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Re: Can anyone clarify dBic?



I think there is a mistake here.

If you phase two linear yagis for circular polarization and each yagi has a
stand alone linear gain of 10 dB, then the gain of the circular pair is
still only 10 dB....but it is now circular. In other words, it takes TWO
linearly polarized yagis to make circular polarization from the pair.....you
don't get to add things in twice. The gain you would have gotten from a
normal linear stack (3 dB), is actually not realized because of the games
being played to get a pair of linear antennas to act as a circular "array"..

A linear receiving antenna results in a loss of 3 dB. So I agree with the
conclusion that the net receive is only + 7 dB...but I don't think it's
because you are only "hearing" one 10 dB single yagi. Once you are in the
far field of the antenna SYSTEM (no matter how the circularity was
generated) . You ARE hearing the combined result of "both" antennas....it
just turns out that the gain of the two linearly polarized antennas when
forced into circular polarization by phasing or orthogonality and offset is
still only 10 dB...not the 13 dB quoted.

The receive antenna is not "smart enough" to know whether a circularly
polarized signal it sees comes from a helix, an egg-beater, crossed yagis
with phase shift, or yagis with phase and space shift. All the receiving
antenna knows is that it's seeing a circularly polarized signal. The real
confusion comes from calculating the gain of a pair of linearly polarized
antennas properly configured to produce circular polarization. The error is
to include both circular polarization conversion and stacking gain...you
can't get both at the same time. To convert to circular, you lose the effect
(in gain) of the 2nd antenna.

There's no free lunch.

If both antennas were circular in the first place, like a KLM 14C with 10
dBc each, and you stacked two of them, then you would have 13 dBc. I ran a
pair of CC 416TB "twist boomers" that were quoted as 13 dBc each. My total
gain was 16 dBc, even though they were made of crossed yagis because each
antenna was a crossed yagi with proper phasing. This would be the equivalent
of 4 yagis in the example that started the discussion. Two yagis each to
produce circular polarization, and two of these "arrays" to give the 3 dB
gain.

Also...you don't really get 3 dB gain anyway...you would be fortunate to see
2.5 dB for stacking gain in most practical situations.

73
hasan schiers, NØAN
schiers@netins.net

----- Original Message -----
From: "Franklin Antonio" <antonio@qualcomm.com>
To: "Scott Townley" <nx7u@arrl.net>
Cc: "Howard Long" <howard@howardlong.com>; <amsat-bb@AMSAT.Org>
Sent: Tuesday, June 12, 2001 5:01 PM
Subject: Re: [amsat-bb] Can anyone clarify dBic?


> At 01:59 PM 6/12/2001, Scott Townley wrote:
> >(Or look at it another way...if you "CP" a pair of 10dB yagis, and
receive
> >them on a linear antenna, the CP pair will look like 13dBic-3dB
> >(polarization mismatch)=10dBi, which is the gain of one of the linear
yagis).
>
> That is not correct.  If you phase a pair of 10dBi Yagis, and transmit a
> circularly polarized signal to me using that setup, and I receive it with
a
> linearly polarized antenna, I will receive only the portion of the signal
> that you transmitted from ONE of the two transmit antennas.  Yes, that one
> transmit Yagi antenna is a 10dBi antenna, but you only put half of the
> transmit power into it (because you put half into the other polarization
> which is in this case wasted).
>
> Therefore, the result will be the same as if you had transmitted to me
> using a 7dBi Yagi.  10dBi - 3dB = 7dBi.
>
> ----
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>

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