Bill Janssen wrote:
Magnus Danielson 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
So a dish reflector and a sub reflector and the GPS receiver at the dish
would work? What is that
configuration called? I can't remember at this early hour.
Depends on the relative curvatures and focal points:
Cassegrain if the subreflector convex.
Gregorian if the subreflector is concave parabolic.
Dragonian if the subreflector is concave hyperbolic
IEEE Ant and Prop Magazine a few years back had a series of articles on
designing them all.
All of them can be done offset or coaxial
Any would conceivably work.. It's all about what your pattern looks
like, what sort of efficiency you need, any mechanical constraints, etc.
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