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Topband: 160M coaxial loop

To: <topband@contesting.com>
Subject: Topband: 160M coaxial loop
From: W8JI@contesting.com (Tom Rauch)
Date: Sun, 3 Mar 2002 16:21:45 -0500
Hi Frank,

Most people incorrectly assume the shield "filters" electric fields, 
and allows magnetic fields to pass to what they assume is the 
"antenna" inside the shield. That concept is totally wrong, and any 
answers derived from viewing the antenna that way would only be 
correct though some sort of random luck.

The capacitance of the cable is NOT the same as the capacitance 
per foot times the number of feet (unless at DC), because it is a 
fairly long transmission line!

Not only that, the effective capacitance of that transmission line is 
not simply in parallel with the trimmer capacitor you have at the 
"feedpoint"! 

The shield is not really a shield, it is actually the antenna that is 
picking up the signal. The outside of the shield is actually the 
antenna, and is what you are really "matching" when you adjust 
the trimmer capacitor. The trimmer capacitor couples to the 
outside of the shield though transformer action, so you are really 
adjusting that complex **transformer system** to resonance.

Since the shield is on the outside, you can see you have really not 
shielded the antenna for anything. All you really have done is 
modify the feed system.

Enjoy your antenna if you have it tuned and it is working, but ignore 
any technical explanations based on the flawed assumption the 
shield is a shield and the antenna is the center conductor inside 
the loop......that just isn't how the antenna actually works.
73, Tom W8JI
W8JI@contesting.com 

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