Groundplane and multipath

Lost? Use the clue

The main issue we face during signal reception is signal reflections. These reflections can come from buildings, trees, people, and the ground. In our technical language, we call this "multipath." This means that one signal ray comes directly from the satellite to the antenna, while another, slightly longer ray, travels from the satellite to a building and then to the antenna. A third ray may bounce off the ground before reaching the antenna. The antenna receives the sum of all these rays, which significantly reduces the quality of reception.

The strongest reflection comes from the ground below. That's why smart people invented a screen called a groundplane, which prevents reflected signals from the lower hemisphere from entering the antenna. In its simplest form, this can be a CD disc with the antenna placed at the center. In a more complex design, it can be a set of rings called a choke ring.

You can see one choke ring antenna, which weighs about 7.5 kilograms (16.5 pounds) and costs several thousand dollars.

On the other end of the spectrum, there are very cheap groundplanes made from old CD discs. In the photo That Doesn't Work you can see a satellite compass, which measures the vectors from one antenna to another. The silliest mistake here is placing the antennas just a couple of centimeters away from the groundplane, which is the roof of a car. The image is titled "That Doesn't Work." The car roof also acts as a signal reflector. The required accuracy for such a setup is 5mm RMS (Root Mean Square). However, due to the reflections, the signals fluctuated by 1-2 centimeters. Using CD discs as groundplanes immediately improved the quality of the solution. See the photo That Works

There are different opinions about the size of the groundplane for patch antennas. One opinion suggests that the groundplane should be slightly larger than half the wavelength, which is 10 centimeters for L1 and 13 centimeters for L2. Another opinion, backed by charts, can be found in the "GNSS-Antennas_AppNote_UBX" document.

It's important to understand that patch antennas work poorly without a groundplane. Other types of antennas need a groundplane much less.

© Eltehs SIA 2023