The difference between ordinary earth and a concrete rebar roof could easily
account for the difference you observe. This is so, particularly, if the
roof's connection with conducting vertical sides of the building are low
impedance at the frequency in use.
I would try making the adjustment to the feedpoint to get 50 ohms, or use a
tuner at the transceiver (borrow if needbe). The antenna may play like
gangbusters. Check out the performance before you tear it down or give up on
it. Performance and feedpoint impedance are independent concepts.
Guy L. Olinger
Apex, NC, USA
From: Igor Sokolov <email@example.com>
To: firstname.lastname@example.org <email@example.com>
Date: Friday, January 01, 1999 5:31 PM
Subject: Re: [antennaware] Antennas over steel and concrete
>The roof is concrete with rebars in it and I think therefore can be modeled
>as near perfect ground. Anyway that does not explain why the input
>of a single element delta loop fed for vertical polarisation and with apex
>angle of around 100 degrees increased from 60 ohm (when over the ground) to
>nearly 160 over the roof.
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