[time-nuts] GPS backup for the stationary time and frequencyuser

Jim Lux jimlux at earthlink.net
Fri Oct 8 12:52:28 UTC 2010


The other thing is that something like a quad helix or patch doesn't have the same cross-pol over the hemisphere.  It could be real good in one direction and not so good in others.  Just like isotropic antennas, you can't physically realize the same cp in all directions (cf hairy ball theorem)

On Oct 8, 2010, at 1:48 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> On 10/08/2010 03:35 AM, jmfranke wrote:
>> When I said the feed would work, I was meaning it would work if LHC.
>> The illustrations and text imply you could just place a normal GPS
>> receiver at the feed location, but the polarization would be wrong.
> 
> Which was what I reacted on...
> 
> I am by no means a practical antenna expert, and the EM-theory is a bit fuzzy on the edges, but I do distinctly recall that signal is RHC and reflections becomes LHC so an antenna with RHC orientation will provide some first-degree damping of the LHC reflections. For this antenna setup the intended RHC signal is reflected and should become LHC... just as the interference... so it relies on the antenna gain of the dish to out-perform the other reflections for the half-space receiver that a normal GPS antenna is. The choke ring for a dish head has a distinct different pattern (forming an inner cone rather than flat space).
> 
> So, a normal antenna would kind of work since the antenna gain would overcome the poor LHC supression of a simple RHC antenna... yay.
> 
> If an LHC antenna was used instead... now we are talking.
> 
> Cheers,
> Magnus
> 
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