Antenna measurement

Antenna measurement techniques refers to the art of testing antennas to ensure that the antenna meets specifications. Parameters of antennas commonly of interest are gain, pattern, bandwidth, 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 (3-10 wavelengths typically). This measurement is then transformed to the far-field (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.

Far-Field Range (FF)

The far-field range was the original antenna measurement technique, and consists of a placing the AUT a long distance away from the range antenna. Generally, the far-field distance, d, is considered to be

$d={{2D^{2}} \over {\lambda }}$,

where D is the antenna diameter and ${\lambda }$ is the wavelength of the radio wave. Separating the AUT and range antenna by this distance reduces the phase variation across the AUT small enough to obtain a reasonable antenna pattern.

Near-Field Range (NF)

Planar Near-Field Range

Planar near-field measurements are conducted by scanning a small probe antenna over a planar surface. These measurements are then transformed to the far-field by use of a Fourier Transform. Three basic types of plannar scans exist in near field measurements.

• rectangular planar scanning

The probe moves in the Cartesian coordinate system and its linear movement creates a regular rectangular sampling grid (as shown in Figure ) with a maximum near-field sample spacing of Δx = Δy = λ /2.

File:Cartesian planar grid.jpg

• polar planar scanning

More complicated solution to the rectangular scanning method is the plane polar scanning method.

File:Polar planar grid.jpg

• bi-polar planar scanning

The bi-polar technique is very similar to the plane polar configuration. Bi-polar method ..

File:Bi polar planar grid.jpg

Cylindrical Near-Field Range

Cylindrical near-field ranges measure the electric field on a cylindrical surface close to the AUT. Cylindrical harmonics are used transform these measurements to the far-field.

Spherical Near-Field Range

Spherical near-field ranges measure the electric field on a spherical surface close to the AUT. Spherical harmonics are used transform these measurements to the far-field.

Free-Space Ranges

Compact Range

Compact Range

Elevated Range

Elevated range

External links

• AMTA
• RadioWORKS A radio wave propogation and antenna length calculator
• JSC "Geozondas" Manufacturer of antenna measurement systems in Time Domain