing to do with the impedance match at either
end of the line.

What is also an issue is if the symmetry of the source and load isnt
preserved. ie if you feed a dipole direct with coax the "balanced"
aerial and "unbalanced" coax will add up to an amount of line radiation.

That's not entirely correct, either. A coax line can be just as
"balanced" as twinlead, and twinlead can be just as "unbalanced" as
coax. Imbalance is caused by other factors besides physical symmetry --
see "Baluns: What They Do and How They Do It" in the ARRL Antenna
Compendium, Vol. 2, for example.

. . .


You have to think of the antenna as a tuned AC circuit. In fact
completely throw away any thoughts of a DC circuit havning any effect
whatsoever. Same kind of logic as a power transformer not looking like a
short circuit. Think of the antenna as being feed by an instantaneous
voltage that takes a fixed time to get from the feedpoint to where there
is a "short cicruit" in the wire. The signal from one side of the coax
arrives "in phase" with the one from the other side so no current flows
between them. The trick is in the length of the wire in question. . .

I don't quite follow this, so I might be misinterpreting what was said,
but it doesn't sound quite right. A small loop has a small value of
resistance but a moderate amount of inductance. So the impedance
(composed of the resistance and inductive reactance), although low, is
considerably higher than just the low-frequency value. Although the
reactance provides the lion's share of the impedance, even the
resistance is quite a bit higher than the low frequency value due to
skin effect. As the antenna gets larger, the inductance increases and,
due to capacitive effects, the antenna eventually becomes resonant.
That's the frequency at which the inductive and capacitive reactances
just cancel. A short dipole antenna, on the other hand, has a much lower
impedance at RF than its low frequency value, primarily due to the
capacitance between the two conductors. Its impedance is composed of a
small resistance and a large capacitive reactance.

The current on the center conductor of the coax isn't in phase with the
current on the shield -- it's exactly out of phase with the current on
the inside of the shield. So the same amount of current that flows out
of the inner conductor flows back along the inside of the shield. In
this way, an antenna behaves just like any other electric circuit.

. . .

Roy Lewallen, W7EL