Antenna measurement

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, bandwidth, radiation pattern, beamwidth, polarization, and impedance. 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. For a reciprocal antenna, these two patterns are identical. A multitude of antenna pattern measurement techniques have been developed. The first technique developed was the far-field range, where the antenna under test (AUT) is placed in the far-field of a range antenna. Due to the size required to create a far-field range for large antennas, near-field techniques were developed, which allow the measurement of the field on a surface close to the antenna (typically 3 to 10 times its wavelength). This measurement is then predicted to be the same at infinity. A third common method is the compact range, which uses a reflector to create a field near the AUT that looks approximately like a plane-wave. The far-field range was the original antenna measurement technique, and the simplest; it consists of placing the antenna under test (AUT) a long distance away from the instrumentation antenna. Generally, the far-field distance or Fraunhofer distance, is considered to be where is the widest diameter of the antenna in any direction, and is the wavelength of the radio wave. Separating the AUT and the standard receiving antenna by this distance reduces the detectable phase variation across the AUT enough to obtain a reasonably accurate estimate of the antenna pattern in the far distance. The IEEE antenna measurement standard (document i.d. IEEE-Std-149-1979), suggests set-up for measurement and various techniques for both far-field ranges and ground-bounce ranges (discussed below). Electromagnetic near-field scanner Planar near-field measurements are conducted by scanning a small probe antenna over a planar surface.