>
> The only comparisons that are clear, misdirection free, are relative
> to an isotropic antenna in free space = dBi, or a halfwave dipole in
> free space = dBd.
The only ones?
What about dBq ... vs. quad loop?
If dBd just means dBi + 2.1dB, it is completely trivial and should be never
used again.
I don't think it means that, or at least that is not how I would use it. I
think it's a convenient reference point for real world antennas, especially
beams because it's referenced to a buildable object that is easily
constructed and erected under similar conditions to any other horizontal
antenna like a yagi or quad and has very minimal losses that are essentially
negligible until it gets too close to the ground.
Is it a bad reference for verticals? Yes, of course. But it's a perfect
reference for yagi antennas and other directional horizontal arrays. What
is the gain over the dipole you'd get if you stripped off all the other
elements?
I think once you start talking about verticals with ground losses you should
just start using dBi, especially because the reference standard, a 1/4 wave
vertical over a good ground radial system installed at zero base height on
average dirt will tend to be close to 0dBi gain once you take into account
the Fresnel zone ground reflection losses.
So 0dBi and 0dBgmv (ground mounted vertical, with excellent radial system)
are often close to the same thing, maybe +/- a couple dB depending on the
dirt.
If you want to make practical comparisons with real world antennas, use dBd
for horizontal ones with the assumption that changing mounting height, etc,
is cheating.
If you want to specify the gain of the whole system relative to an absolute,
invariant standard so that you can directly compare whole antenna systems of
any type, that is what dBi is for.
73
Dan
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