I have collected measurement results for many different 5.8 GHz FPV antennas. Most of these are commercially available antennas, some are my own designs.
Vector network analyzer results
A vector network analyzer (VNA) is an instrument that sends a signal to a device under test (in this case the antenna) and analyses the amplitude and phase of the returned signals. Most VNAs have two or more ports, but for antenna measurements we only need one port. The amplitude of the returned signal tells us how much power was reflected by the antenna as a result of mismatch - this is called return loss, also known as s11. This number should be as low as possible, ideally below -20 dB (1%). An antenna is considered functional as long as the return loss is less than -10 dB (10%). The VNA measures the return loss at a range of frequencies, which allows us to determine the bandwidth of the antenna (which is generally defined as the frequency range where the return loss is less than -10 dB).
Low return loss is important, but it is certainly not the only thing that matters. Other performance metrics such as a good radiation pattern and axial ratio are just as important. In order to make it easier to estimate the real-world impact of antenna matching, I have also calculated the matching efficiency of each antenna. A return loss of -20 dB corresponds to a matching efficiency of 99%, which means that 99% of the power is accepted by the antenna and 1% is reflected. For most practical purposes, -20 dB and lower can be considered 'perfect'.
Anechoic chamber results
For a few antennas in this list, I have been able to do measurements of the radiation pattern in an anechoic chamber. An anechoic chamber is a room in which the walls, floor and ceiling are covered with RF absorbing foam pyramids. This greatly reduces multipath interference, which is necessary in order to accurately measure the radiation patterns of the antennas. The anechoic chamber used for these tests has a 'noise floor' of -20 dB. This may not sound very impressive, but it is orders of magnitude better than what you would get in a regular room. Still, actual measurements never look as perfect as the simulation results.
There are no comments yet.