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[TowerTalk] Tin roof blues

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Subject: [TowerTalk] Tin roof blues
From: (Barry Kirkwood)
Date: Tue, 12 Jun 2001 21:11:34 +1200
Some suggestions on using metal roof as groundplane.
When I lived back in the city I had a small two story town house with a
galvanised corrugated steel roof.
I used various quarter wave verticals with their base on a stub mast working
against 3-4 slightly sloping quarter wave radials.
For 80m I used a quarter wave inverted L (more like half an inverted v).
This was alwys used with at least one quarter wave radial. These longer
radials would run to a corner of the roof and then along the fence tops at
the sides of the lot.
I bonded the roof iron by spot soldering thin tinned copper wires at the
corners of the metal 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.
Current flowed in this wire when I worked the 80m inverted L. I measured the
drop in feedpoint impedance as extra 1/4 wave radials were attached to the
80m feedpoint.
I came to the conclusion that the relatively small metal roof was equivalent
to at least 5 radials so far as currents were concerned.
I never tried the higher bands against the roof alone.
The 80m L worked very well, better than might be reasonably expected.
The little link wires corroded out very rapidly, so use a better bonding
technique if going this road.
I would certainly advise giving the metal roof a try as a groundplane.
Barry Kirkwood PhD ZL1DD
Signal Hill Homestay
66 Cory Road
Palm Beach
Waiheke Island 1240

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