The Patch Antenna
This is the antenna I made once.
A patch antenna consists mainly of a square conductor mounted over a groundplane.
The dimensions of the square are roughly 1/2 wavelength.
The insulating space between the two planes may be air. The operating frequency of the GPS
satellites is 1575 MHz. So half a wavelength is about 300000/1575/2 = 95 mm. Such an antenna
'looks' up, and is not very sensitive to the satellites on the horizon. But we are mainly
interested in satellites above us. Some additional gain can be obtained by
making the antenna not square, but shortening one side, and making the other side somewhat longer.
In this way the antenna matches the circular polarisation of the satellites. Also the position
of the feedpoint is important. It should present a 50 Ohm impedance. The exact calculations
are much more complicated then those of a dipole. I found a sketch of such an antenna in an
amateur radio magazine called QST (October 1995).
Why are the commercial available antennas so much smaller?
Readily available patch antennas, like the Trimble and Lowe, are nearly three times smaller,
then this design.
This is made possible, because the antenna elements are not separated by air, but some ceramic material
with a much higher dielectrical constant. All dimensions are much more critical.
Some constructional details.
This antenna is made of 'printed circuit' material. Use the one sided variety. In the
4 corners you need non-conductive spacers. I made these by cutting off the nylon(?) shaft
of an old potentiometer. Also the bolts and nuts are made of nylon.
(the V-shaped marking is just a felt-tip scratch for orientation)
All measurements in mm.
It turned out that
the antenna had even less gain than the (helical) antenna supplied with
the GPS III. But that is not surprising. So a small rf amplifier was constructed.
This small (about 2x1,3 mm!) monolithic amplifier produces a gain of about 13 dB. That is nice,
but not as much as the commercial antennas, which claim 23 dB.
The amplifier can be mounted in line with the coaxial cable. The power supply, out of the GPS, is
used to feed the amplifier via the inner conductor.
This is not a project for a beginner. The SMD components are that small,
you need special equipmentto handle and solder them.
But now I receive more satellites then on the standard antenna.
I have to mention one big disadvantage of this home construction, it is not weatherproof.
(so after all, I ordered a low profile Trimble antenna, with magnet mount)
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