Meaning
Electromagnetic radiation patterns of antennas stabilize only after the wave front travels a specific distance from the radiating aperture. Utilizing a direct far field measurement approach requires placing the test antenna at a distance where the incoming wave front is essentially planar. This condition allows engineers to measure radiation patterns, gain, and polarization directly without complex mathematical transformations.
The technique is the standard method for characterizing small antenna arrays.
Antenna Characterization
Radiation properties measured in the plane-wave region accurately represent how the device will perform in real-world environments. In a direct far field test, the angular distribution of the radiated energy is recorded by rotating the antenna under test relative to a fixed probe. The resulting data provides a direct map of the main lobe and side lobes.
These measurements are used to validate the radiation efficiency of the module.
Physical Setup
Implementing this measurement method requires a large physical space because the boundary distance is proportional to the square of the antenna dimension divided by the wavelength. For a direct far field setup, the testing chamber must be lined with high-performance electromagnetic absorbers to eliminate reflections from the walls. The positioning system must be constructed from non-metallic materials to avoid disrupting the field structure during the test.
A specialized turntable rotates the device across both azimuth and elevation axes to capture the full radiation sphere. This setup requires precise physical alignment between the transmitting and receiving antennas to avoid polarization mismatch errors.
Calibration Method
System losses must be compensated before the final antenna characteristics can be calculated. The calibration of a direct far field chamber utilizes a substitution method with a known reference antenna. The reference antenna is placed at the test location, and the system path loss is measured across the operating frequency band.
This procedure establishes the baseline for all subsequent measurements.
