Marinus,
I suggest you look on the web for copies of the series "Reflections".
By Walter Maxwell, noted antenna inventor and author. It was also put
out as a book, first by ARRL, and then World Radio magazine in a later
revised edition.
The radiation from coax is not a function of the swr, which is measured
on the signal INSIDE the coax. Radiation from coax might be from
induced antenna signal upon the outer of the shield, if the coax is not
symmetric to the antenna elements. In that case, it being near to the
antenna element, current is induced upon the shield and then can radiate
as it moves along the shield length.
It also happens if the coax length presents a low impedance from the
outer of the shield to ground, at the antenna end of the coax. This can
happen if the coax length is a half wave at the frequency of operation,
and the impedance of the outer shield is low to currents exiting the
coax, than the antenna element impedance connected to the coax shield.
The current will divide with more going to the lower impedance conductor
given such a choice of two paths. Low impedance on the outer shield
surface means a high current, thus leading to radiation from the outer
part of the shield.
SWR simply means the current inside a coax either encounters a matched
source and load, (SWR 1:1), OR the current encounters a reactive load,
(impedance not matched to source), and the either inductive or
capacitive reactence causes a reflection at the antenna end of the
coax. (SWR other than 1:1). This is a reflection back into the inside
of the coax, separate from the outside shield radiation due to the other
factors mentioned.
The key thing to remember about coax, is it really is THREE conductors;
center wire, inside part of the shield, and outside part of the shield,
that comes into play when its impedance is in a certain relationship to
the antenna impedance Z. (Lower Z than the antenna, which leads to
radiating current on the outer shield section).
Ten Tec uses sturdy finals to withstand the occasional SWR excursion,
and also in some of the models, a fast acting magnetic breaker is used
to interrupt the power supply current before finals can be damaged.
-Stuart Rohre
K5KVH
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