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[TowerTalk] Tin roof blues

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Subject: [TowerTalk] Tin roof blues
From: (Tom Rauch)
Date: Tue, 12 Jun 2001 06:59:51 -0400
Hi Barry,

> sheets. Using a simple home made clip on rf current meter showed no
> current in a wire from the junction of the high band radials to the roof.
> i.e. the fan of radials effectively isolated the system from the metal
> below.

What that might show is the radials and roof are almost perfectly 
coupled. If they were isolated, if there was a voltage difference, 
current would definitely flow. When there is no current via a low 
impedance connection, they are at exactly the same potential.

There is considerable current of the feedline to a 1/4 wl 
groundplane with four or less radials, because four radials do not 
"hold" the feedline shield to zero voltage. You can actually 
measure an SWR change on a 1/4 wl groundplane as a low 
impedance ground wire is touched to the shield of the coax just 
below the groundplane, and you will measure current in that wire.

If you do not see that effect it is most likely because the roof and 
radials are already acting like one big ground, connected through 
capacitance and mutual inductance! 
73, Tom W8JI 

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