Ever wonder how wireless signal gets to a specific area? Connections and networks live and breathe by antennas. If an antenna goes down, the wireless network would be in trouble. Thus, it’s extremely important to test antennas! Antenna measurement techniques refers to the testing of antennas to ensure that the antenna meets specifications or simply to characterize it. Typical parameters of antennas are gain, radiation pattern, beamwidth, polarization, and impedance.
How do antennas work? The antenna pattern is the response of the antenna to a plane wave incident from a given direction or the relative power density of the wave transmitted by the antenna in a given direction. When the antennas are reciprocal antenna, the patterns are identical. Many techniques have been created to help with antenna measurement. One of the very early techniques was the far-field range, where the antenna under test (AUT) is placed in the far-field of a range antenna. Because the size needed to test for a far-field range for large antennas was so massive, near-field techniques were developed, that allow the measurement of the field on a surface close to the antenna (typically 3 to 10 times its wavelength). Another common method is the compact range, which uses a reflector to create a field near the AUT that looks somewhat like a plane-wave. [1]
“Testing of real antennas is fundamental to antenna theory. All the antenna theory in the world doesn’t add up to a hill of beans if the antennas under test don’t perform as desired. Antenna Measurements is a science unto itself; as a very good antenna measurer once said to me ‘good antenna measurements don’t just happen.'” [2]
What exactly is being looked for with antenna measurement? The most common and desired measurements are an antenna’s radiation pattern including antenna gain and efficiency, the impedance or VSWR, the bandwidth, and the polarization.
In order to analyze antenna test equipment, let’s first look at the test antenna (often called an Antenna-Under-Test) with a plane wave. This is be approximated by using a source (transmitting) antenna with known radiation pattern and characteristics, in such a way that the fields incident upon the test antenna are approximately plane waves. The required equipment for antenna measurements include:
- A source antenna and transmitter – This antenna will have a known pattern that can be used to illuminate the test antenna
- A receiver system – This determines how much power is received by the test antenna
- A positioning system – This system is used to rotate the test antenna relative to the source antenna, to measure the radiation pattern as a function of angle.
To give you a more complex look, let’s go over some of the necessary testing procedures for antenna measurement:
The Source Antenna should radiate well at the desired test frequency. It must have the appropriate polarization and a suitable beamwidth for the given antenna test range. Source antennas are usually horn antennas, or a dipole antenna with a parabolic reflector.
The Transmitting System should be capable of outputing a stable known power. The output frequency should also be tunable (selectable), and reasonably stable (stable means that the frequency you get from the transmitter is close to the frequency you want).
The Receiving System simply needs to determine how much power is received from the test antenna. This can be done with a simple bolometer, a device for measuring the energy of incident electromagnetic waves. The receiving system can be more complex, with high quality amplifiers for low power measurements and more accurate detection devices.
The Positioning System controls the orientation of the test antenna. Since we want to measure the radiation pattern of the test antenna as a function of angle (typically in spherical coordinates), we need to rotate the test antenna so that the source antenna illuminates the test antenna from different angles. The positioning system is used for this purpose. [2]
There are many ways to test antennas, along with a multitude of equipment to do so. Understanding each facet of antenna measurement is not the easiest thing to do. However, just knowing the importance of antenna testing is the first hurdle.
[1] http://en.wikipedia.org/wiki/Antenna_measurement
[2] http://www.antenna-theory.com/measurements/antenna.php
