Re: [amsat-bb] Seeking info on stacked helices

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

>From: "Bill Magnusson" <williamb@netc.net.au>
>>I would like to talk to those who have stacked helices for 2.4GHz.  I have
>>one 20T helix and I want to improve things. How difficult is it?
>>How is it done?  Is there much to be gained, besides 3DB?
>
>Hi Allen,
>
>There was a  good article in one of the ARRL publications ... the Antenna
Handbook, I
>think. It described a method of stacking 4 helix antennas against a common
screen
>reflector and used tapered lines running in to a central "N" connector to
achieve the feed
>and matching section. Very simple looking and easy to duplicate. I don't
have the article
>here but I'm sure someone will give you a detailed reference. On 2.4 GHz
you could hold it
>one hand ... a definite possibility for S-band. I built one several years
ago for 70cm and
>it worked well on ao-10.
>
>Bill ... vk3jt
>Milawa
>Australia

Bill and Allen,

That design is right from John Kraus' textbook "Antennas".  Dr Kraus
decribes his development of the helix antenna in chapter-13 of his book
"Big Ear".  In chapter-14 there is a picture of his 96 helix array.  

Four will give more of a measurable improvement over one helix {6 dB
theoretical...~5 dB actual due to losses}.  But John Stephensen is correct
that a dish will give you a cleaner pattern and is much simpler to build
for more gain.  {When I upgraded from an 18-inch diameter dish to a 33-inch
dish I improved my gain about 6 dB} 

The tapered lines work as micro-strip transmission lines.  They taper away
from the common reflector plate to connect at a central connector.
Probably use 1/4 inch wide brass strips spaced about 1/8 inch at the helix
and 3/16 inch at the connector.  That should transform your 140/4 ohm
impedance to 50 ohms.  If you can make the helixes movable you could vary
the spacing to obtain a best match.

But even with four 20-turn helical antennas your gain will be low for good
receiving of AO-40.  I would guess a single 18 inch dish would equal them
[~19 dBi gain].

Ed     

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