[TowerTalk] Rai Beam
wa4dou@juno.com
wa4dou@juno.com
Wed, 12 Apr 2000 00:39:25 -0400
Hi Ed,
Comments below! 73 Roy WA4DOU
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On Tue, 11 Apr 2000 15:14:49 +0000 K4SB <k4sb@mindspring.com> writes:
>
> wa4dou@juno.com wrote:
> > I don't believe for a second, that the 2 element Raibeam
> produces a
> > gain of 8
> > dbi, which translates into 5.9 dbd., but for the sake of the
> arguement,
>
> Roy made a very good presentation in his arguments. However,
> along with a lot of others, he is mixing apples and oranges with
> the dbi and dbd. You simply cannot subtract the 2.15 dbi gain of
> a free space dipole, and come up with dbd.
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I subtracted 2.1 db from 8 dbi, leaving 5.9 db of gain referenced to
a dipole in free space. Unless i'm wrong, dbd means gain in db
referenced to a dipole. I'm unaware of that dipole having to be in the
real world. Were it to be in the real world, then you are correct, there
is ground reflection gain to consider. However that ground reflection
gain applies equally to both antennas, and therefore does not come
into play, as my comment was about the Raibeam as compared to
a dipole.
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A common dipole over
> real ground will have a gain of about 6.5 to 8 db. Against a
> similar antenna, it gain in DBD is 0. I have 3 wide spaced long
> boom yagis for 10, 15, and 20. They generally average out at
> about 15.7 dbi gain over real earth but, to get to dbd, you have
> to subtract the gain of a dipole at the same height, and at the
> same angle of radiation. As Roy noted, that leaves us with about
> 8 dbd gain.
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I won't take issue with your real world gain claim of 6.5 to 8 db of a
dipole versus in free space, although conventional wisdom says 4-6 db.
Whatever it is, the gain of an antenna to a reference standard only
requires that the reference standard be clearly understood. An isotropic
point source or a dipole in free space serves fine. So does a real world
dipole, as long as that is clearly stated. To include ground reflection
gain
in the antenna stats without also including them in the reference
standard
is likely why so many antenna manufacturers "triband" trap yagis looked
so good. Also its not my idea of being ethical.
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> Again, taking Roys general position, there is no such thing as
> something for nothing. I don't care how you phase them, space
> them, or what, the simple truth is that every compromise you
> make, whether using traps, shortening the boom, or any other will
> result in a decrease of gain from a reference standard.
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Bravo! :o)
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> And another factor, reports such as I worked XX1XX with a 20/9
> signal mean absolutely nothing, especially as you go higher in
> frequency to say 28 Mhz. When propagation is right, 1 watt into a
> bedspring will sound like a wrong coast kilowatt.
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Again, Bravo!
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> You can believe these inflated gain figures, and a lot of us
> "want" to, but the simple truth is antennas follow the same laws
> as anything else.
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Right on Ed!
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> 73
> Ed
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