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Subject: Re: [time-nuts] GPS Antenna Feed Line Decision

kb8tq@n1k.org said:

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?

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.

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

--
These are my opinions.  I hate spam.

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.

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.

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.

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

Hi

It’s a cascaded noise figure problem rather than an attenuation problem. Given
the noise figures involved, once you get to about 10 db of gain, the impact on the
overall noise figure is negligible.

If your antenna location is “bad” your sky view is likely cluttered up with terrestrial
“stuff”. The clear sky is “cold” and terrestrial “stuff” is hot relatively speaking. The
higher the noise temperature, the less your noise figure matters.

Mostly what you will measure on antenna A and antenna B is the effect of
local multipath. Yes, if you have two “ideal” antenna locations to compare
that will be a bit less than with normal ones. Not many of us have clear view
30 M tall concrete towers to mount our antennas on.

At some point you simply have to dig into the theory and the math. Measure the
components on a network analyzer / noise figure meter / whatever. Run the numbers
and see what you get.

Somewhat more to the point of Time Nuts. Your best timing solution comes from the
sat’s that are straight overhead. Those are the ones with the strongest signals and
the least impact of all these noise figure issues (and a bunch of other stuff). The close
to the horizon sat’s are  what help with the X/Y part of a survey solution …..

Bob