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

From: Gary E. Miller
Subject: [gpsd-users] Fw: ✘Grandpa got a new hammer
Date: Fri, 7 Sep 2018 15:14:58 -0700

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

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

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.

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

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