At 7/23/2004 01:28 PM, you wrote:
>At 12:19 PM 7/23/2004 -0500, you wrote:
>
> >Once again we see references to gain without the actually reference pointed
> >out. There is a big difference between dBi (dB over an Isotropic,
> >non-existent antenna) and dBd (dB over a dipole, real antenna).
>
><---Well, 2.8 Db difference to be exact but most folks simply round it off
>to 3 Db.
Actually it's 2.1 dB.
I actually prefer dBi. Sure, there's no such thing as an isotropic
radiator in real life, but for some reason theoretically it makes more
sense to me to express an antenna gain that way. Same for ERP (actually EIRP).
> And one correction if I may. A 1/4 wave antenna is a "real
>antenna" and many times can be used as the isotropic equivilent. A dipole
>is a 1/2 wave and demonstrates 2.8 Db over a 1/4 wave.
Then the 1/4 wave would have -0.7 dBi gain. Doesn't sound right. The 1/4
wave ground plane is simply a ground-imaged dipole, & should have the same
gain as a 1/2 wave dipole.
> >And both seem to be less than dBC (dBCushcraft - gain over a mythical
> >worse-than-isotropic reference).
An antenna's effective aperture (related to length for a colinear)
establishes the directivity, & hence the maximum possible gain. If an
antenna is only 10 ft. long, it can't possibly have 7 dBi of gain at 146 MHz.
Whenever I see an antenna gain figure expressed only as dB, I assume dBi.
Bob NO6B
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