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| 58 Basics of Antenna Testing RF antennas often undergo rigorous testing, as these transducers are a key element in the behavior of RF systems that capture or radiate signals and are subject to regulatory and standards certification. Antenna testing involves transmitting or receiving signals between the antenna under test (AUT) and one or more reference antennas. In the simplest type of antenna testing, a single highly directional test antenna is placed in the desired field region of the AUT. Engineers often conduct this test in an open-air test range or in an RF shielding chamber to reduce external interference. More sophisticated antenna testing uses an array of antenna probes, often in the radiative near-field region, and a test apparatus that can distinguish the signals from each probe (Figure 4). The signal energy captured from the probes is then digitized and mathematically processed to extract the antenna behavior, including gain, directivity, efficiency, and antenna pattern. There are also dedicated antenna test chamber designs with positioners that control a test antenna to capture the AUT's response at different azimuth and elevation positions. Anechoic- style chambers are often used for antenna testing to minimize internal reflections while providing a high level of RF shielding from external interference. Because an antenna's field region depends on the wavelength of the frequencies at which the antenna is tested, testing low-frequency antennas requires larger separation distances between the test antenna and the AUT, as well as between the antennas and the chamber walls, if a chamber is used. Antennas being tested over a wideband operation often need to be tested in larger chambers to accommodate the lowest frequency being tested. RF Test Equipment RF test equipment comprises hardware and software systems designed to generate, capture, and process the signals to and from RF SUTs. Common, general-purpose RF test equipment used throughout the Figure 4: The NSI-300V-30x15 planar near-field scanner sits in the 60-foot anechoic chamber at the NASA JPL MESA facility. This system captures near- field antenna data using an array of probes and processes it to extract antenna characteristics. (Source: NASA)

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