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

jmfranke jmfranke at cox.net
Fri Oct 8 17:34:21 UTC 2010


The Doppler is dramatically reduced by looking only at the WAAS bird(s).

John WA4WDL

--------------------------------------------------
From: "J. Forster" <jfor at quik.com>
Sent: Friday, October 08, 2010 1:27 PM
To: <brooke at pacific.net>; "Discussion of precise time and frequency 
measurement" <time-nuts at febo.com>
Subject: Re: [time-nuts] GPS backup for the stationary time and 
frequencyuser

> 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.
>
> Best,
>
> -John
>
> =============
>
>
>> 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
>> http://www.PRC68.com
>>
>>
>> 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.
>>>>>
>>>>> 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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>>>
>>
>> --
>> Have Fun,
>>
>> Brooke Clarke
>> http://www.PRC68.com
>>
>>
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>
>
>
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