Antenna Measurement FAQ
This page shares some of the frequently asked questions about Antenna Measurements.
What are Antenna Measurements
Like any component we need to be able to characterize the performance of an antenna. The ultimate performance criteria are the radiation pattern and magnitude. Does the antenna radiate in the desired pattern (directional, omnidirectional)? Does it radiate with high efficiency? This measurement is performed with a system that can measure the Electromagnetic output of the antenna, usually in combination with a rotation system for the antenna under test.
But in order to perform well, the electrical connection of an antenna to the wireless device needs to be optimally matched. This is measured with a device that measures the electrical impedance of the antenna.
Antenna Measurement with a Vector Network Analyzer (VNA)
A VNA is the device of choice to measure the impedance of an antenna, in order to access the electrical match of the antenna to its feed (usually 50 Ohm), and to devise a matching circuit to optimize this match.
A 2-port VNA can also be used to measure the radiation of an antenna by using a reference antenna on one port and the antenna under test to the second port of the VNA. It is then a matter of pointing the antennas toward each other or rotating the antenna under test to characterize the radiation (pattern) of the antenna under test. Usually two measurement are required to measure both horizontal and vertical polarization of the antenna under test. It is needless to say that this is a very cumbersome procedure and prone to large errors because of the (reflective) measurement environment, and the possibly unknown characteristics of the measurement antenna.
Antenna Measurement with a Spectrum Analyzer
A spectrum analyzer can be used to measure an antenna if the antenna under test is connected to a signal source (a signal generator or a transmitter chip that may be part of the whole device). This measurement also requires a measurement antenna with known characteristics, and is prone to the same problems and errors as an antenna measurement with a VNA.
Antenna Measurement without an Anechoic Chamber
If antenna radiation measurements are done in any normal space the signal that is radiated by the antenna under test is reflected by all kinds of objects like metal desks, walls, computer screens and even by the people in the space. When these reflections reach the measurement antenna they can add to the actual measurement signal or cancel them out. This can lead to measurement errors as large as 10 or 20 dB.
The MegiQ Radiation Measurement System (RMS) implements features and protocols to minimize the sensitivity for reflections and bring down the measurement uncertainty to desirable values, even in fairly reflective environments.
Antenna Measurement in an Anechoic Chamber
Because of the problems and errors with the measurement methods above, antenna measurements are often performed in an Anechoic Chamber. This is a room with the walls covered with absorber material to suppress the reflections against the walls, floor and ceiling. Without reflections the antenna measurements with a VNA or Spectrum Analyzer become stable and predictable.
While an anechoic chamber usually requires a high budget, it is still necessary to use a calibrated measurement antenna, and a rotational device is also highly desired. To measure both horizontal and vertical polarizations the measurement procedure needs to be done twice with different measurement antenna orientations.
Antenna Measurement in the Field (Walker Method)
One way of measuring antenna radiation performance is to determine the ‘range’ of the communication system. This is often done by placing one half of the system (transmitting side) in a fixed place and position and walking around with the other half (receiving side) and determine how far (in the building or in the field around) the communication link stays reliable.
This method is very prone to errors because of many reflection sources (including the ground), variables like traffic, weather, people, animals and the uncertainty in polarization of both sides of the link. It is also very time consuming to test an antenna in this manner.
Antenna Measurement with a Radiation Measuring System (RMS)
MegiQ had developed a standalone antenna measurement system that reduces the error sources to acceptable levels (not unlike certified test labs) even when it is used in fairly reflective environments. The RMS measures both signal polarizations simultaneously to cut the measurement time in half. The system provides a rotation table and a measurement protocol to produce concise and stable measurement results.
By eliminating the need for an anechoic chamber while providing accurate results greatly reduces the cost of performing antenna radiation measurements. Its compact size makes it possible to perform many measurements during the development of a wireless device and thus provide optimal performance of the product.