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Re: Question "nesting Helix antennas" (was ) JavaScript Axial-Mode Helix Calculator

The nesting idea has a certain appeal, especially for frequencies that are 2x or 3x apart, but the size of groundplane reflector become a problem.  The shape of the main lobe is greatly affected by the surface area of the reflector.  Modeling in NEC4Win95 (virtual wires version), shows reducing the diameter of the reflector below 0.75 wl greatly reduces the gain of the main lobe and increasing the reflector diameter beyond 1.25 wl decreases the main lobe and greatly increases the side lobes at +/- 45 degrees.  I can only envision this working well if you are willing to accept significantly reduced performance on one of the helices.  Alternately, this may be a good application for the Kraus tuned ring-reflectors in lieu of a conventional groundplane reflector, as separate reflectors could be installed concentrically for each helix.  Lots of room for experimentation here...
Jerry, K5OE

Standard Time, "Frederick M. Spinner" <fspinner@hotmail.com> writes:

<< I suspect there will be interaction.  You would either need to try it, or 
model it in NEC.  Also, I would be positive that you would not
get away with transmitting into one nested turn without blowing the 
preamps/converters of another turn.  But you probably could pull off a
multiband RX antenna.  The really good thing about AO-40 and microwaves is 
that the 23 cm antenna is the largest.. the skeleton
for the helix I'm building for that band (15 turns, about 15 dBic gain) is 
that the dimensions are only 35" long, 3.25" diameter of the turns, and 9.25 
in radius of the reflector.  Couple that with a
13cm helix 20 turns, it will be nearly half that size-- reflector 5" 
diameter, 25" length, 1.75" diameter.

Above those bands, IMHO, is dish territory.  (and I bet I'll end up using a 
dish for mode-s.  Dishes are wonderful.. even less picky than helices)  18" 
RCA dishes (even two if necessary) are DIRT cheap now.  Feeds can be horns, 
dual band horns, helices, dipole splash feeds, etc.  So there is many 
different configurations that can be used.  One of the coolest ideas is to 
rotate a "turret" at the feedpoint of an offset fed dish and have feeds 
rotate in and out.. only one is active at a time-- but just rotate, and 
bands change.
This really can only work if the dish is offset fed, though.

So don't be discouraged, there are going to be MANY ways of skinning this 
cat.  Its going to be soooo cool...

Fred W0FMS

>From: Joe Leikhim <jleikhim@nettally.com>
>To: "Frederick M. Spinner" <fspinner@hotmail.com>
>CC: amsat-bb@AMSAT.Org
>Subject: Question "nesting Helix antennas" (was )[amsat-bb] JavaScript 
>Axial-Mode Helix Calculator
>Date: Sun, 10 Dec 2000 16:34:13 -0500
>If I wanted to build a helix for each of the bands:13cm, 23cm and 70cm. 
>Could I "nest" them
>together and use a single groundplane for 70cm?, Would there be any 
>negative interaction? I
>would use a separate feed for each.
>Joe Leikhim K4SAT
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