On 1/7/2011 4:29 PM, Steve Hunt wrote:
> 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
I fear its not a commonly held view though I'd think in broadcast
circles NEC would be held in contempt because it seems by my analysis to
predict twice the field power than would be measured in the real world.
And if it was a commonly held view it would be very easy to fix in the
base NEC code or in the user interfaces that always have the flag for
free space or over ground. It was easy to fix in the Apple II+ version
of basic source code for MININEC.
Yesterday, ND3K on this forum said:
> Nevertheless, frequent users of NEC know about this idiosyncracy, and it
> is a simple matter to subtract 3 dB to get a more realistic picture of
> the performance of an antenna over ground. You just have to be aware of
> the issue.
I stick with my analysis below. That the ground plane splits the
isotropic source in half shielding half of its energy from the far field
intensity comparison that defines antenna gain. Otherwise why would the
quarter wave and its image put out a stronger
field than the free space vertical dipole?
> 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.
73, Jerry, K0CQ
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