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Re: [TowerTalk] Fwd: Fwd: Dbi vs DBd

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Fwd: Fwd: Dbi vs DBd
From: Steve Hunt <steve@karinya.net>
Date: Tue, 03 Nov 2009 23:16:13 +0000
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Dan,

Amen to that !!!

Steve G3TXQ

Dan Zimmerman N3OX wrote:
>
> I think dBd should ALWAYS be defined this way:  as "decibels with respect to
> a dipole antenna installed at the same height in the same location
> and orientation as the other antenna in the comparison"  And when "the same
> height in the same location and orientation" doesn't even make sense, you
> need to drop back to the dBi comparison.  The problem with dBd = dBi +
> 2.15dB is that it makes a dipole that's installed at a real height over
> ground have lots of gain in "dBd"  ... a dipole could be 8dBi or 10dBi or
> 5dBi maximum gain.
>
> That doesn't make sense to people and it leads to all manner of confusion,
> because that last sentence could easily read:
>
> "A dipole could have six or eight or three decibels of gain over a dipole"
>
> This is the big pitfall of dBd, exploited by many (but not all) beam antenna
> marketers.   A dipole installed on top of a tower has a big fat double
> handful of dB gain over a dipole in free space.  A beam installed on that
> tower has almost exactly the same extra gain compared to its free space
> gain.
>
> But some manufacturers like to compare the INSTALLED gain of the beam to the
> FREE SPACE dBi+2.15dB value.  Technically true for that definition of dBd.
> Totally misleading and useless nonetheless.
>
> So I think we should banish the idea that dBd = dBi + 2.1dB.  It's only true
> in free space, and our antennas are
> never in free space.  When we buy beams, we want to know how much stronger
> they are vs. the dipole on our tower.
> That's the only dBd definition that makes real sense.
>
> Otherwise all we need to know is how much power the new antenna sprays in
> the desired direction vs. what would happen if we sprayed
> the entire transmitter power in all directions equally, and that's the
> absolute number that dBi gain gives us.    If you know the dBi gain of ten
> different antennas, you know exactly how much stronger and weaker they are
> mutually in the desired direction.  dBi never lies, because it's *always
> relative to the same thing*
>
> And since dBi has one single fixed defintion, you can add and subtract dBi
> gains without ever worrying if someone used an inappropriate definition of
> dBi.  There's only one.  It means one thing.  It doesn't matter if you can't
> figure out how to build that thing, because it's just a reference point  for
> easily comparing very different antennas to each other.
>
> 73
> Dan
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>   

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