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Re: Question "nesting Helix antennas"



>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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