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Re: [TenTec] NEC, ground, grounds, and radials.

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] NEC, ground, grounds, and radials.
From: Steve Hunt <steve@karinya.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Fri, 07 Jan 2011 22:29:48 +0000
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>

Please don't think I'm trying to be contentious, but your comment 
challenged my established view of what "dBi" represents. That led me on 
a literature search starting with my (very old) Masters Degree notes, 
antenna engineering reference books, and web sites. I can't find any 
material where the Isotropic reference power density is defined as 
anything other than the transmit power spread over a *complete sphere* - 
not a hemisphere - even where the antenna being compared is over real 
ground. In other words, consistent with EZNEC.

In some cases the interpretations were explicit - for example "...... 
compared with the power density of an isotropic radiator in Free Space"; 
in others it could be inferred from the underlying maths.

That's the interpretation I was taught, and yes it would certainly lead 
to a 3dB higher figure than your interpretation.

I'm trying to understand whether your view is commonly held, or rather 
something you feel strongly about; if it's a commonly held view perhaps 
you could point me to some literature. I presume that someone like IEEE 
must have an unambiguous definition?

Steve G3TXQ

On 06/01/2011 17:07, Dr. Gerald N. Johnson wrote:
> As for the effects with ground planes and my claim of error. I base it
> on this: Model a quarter wave vertical on a perfect ground plane. It
> will show 3 dB more gain than a half wave dipole in free space. Yet the
> theory of images in the ground plane insists that the quarter wave
> vertical on the ground plane has a image of the other half making it the
> exact equivalent of a half wave dipole. I claim that while the program
> in free space is comparing the signal intensity from the antenna to that
> of a perfect isotropic radiator located at the 0,0,0 origin of the axes,
> that when the ground plane is present it cuts that isotropic radiator in
> half, shielding half of its radiated power and so the reference to a
> full isotropic radiator is 3 dB in error. 3 dB too much gain.
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