[TowerTalk] 40 Meter Dipole Question

[Robert Chudek]

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 at erols.com>
Subject: [TowerTalk] 40 Meter Dipole Question
To: <towertalk at contesting.com>
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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