Near-Field Measurements
Near-Field Antenna Test Ranges
 Near-Field
test ranges are typically indoor configurations that occupy a
relatively small space. This type of range uses a small RF probe
antenna that is scanned over a surface surrounding the test
antenna. Range lengths can be very short even to the point to
where the probe nearly touches the antenna structure. During the
measurement, near-field phase and amplitude information is
collected over a discrete matrix of points. This data is then
transformed to the far-field using Fourier techniques. The
resulting far-field data can then be displayed like other
conventional far-field antenna measurements.
In addition to obtaining far-field data,
back-transforming to the antenna’s aperture produces aperture
field distribution information. This offers the ability to
perform element diagnostics on multi-element phased array
antennas.
 In
near-field testing the test antenna is usually aligned to the
scanner’s coordinate system and then either the probe or the
test antenna is moved. In practice it is easier and more cost
effective to scan the RF probe over linear axes or the test
antenna over angular axes. Three techniques are in common
usages:
Planar Near-Field Testing - With
planar near-field scanning, the probe usually is scanned in X
and Y linear coordinates over the aperture of the test
antenna. A planar scanner is used to move the probe over a very
accurate plane located in front of the test antenna’s aperture.
Once aligned to the scan plane, the test antenna is not moved
during the collection of the near-field data.
 Cylindrical
Near-Field Testing – For this method the probe typically is
scanned in one linear dimension using a single axis linear
scanner. The test antenna is stepped in angle on a rotary axis
oriented parallel to the linear axis. The resulting scan
describes a cylindrical surface around the test
antenna. Cylindrical near-field scanning can provide complete
angular coverage of the test antenna’s field in one plane.
Spherical Near-Field Testing–
Spherical near-field scanning normally involves installing the
test antenna on a spherical scanning positioner. The probe
antenna is supported fixed in space. The test antenna is scanned
in one angular axis and stepped in an orthogonal angular
axis. The resulting data is collected over a spherical envelope
surrounding the test antenna. Full or nearly full coverage of
the test antenna’s radiating field can be evaluated with this
type of near-field system.
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