[TowerTalk] Rai Beam

[Al Williams]

This dbi, dbd, dbD etc has bugged me for a long time.  I think I fully
understand the dbd meaning ~2db
gain of a dipole in free space over a point source in free space; and a dbD
adding another ~4 db for a
dipole over earth ground to that of the dipole in free space.

What I dont understand is what the manufacturer is using e.g. if the
manufacturer claims 7 dbi gain, does
this mean 7db gain for his antenna if it were in free space (which is a true
7db gain) or does he mean his antenna, when over ground, had 7db gain (which
is ~ 1db gain!)

Unless the antenna location is spelled out, it seems to me references to
dbi, dbd, etc are worthless?????  Am i missing something?


k7puc
-----Original Message-----
From: K4SB <k4sb@mindspring.com>
To: towertalk@contesting.com <towertalk@contesting.com>
Date: Tuesday, April 11, 2000 8:17 AM
Subject: Re: [TowerTalk] Rai Beam


>
>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. 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.
>
>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.
>
>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.
>
>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.
>
>73
>Ed
>
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