Antenna Measurements: Near-Field Vs Far-Field

To make sure a given antenna and application meet character specifications, transmission, coverage and configuration requirements, they will need to undergo radiation frequency testing and a wide-variety of measurements by electromagnetic-compatibility (ECM) antenna engineers.

Probing the strength of the electric and magnetic fields, checking an antenna’s RF-performance, azimuth and elevation coordinates, radiation, beam patterns, parameters, polarization, gain, distortion, impedance, range (far-field and near-field testing measurements) and distance (given by calculating the ratio of the electric and magnetic fields of an antenna – far and near-field parameters – and/or separated by the Fraunhofer diffraction equation of groups) are some of the tests antenna engineers conduct.

Amplitude and/or phase characteristics of an antenna under test (AUT) – placed and aligned in an antenna test range – in addition as size, size, energy fluctuations are also measured using a digitizing oscilloscope connected to the antenna.

The two terms that depict the range (far and near field) of an AUT describe the changes (in respect to their distance produced directly by currents and charge-separations) of electromagnetic (EM) fields around the antenna. These are the two vicinity boundaries that exist with relation to how far or near they are from the surface of the antenna (or surrounding any electrically charged object for that matter).

Found radiating closest to the antenna is the near-field vicinity. This reactive field prompts current in near-by objects – it represents zero energy flow as a consequence of the 90-degree phase difference between the electric and magnetic fields.

Electromagnetic radiation of radio frequencies in the near-field vicinity is apt for short range communication. It is ideal for wireless technology like radio-frequency identification (RFID) and real-time locating systems (RTLS) and commonly used for small antennas operating within the AM broadcast band (525-1715 kHz), offering low-strength and probability of intercept of RF signals in propagation environments.

Other than for planar, cylindrical and spherical testing and to perform diagnostics on a variety of microwave radar antennas and characterizing antennas, measuring their performance (such as its accuracy and throughput) or being ideal for antenna measurement applications, to include wireless, PCS/cellular, satellite and radar systems, near-field systems have been developed to lower costs.

The “far-field” is the more distant range from the antenna (or any EM field source) and is known as the “radiation zone” or “free space” of this electromagnetic spectrum – it’s where engineers will find radio groups and microwaves of shorter-wave EM radiation.

Major distinctions in the “far-field” vicinity of an antenna is the ratio of the electric and magnetic fields strengths do not vary with distance – rather, they are fixed and in phase; while – “near-field” phase relations between the EH elements (E=electric and H=magnetic) are not – they vary as the distance from the antenna increases.

For a shorter wavelength and distance, “far-field” can use shaped reflectors – employing single or dual reflectors – to minimize radiation, give antenna directivity while producing a uniform plane wave in the test vicinity.

Far-field measurement techniques are best suited for lower frequency antennas and appropriate to provide functional simple pattern measurements. Near-field measurement techniques are suited for high frequency gain antennas and when considering the accuracy of planar, cylindrical and spherical measurements (in terms of angular spectrum and distance of groups from tests and configurations).

Near-field is also appropriate for complicate patterns and polarization measurements where horizontal, vertical, circular, or elliptical types can be present.

For certain applications, installations and upgrades in addition as for lower frequencies and real-time measurements far-field is the most appropriate option. Also, far-field configurations work best for indoor or compact range testing.

However, for high gain antennas the most appropriate option is near-field configurations: For high directivity antennas as with planar near-field ranges at 75-degrees or less, angular coverage is desired. If, however, lobe measurements are required and there is to be more than 75-degrees coverage, cylindrical or spherical configurations are the appropriate choices.

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