Hans, you raised a very good point that needs to be discussed - the need for
ferrites on the antenna cable. I have read articles indicating that adding
the ferrites when taking NSA measurements was the only way they could make
their site compliant. But I have also heard that the reason the ferrites are
needed in those cases is that the antenna - most likely a bicon - is poorly
balanced. I have also heard that a properly balanced bicon does not need
ferrites. Comments anyone?
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From: [email protected] [SMTP:[email protected]]
Sent: Thursday, November 12, 1998 10:13 AM
To: [email protected]; [email protected]
Subject: Re: ANSI C63.4
A1. Since concrete is partly conductive and varies with mositure
content and
ambient conditions, the reflected path with vary substantially. The
solution
is to place a screen mesh over the top of the concrete.
A2. The specified size is that of the elipse which is 2d by root3d.
A
practical size is 20m by 17m rectangle.
A3. running coax on the top surface most likely will not affect the
measurements but for NSA measurements, to obtain maximum
repeatability ferrite
cores are placed every 1m on the mast cable. It would be best to
feed the
cables diametrically opposed from each other so as to minimize any
effects.
A4. Theoretically the conrete curbs, the hedges and metal object can
cause
additional unwanted reflections. Those may or may not be a problem.
Metal
objects will have the greatest effect since most of the wave will be
reflected. Hedges will most likely act as absorbers at certain
frequencies,
and the curb will cause reflections or absorbtion at certain
microwave
frequencies.
Hans Mellberg
EMC Consultant
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