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Re: [TenTec] OT SWR VS Power Loss

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] OT SWR VS Power Loss
From: Stuart Rohre <rohre@arlut.utexas.edu>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Tue, 08 Jul 2008 13:28:39 -0700
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
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

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