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

To: "'Steve Hunt'" <steve@karinya.net>, <towertalk@contesting.com>
Subject: Re: [TowerTalk] Fwd: Fwd: Dbi vs DBd
From: "AD5VJ Bob" <rtnmi@sbcglobal.net>
Date: Tue, 3 Nov 2009 18:48:51 -0600
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
 
Hi All

Thanks to everyone for all the replies.

I guess what Steve said is what I have done, but by accident.  

> > 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

When I installed my Hornet Tribander on the Tower at 30 some odd feet, I began 
comparing it to my Butternut Vertical which I had
been using pre-Tribander.

I noticed around a 3-6 S unit increase most times when in the Short Path 
direction and (2-7) S units less off the sides and a little
less than that (2-4) off the back (on average). 

At other times the Vertical actually won out around (3-4) S units over the Beam 
(propagation difference I think on those).

But the point is I was comparing it to an antenna I had already been using here 
before, to see what the difference was.

I know there is a difference in polarity, but comparing the two made better 
sense to see if the Beam had actually done any real good
to install.

Hence my question abt the DBd vs DBi. Since the figures on the Hornet manual 
say it has anywhere from 7.5 to 8.5 DB gain (depending
on the band), it just doesn't say what it was compared to.

Now if I am understanding all I have read here, I am taking it that it was 
being advertised as the DBi figure since it was made in
1959. So the gain should actually be a couple DB lower when compared to a 
Dipole (which makes better sense).

Bob AD5VJ

> -----Original Message-----
> From: towertalk-bounces@contesting.com 
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Steve Hunt
> Sent: Tuesday, November 03, 2009 5:16 PM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Fwd: Fwd: Dbi vs DBd
> 
> 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 definition, 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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