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