Antenna measurement
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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.
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[edit] 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
,
where D is the antenna diameter and λ 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.
[edit] Near-Field Range (NF)
[edit] 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 planar 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.
• polar planar scanning
More complicated solution to the rectangular scanning method is the plane polar scanning method.
• bi-polar planar scanning
The bi-polar technique is very similar to the plane polar configuration. Bi-polar method ..
[edit] Advantages and disadvantages of planar NF scanning.
[edit] 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.
[edit] 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
[edit] Free-Space Ranges
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[edit] Compact Range
Compact Range
[edit] Elevated Range
Elevated range
[edit] Slant Range
[edit] Antenna parameters
[edit] Antenna Radiation pattern
[edit] Gain Measurement Techniques
[edit] Directivity
Antenna directivity is usually measured in dBi, or decibels above isotropic. This number is obtained by measuring the power delivered in the strongest lobe, and comparing it to the total power (as if all power were radiated uniformly in all directions:
dBi = 10 * log10(Directional Power / Total Power)
[edit] Polarization Measurement Techniques
ff
[edit] Impedance Measurement Techniques
[edit] Efficiency
[edit] External links
- AMTA
- RadioWORKS A radio wave propogation and antenna length calculator
- JSC "Geozondas" Manufacturer of antenna measurement systems in Time Domain
- ETS-Lindgren Manufacturer of antenna measurement systems for wireless telecommunication devices
