[time-nuts] Choke Ring Design for L1

[Magnus Danielson]

Peter Vince wrote:
> As I understand it, the GPS signals are circularly polarised, and so
> surely reflections will reverse the sense of that polarisation such
> that the antenna will be insensitive to them?

Yes, as a first degree analysis. However, cancellation will not be 100% 
since neither the wave, the reflection and the antenna is completely 
according to that theory.

>  Maybe Warren's simple
> pie dish is working by shielding the antenna from the true MULTI-path
> reflections, and any direct SINGLE reflections it produces are ignored
> due to the polarisation reversal?

It at least provides the function of the ground plane. Ground-plane only 
antennas have been widely used in survey applications.

There are many ways to improve the situation for a patch antenna. A 
ground plane is certainly one of the ways. Recall that wavelength is 
about 19 cm so anything near optimal antenna should be expected to have 
comparable size.

Oh, the choke ring antennas not only suppress reflections from below, 
but also provides spatial nulls towards the horizon, such that low 
elevation reflections is also suppressed. The more nulls, the wider lobe 
and thus lower elevation angle gets can be accepted. For traditional 
choke rings 3 or 4 is used. I have three rings in mine.

Cheers,
Magnus