[TowerTalk] Fwd: Fwd: Dbi vs DBd

[Dan Zimmerman N3OX]

On Tue, Nov 3, 2009 at 4:31 PM, <hanslg at aol.com> wrote:

> dBi, (gain over isotropic antenna) refers to something that radiates
> equal in ALL directions. (If you cross two dipoles and feed them 90
> degrees out of phase, you get something very close.) The dipole
> radiates its power in a doughnut shaped pattern with about double the
> field in the maximum direction and (theoretically no field along the
> axle). You may therefor add 3 dB when you "translate" dBd to dBi.
>

It's not 3dB, it's 2.15dB

And as far as I'm concerned dBd = dBi +2.15 is a totally useless
definition.   No offense intended to you,
it is a "true" thing and is indeed indicative of the FREE SPACE situation
where you compare a dipole's donut
to the equal radiation in all directions of the isotropic.  But the utility
stops there.

Saying dBd = dBi + 2.15 is  kind of like me defining a new temperature scale
: Degrees Zimmerman = Degrees Fahrenheit + 9.3

it gives no new information and is not done to make anything easier to
remember.

dBd is often used to mean something useful.  You can't build an isotropic
comparison antenna for your test range, so you build a dipole instead.  dBd
gain is the gain vs. that dipole.

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