Spring 2006 Issue

ANTENNAS

GPS Antenna Project

 By Kent Britain, WA5VJB

 

 

 

Photo A. Ceramic and homebrew GPS receivers.

 

There have been a couple of requests for a GPS (global positioning system) antenna construction project, but I am not sure how much it will help many of you. The problem is that those expensive little external antennas for your GPS receiver contain a high-gain, low-noise preamp. This preamp has been designed to work with the control voltage your GPS receiver sends back up the coax. I am unaware of any universal design for GPS antennas with integral preamps.

Shown in photo A is the ceramic element from an external GPS patch antenna. The dielectric constant of the ceramic material greatly reduces the size of the antenna. The trimmed corners create an imbalance in the patch, which gives you circular polarization. If you donít trim off enough from the corners, the polarization will be more vertical than horizontal. If you trim off too much, the polarization will be more horizontal.

For our prototype, the dimensions for the patch were tweaked on a network analyzer and the corners were trimmed for circular polarization on the antenna range (figure 1). Since I am using air dielectric, the antenna is much larger than the ceramic version.

Normally, we would have a much bigger ground plane under the patch. However, for GPS most of the birds are near the horizon, so we donít want the higher gain of a large ground plane. Again, the corners were trimmed to make the patch circularly polarized.

Construction

I used double-sided .031-inch PC board, but sheet brass or sheet tin would work just as well. You can use aluminum, or most any sheet metal for the base, but itís a lot easier if the patch is made out of something you can solder, too.

The very center of the patch is a null point. I used a 4-40 screw as the patch support, but if for some reason you need the patch to be electrically isolated, a plastic screw could be used, or the patch just could be supported on a small block of plastic. Again, the very center is a voltage null point, like the center of a Yagi antenna element.

The corners are cut for receiving GPS signals. If you are building a GPS simulator and want to transmit to a GPS receiver, you need to reverse the circular polarization. In this case, just trim back the opposite corners off the patch .6 inch.
 

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