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[TowerTalk] dBd vs dBi misuse

To: "Chuck O'Neal" <>,"Tom Jednacz" <>, <>,"Jim Lux" <>
Subject: [TowerTalk] dBd vs dBi misuse
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Wed, 23 Jun 2004 06:38:04 -0400
List-post: <>
> F/B, and as modeled over ground, a gain of over 16.6 dBd
> the stack fed in phase.

The proper way (or at least the accepted way in HF BC
antennas)  to apply dBd is for a dipole at a the same mean
height as the array being spec'ed.

Deducting ~2dB for isotropic dBi is very misleading. We
still have grossly inflated gain figures because "ground
effects" are not applied to the dipole and they are applied
to the stack. You either have to move the stack to freespace
or the dipole to earth, otherwise it is nearly as misleading
and inflated as dBi.

My 40 meter three element full size Yagi, for example, has
~13dBd gain when I do an unfair comparison to a dipole in
freespace by converting a "freespace" isotropic to an
equally impossible to obtain freespace dipole. Anyone out
there think I really have a 13dBd gain 40 meter Yagi?

Plant a dipole at the same mean height, and then subtract
that "gain" from the antenna you want to spec in dBd.

If I do that, my 40m Yagi has about 6dB or so gain.

On the other hand if I compare a dipole over ground to an
isotropic I can make the dipole have around 7dBd gain. Does
that make sense? Dipole references should always be at the
same mean height.

73 Tom


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