Dave,
The roof as you describe it should be pretty transparent to RF, unless the
insulation has a metalized skin on one side or both. Have you tested the vswr
with your transmitter and a swr meter?
With such a good (high) antenna position there may be a local broadcast or
other RF source that is swamping your analyzer. My analyzer was useless near
downtown Minneapolis in a similar high location. In my case it was high power
FM stations in line of sight, about a half mile away.
My analyzer also was useless near a 15 kW AM broadcast station in a suburb of
St Paul.
My first experience with RF swamping made me think there was an intermittent
with my analyzer. This was at a Field Day event where I finally correlated the
CW operation with the "intermittent" in the analyzer! When they transmitted,
the analyzer went berserk!
73 de Bob - K0RC
Message: 1
Date: Thu, 8 Dec 2005 20:28:08 -0500
From: "WA3GIN in Alex. City, VA" <wa3gin@erols.com>
Subject: [TowerTalk] 40 Meter Dipole Question
To: <towertalk@contesting.com>
Message-ID: <000a01c5fc5f$d4493f70$6401a8c0@TS480WA3GIN>
Content-Type: text/plain; charset="iso-8859-1"
Question:
Installed a ? wave 40 meter dipole 10ft above the penthouse roof which
is 410ft above sea level and 210ft above average terrain. The dipole is
almost perfectly horizontal.
We get a dip on the antenna analyzer around 7Mhz but the SWR is 7.5:1 at
the dip and just goes up from there. I presume there is something going
on with the roof which is concrete covered with 4? Styrofoam and a layer
of creek stones.
Thinking about installing a lumped inductance across the feed-point and
using the center conductor of the 50 ohm coax to tap the coil where we
find a 1.1:1 SWR.
Looking for other ideas and suggestions as to how best to match the
input impedance.
73,
dave
wa3gin
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