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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

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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