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Topband: Inverted Ground Plane

To: <topband@contesting.com>
Subject: Topband: Inverted Ground Plane
From: W8JI@contesting.com (Tom Rauch)
Date: Wed, 25 Sep 2002 13:53:50 -0400
> My opinion is that radials can only function below the vertical.  The
> wave radiated from the vertical must immediately become 511 feet tall
> to propogate efficiently.  Since most verticals are less than a full
> wavelength tall, the ground or raidals must act as a reflective
> surface to prevent the wave from digging into the ground and loosing
> power.

The wave does not become a full wavelength tall, and certainly does 
not need to become that tall to propagate efficiently.

Visualize the waves from a pebble hitting a pool of water. The period 
of the wave has nothing to do with the total frontal area of the wave 
as we move out from the point source. 

Consider any antenna in freespace receiving a distant signal on 
direct wave. We have a wave front geometry that is determined solely 
by the distance to the source, it is a geometry problem and not a 
wave-period problem. A two-meter signal and a 160 meter signal would 
have the same wavefront, just as it would be with a pebble or a rock 
in a pond of thick oil or a pond of thin solvent. If it didn't work 
that way, collinear antennas beyond 1 wl long would not work. 

The real mechanism of loss is due to currents induced or coupled to 
the earth. Whenever we intentionally or unintentionally increase 
those currents, we increase the power that is lost as heat.

Four radials concentrate the fields around and even below each 
radial. The more radials we use, the more area we would spread those 
fields over. Each radial would share current, and so the magnetic 
induction fields would decrease near each radial. The voltage along 
each radial would be reduced (for a given power), and the so electric 
fields would be weaker. Losses would be less, as we use more and more 
radials.

In order to use a few radials effectively, the radials must be a 
large fraction of a wavelength above earth. At 1/4 wl, eight radials 
measure to be just the tiniest amount below 100 radials on the earth 
surface...for all purposes they are nearly the same. But that is with 
the antenna extended up away from earth. There are published measured 
FS data for this from an actual BC station system.

If you look at any old antenna handbook, they always warn that a 
groundplane antenna must be at least 1/4 wl high or higher above 
earth. Based on a few articles that were never verified by actual A-B 
measurements of real systems, we now tend to dismiss all the real-
world work over the past century.73, Tom W8JI
W8JI@contesting.com 


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