[time-nuts] How do I compare GPS antennas?

[Mike Cook]

> Le 6 sept. 2017 à 04:23, Hal Murray <hmurray at megapathdsl.net> a écrit :
> 
> Was
>  Subject: Re: [time-nuts] GPS Antenna Feed Line Decision
> 
> kb8tq at n1k.org said:
>> There is pretty much no experiment you could run that would show a
>> difference  between the two. With a normal GPS, the “front end” of the radio
>> is in the antenna. The filtering and RF amplification there determine a lot
>> of things. The cable is just a  chunk of wire in the middle of the system.  
> 
> Does that depend on the antenna (and location) being "good" and both coaxes 
> being good-enough so that the receiver always has a good signal?  
> Alternatively, if the signal is good, you can't tell the difference in a few 
> db of attenuation.
> 
> But suppose the antenna location isn't good.  How can I tell if it is 
> good-enough?  Or how can I compare location A with location B?

  Good location depends on a couple of things.
       How much sky your antenna can see clearly.
       Are there any near/far objects that can create reflections.
  The first is easy with an eyeball.
  The second you can determine by monitoring the satellite map to see if it is reporting signals from satellites that should not be visible.  
  The receivers I have tried are good at ignoring reflections when there is ALSO line of site visibility but they can get confused when there is only a reflected signal.
  In my case, I have learned to do to site surveys and any experimental measurements in the periods when there are no reflections. 

> 
> The best I have been able to come up requires two identical receivers.  You 
> can verify that they are identical, or at least close enough, by running them 
> from a single antenna with a splitter.  I haven't gotten past that.

 I doubt that you need to measure the two locations simultaneously. Using the same receiver at both in succession for comparison should be good enough.

> 
> Assuming you had a not-good antenna, is there any numerical scale that would 
> be useful to describe its goodness?

 The signal quality reported by the graphic utilities in comparison with a « good » one.
 I tried a few and picked those which got the highest quality. 
For SVs that have LOS I get 36-47 whatever the scale is measured in (Motorola use C/No «  carrier to noise density » , while U-blox use Db (SNR)).
These seem to be typical values from some doc I saw on the web. It seems to be good enough. 

> 
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"The power of accurate observation is commonly called cynicism by those who have not got it. »
George Bernard Shaw