[time-nuts] Antenna question about RHCP/LHCP I'm sure a time-nut can answer

David Kirkby david.kirkby at onetel.net
Tue Jun 5 07:28:04 UTC 2012


On 5 June 2012 01:12, Dave Martindale <dave.martindale at gmail.com> wrote:
> I don't think that's correct.

This is a funny topic. No matter where see it discussed, there are
people with different views on it. I looked on the edaforum

http://www.edaboard.com/forum26.html

and found a thread (can't find it now unfortunatey), where someone was
adament they needed to be one way (I forget whether both RHCP or
RHCP+LHCP), and someone else was adament a colleague nearly lost his
job after making that mistake. I think there was about a 50:50 mix of
views on the topic

I think I might have to make 3 axial mode helical antennas and test
this myself. If I wind two in one way, and one in another, it should
be possible to determine if the strongest signal is received with them
wound the same way, or wound the opposite way.

I don't know much about helix antenna design, but I know there are two
modes - "normal" and "axial". I have some software able to design
either, as well as some complex types. Sticking a frequency of 300 MHz
(so lambda is a convenient 1 m), I get:

Normal mode: helix diamater 0.05 lambda, helix spacing 0.05 lambda.
1.818 turns (no option to set gain).
Axial mode: helix diameter 0.318 lambda, helix spacing 0.222 lambda,
3.341 turns for 10 dBi gain, 6.667 for 13 dBi, 13.302 turns for 16 dBi
and 26.541 for 19 dBi and 33.413 turns for 20 dBi.

So the diameter of the helix does not tell you much on its own, as
there are two different types, one of which has a very different
diameter to the other.

Note the axial mode helix has a diameter of 0.318 lambda, so a
circumference of Pi*0.318 = 1.0 lambda. I think there is some story
that when Krauss invented this antenna, he made the first one with a
circumference of 1 wavelength, and more by luck than anything else,
got it right first time.

(Note, this software is designed to create a model for a 3D
electromagnetic simulator, so the results are not perfect, and one is
expected to tweak the design using the 3D electromagetic simulator).

Dave



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