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. See the diagram in graphics file GPSPATCH.BMP or GPSPATCH.TIF. 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