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Re: Re: Question "nesting Helix antennas"

I have done some modelling of concentric helices recently, using NEC2;
in particular, a 13 cm helix inside one for 23 cm.  I found that it
works fairly well provided the 13 cm helix is a bit longer than the 23
cm one, so it sticks out (I found that 12 turns for the L-band helix and
24 turns for the S-band one gave only 1 dB degradation in gain at S-band
(14.4 dBi versus 15.6 dBi for the S-band helix alone; 1 wavelength
circumference & 10 deg pitch).  The impedance was altered some, but no
matter.  The 23 cm helix is hardly affected at all.

 The problem I have with this structure is I expect (but haven't
verified) that there is considerable coupling between helices, possibly
making full duplex operation difficult.  A good diplexing filter would
solve that problem, however, I think.

I am using the old NEC Wires program that Brian Beezley used to sell. 
It came with a wonderful DOS program that generates helix wire files
quickly and easily.

Bob Snyder KD1VV

"Edward R. Cole" wrote:
> >From: Joe Leikhim <jleikhim@nettally.com>
> >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,
> If I understand by the term "nest them", you mean to mount them
> concentrically on the same axis, I would guess you would have considerable
> interaction.  Without modeling such an array I can not say for sure.  To
> give you an example, there are significant issues when stacking antennas
> for different bands in close proximity to each other due to mutual
> coupling.  I don't think this idea will "fly", sorry.
> But the only way to be sure, would be to build each helix separately and
> test them, then mount them together and test again!  I would try a 70 cm/13
> cm "nested" design, first, since there is a bigger size differential.
> Maybe such a dual-band model would work?  {an idea to try would be is
> making the higher freq. helix longer than the other?}
> Probably you will have to separate them on a cross-boom mounted on separate
> axes.
> I have conceptualized {but not yet built} a dual band 13 cm/3 cm dish feed
> for P3D {Rx only}.  It consists of a 13 cm 3 turn helix wound around a 3/4
> inch copper tube axial-support.  The 3/4 inch tube is fashioned to be a 10
> GHz circular waveguide and either fed with a coaxial probe near the
> back-end, or transitioned to rectangular wr-90 w/g.  The 3 cm antenna would
> be linearly polarized.  Both antennas would be separately fed.  I think
> this will work since helix design practises allow the use of a metallic
> support rod if it is well centered.  The problem will be adjusting the dish
> focal point for both bands and getting a suitable dish illumination
> pattern.  I plan to run this design on NEC4WIN  to see what it analyses like.
> So Joe, your 70 cm helix may ignore the smaller helix like a centered
> support, but what will the smaller helix see the large one as?  I don't
> know.  Don't be discouraged from this.  A lot of great ideas sound strange
> at first!
> ED
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