[time-nuts] GPS backup for the stationary time and frequencyuser
brooke95482 at att.net
Fri Oct 8 17:18:18 UTC 2010
I've got a spare Ku band satellite dish and would like to use it for GPS.
In an ideal application the GPS antenna would be mounted in the normal
manner and above it would be a sub-reflector aimed at the Ku dish.
That way the antenna might pickup sats near the horizon directly and
from a narrow part of the sky by means of the dish.
The dish might be aimed at a WAAS GPS sat.
I've heard that you can just use the TV dish with a normal GPS antenna,
and it gas gain even though the polarization is reversed.
> 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.
>> 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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