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General Explanation
GPS signals come in at ± 1500MHz, the wavelength is ±190mm.
I had a small h/h GPS that did not work satisfactorily when placed on the dash
of my Series 3 - so I made an external antenna for it.
Basic
system drawing picture
The receive antenna is an alluminium "patch"
antenna about 130mmx120mm. .
It has a smaller element about 80x90mm mounted 8mm above this. See the
explanation below
The coax from the antenna then feeds into a wide band (0-2010 MHz) satellite
in-line amplifier with a slope gain of up to 26dB. This amp costs about R50.00
and is available from any shop that dabbles in TV/sat etc. The amp is small
-about two match boxes in line type size and sits about 200mm from the rx
antenna. The coax then goes into the Landy and terminates in a simple loop of
wire-1 wavelength long. This effectively "retransmits" the GPS signal
in the vehicle cab. The h/h GPS lies on top of this loop, which sits on my cubby
box/armrest. I get brilliant reception and the system does not tie up the h/h
GPS i.e.. I can just pick it up and go walkabout.
Bruce Molzen E-mail: molzenba@sabc.co.za
W1GE Patch Antenna for GPS
Patch Antenna Picture
Harold R. Ward E-Mail: hrward@ultranet.com
Excerpt from QST Magazine, Oct. '95, p. 45
Construction
We have chosen to build the antenna using aluminum sheet and an air dielectric with nylon bolts and washers to support the patch.
The final dimensions of the patch and its ground plane are shown in the diagram. The critical dimensions are those of the patch and the location of the feed point. The dimensions of the ground plane are much less critical. Separation of the patch from the ground plane is 5 mm (0.197 in) and is determined by the three No. 8 nylon washers. The sheet aluminum should be at least 1.27 mm (0.05 in) thick to provide sufficient mechanical strength but the exact thickness is not critical. The bandwidth of the two modes of the patch are 8 and 10% respectively which means the critical dimensions of the patch should
be held to within about 3%. Having determined the design dimensions of the patch, we will now turn our attention to its construction.
The first step is to cut the aluminum sheet metal pieces needed for the patch and the ground plane. One means of controlling the dimensions of the pieces is to scribe the exact dimensions on the aluminum surface, then cut the piece somewhat larger than the necessary dimensions and grind the edges to obtain the final size.
After the aluminum is cut to size, clamp the patch so that it is centered on the ground plane and drill the four 4.76 mm (3/16")corner holes for the #8 nylon bolts as well as the small 1.59 mm(1/16") hole in the patch for the center conductor of the coax at the feed point. Then separate the aluminum pieces and enlarge the ground plane hole at the feed point to 4.59 mm (3/16") diameter (for RG58/U coax). At this stage round the corners of the patch and the
ground plane to a radius of about 4 mm ().157") and then bolt the patch to the ground plane.
The next step is to attach the coaxial cable. A 10 ft. section of RG-58/U will satisfy most applications.
Since soldering to aluminum is difficult I attached a 1" square of copper tape to both the top of the patch and the bottom of the ground plane centered at the feed points where the cable should be soldered. The tape that I used has a capacitance of 143 pF or 0.7 Ohms reactance at 1575.42 MHz. A DC connection to the aluminum is not necessary because the capacitive reactance is small enough to be negligible. An alternative to using copper tape to make the solder connections is to use copper or brass rather than aluminum sheet metal.
TECHNICAL DETAILS, build
your own Amplifier and power supply etc
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