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Re: QFH Gain



Domenico,
As the QFH is circular the gain should be dBic. As the QFH pattern is never
a hemisphere, but either a cardioid or a modified Cardioid making the gain
over 3 dBic possible. If you compare it with a dipole on a circuar
transmitted signal you get an extra 3 dB as you lose 3 dB when you use
linear polorization.

Art, KC6UQH

----- Original Message -----
From: "i8cvs" <domenico.i8cvs@tin.it>
To: "Dave Guimont" <dguimon1@san.rr.com>; "KC6UQH" <kc6uqh@cox.net>
Cc: "AMSAT-BB" <amsat-bb@amsat.org>
Sent: Tuesday, March 15, 2005 4:53 AM
Subject: [amsat-bb] QFH Gain


> Hi Dave,WB6LLO and Art, KC6UQH
>
> After writing my article "Experimental Investigation on Quadrifilar Helix
> Antennas for 2400 MHz" in the AMSAT-DL Journal dezember/februar
> 2003/2004 and AMSAT Journal May/June 2004 I got a very interesting
> comment from G3BVU but unfortunately I was not able to answere
> directly to John because every time my email to him bounce back to me.
>
> Since the John explanation on how to evaluate in theory the QFH gain is
> very simple and very interesting I finally have decided to disseminate the
> G3BVU letter through the AMSAT-BB reflector.
>
> Best 73" de
>
> i8CVS Domenico
>
>
> ----- Original Message -----
> From: John <g3bvu@arrl.net>
> To: <domenico.i8cvs@tin.it>
> Sent: Friday, June 11, 2004 12:54 PM
> Subject: QFH Gain
>
>
> 11 June 2004
> I8CVS de G3BVU     BT
> Dom,
>
> I have just read your QFH article in The AMSAT Journal, May/June 2004.
> I previously read a copy of your report in the AMSAT-DL Journal.
>
> Congratulations. It is a very good report.
>
> I do have one comment.
>
> You state the gain of a QFH is about 5 dBi. I believe this is too high for
> the following reason.
>
> Let us consider the ideal theoretical isotropic radiator. It has unity
> gain, 0 dBi, in all (3D) directions.
> If we remove all of the the energy from one hemisphere and add it
> uniformly to the other hemisphere, we have doubled the energy in the
second
> hemisphere. Hence the gain at all directions in the second hemisphere is
> now increased by a factor of 2 (3 dB) to 3 dBi.
>
> The ideal QFH has the same characteristics as I have just described. It
> has 100% response in one hemisphere and zero response in the other
> hemisphere. The only way to obtain higher gain is the restrict the
> radiation pattern to be less than a hemisphere.
>
> In practice a QFH has some small amplitude radiation in the "suppressed"
> hemisphere; hence this energy is not available to increase the gain of the
> main hemisphere. For uniform illumination of the main hemisphere the gain
> would be a little less than 3 dBi.
>
> However, the radiation intensity in the main hemisphere is not uniform.
> There is reduced gain near to the horizon (90 degrees from boresight). The
> "missing" energy is not lost; it is added to the radiation at angles near
> to boresight. So, depending on the illumination taper, the gain of a QFH
at
> boresight will be greater than 3 dBi.
>
> Your excellent patterns show about 6 dB gain reduction at the horizon,
> which quickly increases to only 2 dB reduction at 30° above the horizon.
>
> I believe about 3.5 dBi is a more honest value for the gain of a good
> quality, uniform distributed radiation, Quadrifilar Helix Antenna.
>
> 73s,  de John  G3BVU/W1   SK
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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----
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