[antennaware] Antennas over steel and concrete

Eric Gustafson n7cl@mmsi.com
Fri, 1 Jan 1999 16:52:49 -0700 (MST)

Hi Igor,

>From: "Igor Sokolov" <ua9cdc@dialup.mplik.ru>
>Date: Sat, 2 Jan 1999 03:04:54 +0500
>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 impedance 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.

Why not?  It is closer to the roof and farther from the ground.
Either or both of those changes may easily have caused the change
you observe.

>My another question:
>Is there a point to install antenna with vertical polarization
>in such a circumstances?  I mean vertical 30 meters above the
>ground radiates too much straight into the clouds.

What gives you the idea that an elevated vertical is necessarily
a high angle radiator.  This isn't necessarily so.  The real
question is: "If you can get a horizontal radiator high enough
above earth (1/2 wavelength or more) to be effective, and it is
physically possible to support it adequately, why would you give
up the benefit of gain from reflection off the distant earth to
use vertical polarization?"

>On the other hand, is it true (and it is according to models)
>that there is no point in putting any stacked antennas on the
>roof just because the lower antenna is too high and therefore
>one does not get "fill in" for certain angles.

How large is this roof again?  What band are you concerned with?

73, Eric  N7CL

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