Re: [gpsd-users] Fw: ✘Grandpa got a new hammer

[nick]

Gary

I was scrapping some old hard disks yesterday and thought about your 
HDD platter experiments.  Here's some comments.

That looks like an active patch antenna in your picture.

The amplifiers in those antennas usually have around 25dB gain and a 2dB 
noise figure.

With that much gain the overall noise figure of the system is determined 
by the noise figure of the amplifier.

So the 4 way splitter does not measurably degrade the overall SNR.

Adding an extra amplifier between an active antenna and the splitter 
cannot improve the overall SNR.

Adding an HDD platter ground plane will raise the antenna 'gain' 
(directivity) which will increase the SNR, but degrade the axial ratio 
which will reduce the SNR.

The platter I have here is 95mm in diameter with a 25mm hole in the middle.

www.u-blox.com/images/downloads/Product_Docs/GPS_Antennas_ApplicationNote%28GPS-X-08014%29.pdf

provides useful information for estimating the performance of patch 
antennas over square ground planes, but not round ones with a hole in 
the middle.

My guess is that you might get 1 or 2 dB of overall improvement in the SNR.

But even this might be significant in terms of number of satellites 
tracked and DOP.

I recently replaced my roof mounted patch antenna with about 150mm x 
150mm ground plane with an AMEC ANT-21 antenna which appears to have a 
ground plane about 90mm diameter.  Although the C/No numbers have not 
gone up much, there is a big difference in the number of satellites 
tracked and the availability of a 3D fix for time synchronisation purposes.

I suspect that an amplified axial mode helix might give the best results 
 in static applications since you don't have to trade off directivity 
against axial ratio.  I plan to build one some day.

http://www.gpscreations.com/NewFiles/GPS%20Helical%20Antenna%20Design.pdf

This is also interesting

https://diydrones.com/profiles/blogs/u-blox-m8n-ground-planes-antennas-and-positional-accuracy

Nick

On 07/09/18 23:14, Gary E. Miller wrote:
> Yo All!
>
> And everything looks like a nail.
>
> TL;DR: don't fear the GNSS antenna splitter.
>
> For years I have been comparing GPS side-by-side, each with its
> own antenna.  This led to complaints that I was a likely measuring
> differences in the antennas instead of differences between GPS.
>
> The classic solution to the problem is to use one antenna, feed that
> antenna into a power splitter, each output of the power splitter feeding
> a single GPS.
>
> To keep the antenna DC power from the GPS's from fighting, insert a DC
> block on the line to all but one GPS.
>
> A two way splitter cuts the signal power in half.  A four way splitter
> cuts that into a fourth.  So some people think they need to add an
> amplifier to regain that lost power.
>
> Not required, maybe even harmful.  I have tried a few cheap chinese
> GHz LNA, with bad results.
>
> The theory is that the GPS cares about the Signal To Noise Ratio (SNR)
> of the signal from the antenna, not the absolute power.  The splitter
> should add very little to the noise and thus the SNR should be little
> affected.
>
> An amplifier can increase the power, but also adds to the SNR.
>
> In practice, just a splitter has seemed to work for me, but I lacked
> hard numbers.  With the new gpsprof polar plots I have some hard
> numbers.
>
> With the new gpsprof polar plot statistics I can finally measure the
> difference a splitter makes.  After running alternating tests, with and
> without, a 4-way splitter, the results are very close.  The differences
> between with/without are smaller than the short term jitter in the SNRs.
> Averaging by eyeball there is maybe a loss of one in SNR.
>
> By placing an HDD platter under the antenna I get back more than the
> splitter lost.
>
> See attached.  Notice the 50 loads and dc blocks on the 3 unused
> splitter outputs.
>
> RGDS
> GARY
> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
>         address@hidden  Tel:+1 541 382 8588
>
>             Veritas liberabit vos. -- Quid est veritas?
>      "If you can’t measure it, you can’t improve it." - Lord Kelvin