[time-nuts] GPS backup for the stationary time and frequencyuser
jfor at quik.com
Fri Oct 8 13:27:25 EDT 2010
I've been half following this thread and can't make out the reason for a
less than hemispheric antenna pattern. GPS needs several birds to lock up,
and if you look at a single bird, Dopplar will make the signal useless as
a frequency reference.
> Hi Jim:
> 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.
> Have Fun,
> Brooke Clarke
> jimlux wrote:
>> 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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> Have Fun,
> Brooke Clarke
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