When I was an undergraduate at MIT, there was a requirement for a
Bachelor's degree thesis. Mine was about bouncing 2m signals off Echo II
QST articles about that in 1962). Anyway, they invited three prominent
professors, all of them hams, to be on my thesis committee. They quickly got
into an argument about whether 20 dB (voltage) was the same thing as 10 dB
(power). I just kept my mouth shut, and got an A. True story.
73 Ray W2RS
In a message dated 1/7/2011 10:30:17 P.M. GMT Standard Time,
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.e
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?
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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