"Jerry Martes" wrote in
news:nPEah.15278$Uz.1717@trnddc05:


"SamSvL" wrote in message
...
Walter Maxwell wrote in
:

On Thu, 23 Nov 2006 19:04:22 GMT, "Jerry Martes"
wrote:


"SamSvL" wrote in message
...
What would be a good (free space, 1 GHz) antenna to achieve
circular polarizarion with a nearly spherical coverage?

Thanks,

Sam


Hi Sam

If you want "half spherical" coverage for receiving a circularly
polarized
signal, a turnstile is probably the easiest to build.
If your requirements for sensitivity are more stringent than the
turnstile
provides, a quadrafilar helix is excellant circular polarization.

Jerry

Hi Sam,

FYI, the quadrifilar helix comprises two bifilar helices fed in
phase quadrature. The result is hemispherical radiation in the
radiation sphere. But the hemispherical radiation results from the
combined radiations from each bifilar helix. Consequently, radiation
from a single bifilar helix is spherical, with the polizarization
sense the same throughout the sphere, unlike the radiation from a
turnstile, which radiates one polarization sense above the equator
and the opposite sense below the equator, where the turnstile
elements lie on the equator.

You can find info on the bifilar helix in Chapter 22 of Reflections,
eds 1 and 2. If you don't have a copy of Reflections available,
Chapter 22 is available for downloading from my web page at
www.w2du.com. Click on "Read Chapters from Reflections 2" and then
click on 'Chapter 22, Quadrifilar Antenna'.

Fig 22-5 shows the radiation pattern for the quadrifilar, but the
radiation in the 180° direction is suppressed due to the effect of
the second helix positioned 90° from the first helix, and fed in
quadrature (90°) relative to the first. In the absence of the second
helix the radiation from a single bifilar is spherical.

Fig 22-8 shows a simple method of feeding the bifilar helix--it's
called an 'infinite balun', because the current that would flow on
the outside surface of the coax when feeding a doublet dipole
without a balun is now flowing on the outside surface of the
radiator, which is what we want.

If you're interested in further info on the helices used in
quadrifilars or bifilars, see the report on my R&D experiment on
quadrifilars, in which I measured radiation patterns and driving
point impedances on more than a thousand different configurations of
the quadrifilar. The R&D report is available for downloading on my
web page. Click first on "Read Appendices from Reflections 2", and
then click on 'Appendix 13, RCA R&D Quadrifilar Helix Antenna'.

Hope this helps,

Walt Maxwell, W2DU

Hi Walt,

Thanks for your reaction. And yes, I found your articles already some
time ago. I missed the notes on the bifilar, but will study them
again carefully.

Again, thanks,

Sam


Hi Sam

I dont want to discourage your study of them, but, bifilar has
significant
nulls for CP.

Jerry


Jerry
Hi Jerry,

The radiation pattern is doughnut shaped. You're right !

sam