W1GE PATCH ANTENNA FOR GPS


Harold R. Ward
23 Hilltop Dr.
Bedford, MA 01730

E-Mail: 71210.1755@compuserve.com



Excerpt from QST Magazine, Oct. '95, p. 45


Construction

        As described above, we have chosen to build the antenna using
aluminum sheet and an air dielectric with nylon bolts and washers to
support the patch. Sheet aluminum can be obtained from most hobby or
hardware stores. The mounting hardware used was supplied by Midwest
Fasteners Corp. which is stocked by ACE and other hardware stores.
The remainder of this section describes the procedure the author used
to make the antenna.    (to see diagram, click below for GIF graphic)

        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 by 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 (see Fig. 4).

        The next step is to attach the coaxial cable. A 10 ft. section
of RG-58/U will satisfy most applications. I purchased a 12' cable
from Radio Shack with connectors at the ends. After cutting off 2'
and stripping the insulation and braid from the cut end, the cable was
ready to be attached to the antenna. 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 was supplied
by Bishop (4) and has a capacitance of 143 pF or 0.7 Ohms rectance 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.

        The final step in the assembly is to attach four small magnets
to the underside of the ground plane to hold the patch to the roof of
the car. The magnet should be slightly thicker than the diameter of
the coaxial cable to allow the cable to run between the ground plane
and the roof of the car. Attach the magnets by first roughening the
aluminum surface then fastening them to the ground plane with hot-melt
glue. Then coat the magnets with bath-tub sealer to prevent them from
rusting or scratching the car roof.

Notes: I obtained the copper sheets from a local electronic supply house.
(4) E-Z CIRCUIT, Cut 'N' Peel copper sheets
    5" x 6" sheet, reorder # EZ7251
    By Bishop Graphics, Inc.
       One River Road, Leeds, MA 01053
       800-628-1910, FAX 800-762-7918