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[TowerTalk] Image antenner Secrets Finally Revealed

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Subject: [TowerTalk] Image antenner Secrets Finally Revealed
From: (Tom Rauch)
Date: Wed, 29 Apr 1998 11:12:53 +0000
Hi Roger,

> and the literature is full of interesting diagrams relating the height 
> of the dipole above, not only physical ground... but also, above a 
> virtual image somewhere below ground.  This in-the-ground image was 
> responsible for radiating an in-phase or out-of-phase signal with the 
> above ground wire creating interesting take-off angles that are 
> responsible for a good, great or a very poorly working antenna.

Your tounge-in-cheek post is important, because it points out there 
really is no such thing as an "in the ground antenna image". The 
mental or paper picture an "image antenna" paints is a very incorrect 
picture indeed!

I e-mailed the ARRL when they, quite incorrectly, explained in 
detail about how a radial system forms a vertical antenna "image" 
below the antenna. The answer was in response to the question "what 
do radials do?" in a "The Doctor is In" column.

(The incorrect response, given to someone trying to understand 
what ground systems do, went uncorrected. My e-mail was received 
by but went unanswered by the author of the "image" response.) 

The image antenna is a "rough tool" useful for calculating 
or mentally visualizing the effects of  ground extending for a large 
distance in many directions of the antenna on patterns.

The image isn't really there, in the dirt below the antenna, at all. 
With a vertical, the image is formed by the ground several 
wavelengths from the antenna, not by the ground near normal short 
(radial L<1/2 wl) radial systems.

73, Tom W8JI

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