An Inexpensive External GPS Antenna (Part 1)

If you operate APRS or just need an external antenna for your GPS receiver, here’s one that is easy to build yet offers surprisingly good performance in a compact size. Best of all, it uses commonly available components and materials.

This antenna design is based on a classic turnstile
configuration (for circular polarization)—two dipoles are placed on the same plane but rotated 90° from each other. These dipoles are then spaced ¼ wavelength above a ground plane. A ¼ wavelength “parallelplate” transmission line (printed circuitboard material) serves as the connection method and mounting post for the dipoles.

Start with the base plate. Cut a 4-inch diameter circle out of thin hobby tin or brass. (It happens that the inside diameter of the container lid is 4 inches, approximately the same width as the hobby tin/brass sheet.) Mark the exact center of the base plate. This is where the parallelplate transmission line assembly is attached (see Figure 1). Cut two 4-inch lengths of #14 solid copper or brass wire and bend each in the exact center at 90°. Make the radius of the bend as small as possible. Set these aside, they will be soldered to the parallel-plate section later. Select an 8-foot length of RG-58/U, RG-174 or RG-188 coax. Attach a male BNC connector to one end (or whatever compatible connector is used on your particular GPS receiver). I used a solderless connector but removed the screw and then soldered the center conductor directly into the screw hole. If your GPS unit has a BNC antenna connection, you can use an Ethernet coax cable found at most computer stores. Just make sure they are 50 Ω. They’ll already have the BNC connectors crimped on each end.

Just cut in the center, trim to length and you’ll have enough
for two antennas. The GPS frequency is 1.57542 GHz so the longer the coax, the greater the loss. Use no more than 8 feet—less if you don’t need the length.

To make the parallel-plate transmission line, cut two 2-inch lengths of single-sided printed circuit board material that are 0.250-inch wide. Make sure it is glass-epoxy (FR-4 or G10 type material) and that it is 0.062-inch (1/16 inch) thick.

On one of the PCB strips, cut the copper foil with a sharp hobby knife or Dremel tool, as shown in Figure 1. This will be the “active” section of the parallel- plate where the other non-modified strip will be the “ground” side, as shown in Figure 2. The 45° cut on the active side is known as a “microwave turn” which allows the signal to effectively turn 90° to the coax. Glue the two strips together (copper outside) and set aside to dry.

I’ve found it easier to cut the PCB strips a bit wide and glue them together first. Then I just file both edges to the correct dimensions. A light sanding with #600 sandpaper finishes off the edges and removes any burrs.

Double-sided 0.125-inch thick PCB material could be used but can be difficult to obtain for the average hobbyist. Conversely, by using a single 0.063-inch thick double-sided material we would be working with a rather small and fragile structure (half the thickness equates to roughly half the width). This might not hold up during handling and operation. By using the two sections glued together, we’ve solved the problem by creating our own 0.125-inch thick material.

Solder the transmission line section to the base plate keeping it as square and plumb as possible. Drill or melt a hole in the plastic container the same diameter as the coax. Feed the end of the coax through the hole and attach the coax to the transmission line active side as shown in Figure 4.

Measure 1.78 inches up from the base end of the parallel-plate
section and scribe a line in the copper foil. Solder one of the #14 wires to the ground side of the parallel- plate section. Position as shown in Figure 4. Do the same with the active side—you may need a helping third hand as it’s difficult to hold the soldering iron, antenna and position the wires all at the same time.

Measure each leg of the horizontal wires and trim to 1.51 inches from the center junctions. Next, trim both the 45° wires to 1.82 inches from the center junction. If all went well, you should have approximately ½ inch between the tips of the 45° wires and the base. If not, carefully resolder or bend the wires to this dimension.

Using a fine saw or a Dremel tool, remove the excess length of the transmission line just above the wire junctions. Sand the exposed junction to remove any burrs and check for a short circuit.

Note that we’ve purposely kept the transmission line section length long, until after construction. The thin copper foil tends to separate from the glass epoxy during heavy duty soldering. The longer length acts as a heatsink to preserve the bond between the copper foil and the glass-epoxy base.

To Be Continue...

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