Compact antenna test range

Communications: radio wave antennas – Antennas – Measuring signal energy

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Details

343753, 343909, H01Q 1502, H01Q 1906

Patent

active

056709650

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a compact antenna test range for performing antenna and radar cross-section measurements. The test range comprises (i) transmitting means for transmitting an electromagnetic wave, (ii) transducer means for converting the wavefront transmitted by the transmitting means to a plane wave, and (iii) a piece to be tested, located in such a way that the plane wave to be obtained from the transducer means hits it.
By means of antenna test methods, characteristics of an antenna are measured, viz. radiation pattern, polarization, gain or directivity, for instance. These methods can be divided into three main groups, i.e. measurements (1) in the far field, (2) in the near field, and (3) in a compact antenna test range.
In far field measurements, a plane wave field is formed by means of a separate transmitting antenna in front of the antenna to be tested. The transmitting antenna shall be located at a sufficient distance from the antenna to be tested so that a wave of spherical phase front type coming from the transmitting antenna would approximate well enough the plane wave in front of the antenna to be tested. In general, it is considered that the far field starts at the distance 2D.sup.2/.lambda., where D is the diameter of an aperture of the antenna to be tested and .lambda. is the wavelength. This is, however, not a sharp limit, because the dependence of the antenna characteristics on the distance at which they are measured weakens when the distance increases. At the determination of the distance of the far field, the important thing is how large an error is allowable with respect to the actual circumstances in the far field. The far field distance mentioned above has been obtained when a phase difference of 22.5 degrees is permitted in the aperture of the antenna to be tested. The distance of the far field for large antennas will be impracticably large anyway; it can be several kilometres and, therefore, the test must always be performed outdoors in these cases.
In the near field measurements, generally performed in an anechoic chamber, the field in front of the antenna to be tested is measured. The characteristics of the far zone are calculated from the measured near-field results. This method is generally used at frequencies below 40 GHz in the first place. At frequencies higher than that, a problem will arise due to phase errors caused by measuring cables and inaccurate measuring points and, especially in connection with large antennas, by an immense number of measuring points, because the field must be measured at intervals of at least half of a wavelength. Each measuring event is cumbersome under these circumstances.
A Compact Antenna Test Range (CATR) is a test range in which an artificial far field, i.e. a plane wave, is formed in front of the antenna under test. The actual antenna test is performed in a manner similar to that used in ordinary far field measurements. The desired plane wave is formed from a spherical wave by means of a focusing element. Up till now, curved metal reflectors have nearly exclusively been used for forming the plane wave. The principle of such a test range has been described e.g. in U.S. Pat. No. 3,302,205 in the embodiments of which either one rotational parabola reflector of two cylindrical parabolic reflectors are used for forming the plane wave.
With respect to far field measurements, an advantage of a compact test range is that it is small enough in size to be located indoors, under protection from the influence of weather conditions, even if the antenna is large. With respect to near field measurements the advantage is that each measurement can be performed easily, after the range is in its place, as a normal far field measurement, e.g. by rotating the antenna under test. A further advantage is also that it enables an indoor measurement of the radar cross-section of even large objects. The use of a compact antenna test range for measurements is in some cases also the only reasonable alternative, e.g. in cases if a near

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