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[TowerTalk] Tin roof blues

To: <>
Subject: [TowerTalk] Tin roof blues
From: (Barry Kirkwood)
Date: Wed, 13 Jun 2001 08:04:08 +1200
Yes, Could be.
I did have an isolating balun at the feed point, made the assuption that no
current =no coupling,
but as you point out no current could mean perfect coupling.
FWIW the higher band monopoles were nothing special but the 80m job really
played. Amazed everybody.
73 es tnx
Barry Kirkwood PhD ZL1DD
Signal Hill Homestay
66 Cory Road
Palm Beach
Waiheke Island 1240
----- Original Message -----
From: Tom Rauch <>
To: Tower Talk <>; Barry Kirkwood <>
Sent: Tuesday, 12 June, 2001 10:59 PM
Subject: Re: [TowerTalk] Tin roof blues

> 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
> > 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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