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RE: Disparaging Helical Antennas for AO-40...some notes

John, and other folks:

I stated in another posting that I do not have or read the literature on
building a cavity reflector for a Helix, but now "Zack, W1VT"
<zlau@arrl.org> has told of his article in the Sept. 97 QEX.

Assuming you do not have that issue, in brief here is what Zack showed:  A
cavity for a 10-turn helix on 2.4G is 2.04 inches diameter and 1.98 inches
high which Zack obtained by scaling the cavity designed by King and Wong in
their study published in IEEE Transactions on Antennas and Propagation,
vol. AP-28, March 1980, pp 291-296, "Characteristics of 1 to 8 wavelength
Uniform Helical antennas".  Zack did not include any formula for determing
the cavity, so applying these dimensions to a different length helix is
unknown to me.  Zack  did not know how King and Wong fed the antenna, so
this is one for you experimeters out there.  You will be going where no Ham
has ventured before :-) 

Please do not ask me for this publication.  I do not have it.  I'm merely
passing along the reference from Zack's QEX article.  Good luck!


>From: "Santillo, John (John)" <jesantillo@lucent.com>
>Hi Ed, can you forward me the equation used for the cavity reflector.  I'm
>with a three turn 2.4Ghz Helix for a 32" dish with a f/D of .4.  The cavity
>I'm using is 
>about a .25 wavelength deep or around 1.25".  This is purely a guess but I
>did notice 
>improved gain of my system while testing it on my home made antenna range.
>TNX & '73,
>	----------
>	From:  Edward R. Cole [SMTP:al7eb@ptialaska.net]
>	Sent:  Thursday, May 24, 2001 3:20 AM
>	To:  amsat-bb@AMSAT.Org
>	Subject:  Re: [amsat-bb] Disparaging Helical Antennas for
>AO-40...some notes 
>	Zack,
>	Thanks!  I pulled out my Sept. 97 issue of QEX.  Good info for a
>helix with
>	cavity reflector.
>	Also interesting how your predictions for P3D are now being seen.
>	Especially found your comments about ground noise and foliage
>	pertinent.  Yesterday morning I attempted to listen to the AO-40 S2
>	shortly after AOS.  The problem is that the dish is looking thru the
>	at elev. <10 degrees.  Previous observation of AO-40 with the 4-foot
>	when elev = 30 degrees produced a signal three s-units over noise
>	~12 to 15 dB].  The noise floor looking at the trees was S-3 vs S-2
>	at cold sky.  My Drake converter NF~6 dB.  Its curious  that filling
>	dish aperture with 290K "hot" trees produced about 3 dB more noise
>[a NF of
>	3 dB is equivalent to 290K]!  BTW the beacon was not heard :-)
>	Ed
>	>From: "Lau, Zack,  W1VT" <zlau@arrl.org>
>	>My September 1997 QEX column (the issue with a cavity reflector
>helix on the
>	>cover)
>	>suggested gains of 15.5 dBi for the G3RUH helix and 17.8 dBi for an
>	>duplication
>	>of the King and Wong 35 turn helix.
>	>
>	>I concluded that you really want a small dish on  for 2.4 GHz
>	>However, if you have
>	>a really fine converter with an EME quality preamp, it should be
>possible to
>	>use a helix.
>	>--Zack W1VT
>	----
>	Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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