[time-nuts] GNSS Antennas (was: Rooftop antenna and splitter)

[Attila Kinali]

On Sun, 27 Jan 2019 14:24:52 +0000
Bill Slade <slade_bill at hotmail.com> wrote:

> The very best GNSS antennas tend to be based on suspended patch antenna 
> (air-dielectric) structures because they give the best 
> bandwidth/radiation efficiency (and hence, noise temperature) 
> performance.

While it is true that air-patch antennas give good bandwidth
and radiation efficiency, they are not really ideal antennas
for GNSS applications. The main drawback is their quite high Q,
which results in high group delay variation. Another annoyance
is that patch antennas form wave guides between the radiating
element at the top and the ground plane at the bottom. This leads
to radiation out at the sides and thus increased side and backlobes,
which in turn limit both multipath surpression and the axial ratio
(ie the ratio between RHCP and LHCP radiation)

The cheap patch antennas also employ only a single feed-point, which,
due its asymmetry, leads to non-uniform radiation and phase patterns
(aka movements of the phase center). The "good" patch antennas thus
employ a four point feed, but this makes the whole antenna quite a
bit more expensive, as a 0°/90°/180°/270° phase spliter/hybrid is
needed. Of course, any such circuit is rather difficult to make
wide-band and thus becomes the bandwidth limiting element.

Of the common geodetic class antennas I am aware of, only the
Trimble Zephyr is a patch antenna[1], but with an rather complex
n-point feed. The Novatel PinWheel[2,3] is a variation of the
archimedian spiral antenna. Most of the single frequency geodetic
GPS antennas are, AFAIK, crossed dipoles, often combined with
a electromagnetic bandgap (e.g. choke rings) to increase impedance
towards low and negative elevations. I have seen bow-tie antennas
as well, but I do not know how common these are.

The jury is still out on what the best antenna structure for geodetic
work is, but it seems that it is more an issue of how much money
someone spends on optimizing the non-ideal behaviour than a fundamental
issue of the structure itself.

For more information on GNSS antennas, I recommend the book
by Rao et al. [4]. It's quite expensive if you buy it normally,
but Artech has some sale every half year. If you wait for the
right one you can get the book for 30-50% less.


			Attila Kinali

[1] "The Design and Performance of the Zephyr Geodetic Antenna",
by Krantz, Riley, Large, 2001
https://kb.unavco.org/kb/file.php?id=135

[2] "A Novel GPS Survey Antenna", by Waldemar Kunysz, 2000
http://meridware.com.tw/NOVATEL/Documents/Papers/gps600antenna.pdf

[3] "High Performance GPS Pinwheel Antenna", by Waldemar Kunysz, 2000
http://webone.novatel.ca/assets/Documents/Papers/gps_pinwheel_ant.pdf

[4] "GPS/GNSS Antennas", by Rao, Kunysz, Fante, McDonald, 2013

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