[time-nuts] Pizza anyone?

[J. Forster]

I sent this thread to a friend, and he replied:

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> I bought a couple of pizza pans for the GPS Patch Antenna Improvement
> Program...  preliminary results:
>
> My  15+ year old Magellan patch antenna (actually made by Murata)
> would barely function with the Tbolt.   It was (barely) tracking three
> satellites and going through long periods of holdover.


A patch antenna is intended to be mounted flush against a ground
plane.  If he tried to use it unmounted, it would not have been happy.


> I then mounted it on the bottom of a 16" pizza pan.  It is now
> tracking 8 satellites with no signal dropouts.

OK.


> If you are using a consumer/automotive type patch antenna,  you might
> want to try a pizza pan ground plane under it.   I am doing a 48 hour
> survey run to see how it performs compared to my other antennas.  I
> cannot compare it to itself without the pan since it would never
> complete the survey.
>
> My geodetic grade antenna is a patch type antenna mounted on a 14.5"
> aluminum disk.  14" diameter is a standard pizza pan size.  I am
> going to
> try mounting the TAPR Motorola patch to it and see what happens.  I
> have a
> good 48 survey with that antenna to compare to.

Unless a ground plane is VERY large in comparison with the signal
wavelength (about 8" for GPS L1), the gain _pattern_ of the antenna is
strongly affected by the ground plane.  (14.5" is emphatically NOT
large in comparison with the L1 wavelength.)  Not just the main lobe
of the gain pattern, but also the minor lobes, including the BACK
lobe(s) -- those directed at the ground rather than the sky.  This
strong sensitivity occurs because incident waves excite standing waves
on the ground plane.  Nothing interferes with the flow of electric
charge/current from the upper surface of the ground plane around the
edge of the plane to the lower surface of the ground plane, so the
amplitude of the standing wave on the underside of the plane is
usually as great as the amplitude of the standing wave on the upper
side.  Therefore, the antenna may have nearly as much gain toward the
ground as toward the sky.  If an antenna does not have much less gain
toward the ground than toward the sky, then multipath interference
from ground reflections will strongly perturb the observed group
delays (code-modulation delays, or pseudoranges) and carrier phases of
the signals received from the GPS satellites.

The antenna pattern will also have fairly deep nulls in the upper
(sky) hemisphere.  So it's not a great idea to stick a GPS patch
antenna on just any old sheet of metal.  The ground planes used with
commercial GPS antennas have dimensions deliberately chosen to
minimize, or at least reduce to a tolerable level, the standing waves
that could cause nasty pattern problems.  (In an archive I have
examples of patterns of GPS antennas with inappropriate ground
planes.  I also have computer-generated graphics showing the
distributions of RF current on various ground planes.  Measured gain
patterns, and gain and current patterns derived by computer
simulation, can be found in the open literature; but it'd take me a
while to dig out references.)

The best antennas for accurate position-determination by GPS have
rather large and massive ground planes (e.g., 36" square) with special
treatment of the edge; or the upper surface of the ground plane is
ringed with quarter-wave chokes; or the antenna has no ground plane at
all, and it achieves an upper-hemispheric gain pattern by means of
arraying.  I.e.,  the antenna comprises an array of several identical,
circularly polarized, elementary antennas, or "array elements," spaced
along a vertical axis; and the feedpoints of these elements are
connected together by a manifold "feed," such that the signals
received by different elements are combined with different relative
amplitudes and phases in the input of the receiver.

======

-John

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> Hi all: for the proper dimensions, see some of the sections of W1GHZ's
> microwave antenna handbook to get the appropriate size for the pizza pan
> choke. Mark may have gotten lucky...
> Don