Jerry,
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?
73,
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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