Topband: Measuring field strength (for the purpose of radialevaluation)

[Richard (Rick) Karlquist]

 -----Original Message-----
From: Telegrapher9 at aol.com [mailto:Telegrapher9 at aol.com]
Sent: Sunday, December 24, 2006 5:12 PM
To: richard at karlquist.com
Cc: zl3ix at inet.net.nz; topband at contesting.com; w8ji at contesting.com
Subject: Re: Topband: Measuring field strength (for the purpose of
radialevaluation)


Rick,

I tried out your 230' x 230' ground screen in simulation. I get around 77
ohms for the 90-degree vertical. Strangely I get 75 ohms for a 3-degree
vertical. I was thinking that perhaps 40 ohms could be attributed to the
ground plane. I can vary the size of the screen in 45 degree increments
without much of a change in the Rin. I'll have to work on this some more.

The ground screen is somewhat similar to PCB power planes and IC tester
heads (that I simulate and modify accordingly) in that they have standing
waves concentric with the excitation source.

dave

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Did you model it as a continuous surface or, as it really is, a wire grid?
At 7 MHz, a 3ft by 3 ft grid is too large to be considered the same
as a continuous surface.  PCB power planes are continuous surfaces.
I am very confident that if you build a 1/4 wl vertical at 1 GHz on an
18"x1'8" PC board plane it will measure 36 ohms.

Rick N6RK