Near-Field to Far-Field Transformation in Antenna Measurements
The near-field technique is the most widely used method of antenna measurement.
An accurate reconstruction of the far-field radiation pattern is made using near-field to far-field transformation techniques from the antenna measurements made at the near-field region.
Planar scanning is the most widely used near-field scanning technique in antenna measurements.
The anechoic chamber is the most popular free space available for antenna measurements
Antennas of all different types—including conformal antennas in mobile phones, horn antennas in radio links, and dielectric resonator antennas in broadcasting systems—are employed in modern electronics systems. Antenna measurement verification for different frequency bands and radiation patterns is critical to increase effectiveness as well as the performance of communication systems.
Antenna measurement techniques use near-field and far-field systems. The near-field antenna measurement technique is the most widely used method of antenna measurements. Near-field to far-field transformations are used to obtain accurate far-field measurements using near-field techniques.
The Ideal Conditions for Antenna Measurement
In antenna theory, testing the antenna is a fundamental task. This testing is important to verify whether there are nearly ideal values for the basic antenna parameters.
Basic antenna parameters, which are commonly measured in antenna testing, include:
- Radiation pattern
- Antenna efficiency
- Antenna gain
- VSWR or impedance
Atmospheric conditions are definitely not ideal for testing antennas; surrounding objects and the ground reflect electromagnetic waves. Free space ranges are the ideal location for antenna measurements, particularly the anechoic chamber, as it simulates free-space propagation by suppressing reflected waves from surrounding objects and the ground. However, far-field distance requirements are difficult to meet in the anechoic chamber.
Near-Field to Far-Field Transformation
The anechoic chamber is not sufficient for testing antennas of large or medium electrical dimensions with far-field distance measurements. To test far-field distance measurements, near-field measurements are carried out first. Next comes the near-field to far-field transformation to characterize the far-field measurement.
The near-field to far-field transformation is typically used in the characterization of antennas or antenna arrays for space applications and radar sensing. In space and radar applications, antenna measurements are taken at the near-field region, and an accurate reconstruction of the far-field radiation pattern is made using near-field to far-field transformation techniques.
The reconstruction of far-field radiation patterns using near-field scanning and near-field to far-field transformation is the most accurate alternative available for conventional, direct far-field measurements. Near-field scanning is a good choice when polarization and pattern measurements are required.
The near-field data acquired is converted to far-field radiation patterns using modal expansion of the spatial field distribution. A suitable modal expansion is selected according to the antenna under test. The modal expansion of the field distribution is defined by the complete set of solutions of the vector wave equation in the outer region of the antenna measurements.
The near-field scanning surface is completely dependent on the choice of modal expansion selected. Electromagnetic waves are considered either planar, cylindrical or spherical for near-field antenna measurements. The modal coefficient is determined using the properties of the modes in the near-field scanning surface. Knowing the modal expansion coefficients leads to the reconstruction of far-field antenna measurements.
Planar Near-Field Measurements
As we discussed above, near-field measurement techniques can be planar, cylindrical, or spherical. As the aperture distribution of most antennas is larger than the antenna projected area, planar scanning is the most widely used near-field scanning technique in antenna measurements.
In planar near-field scanning, the planar wave spectrum representation of the fields is utilized for near-field measurements. A relationship between the far-field pattern and the plane wave spectrum of the antenna is deduced for computation of the radiation pattern of the antenna under test at any point.
Near-field to far-field transformation is the most viable indirect method of far-field antenna measurement. Cadence software offers tools to simulate the near-field distribution of antennas, which aids engineers in far-field pattern measurements.
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