Antenna II LN09_Antenna Measurements 1 /10 Antenna Measurements

Antenna II LN09_Antenna Measurements 2 /10 Antenna Measurements  Many antennas, because of complex configuration & excitation, cannot be investigated analytically.  Experimental results are often needed to validate theoretical data.  It is usually most convenient to perform antenna measurements with test antenna in its receiving mode.  By reciprocity, receiving mode characteristics are identical to TX mode.  Ideal condition for measuring is incidence of a plane waves having uniform amplitude and phase.  Although this ideal condition is not achievable, it can be approximated by separating test antenna from illumination source by a large distance on an outdoor range.  At large radii, curvature of spherical phase front produced by source antenna is small over test antenna aperture.  If separation distance is equal to inner boundary of far-field region, 2D 2 /λ, then maximum phase error of incident field from an ideal plane wave is about 22.5 o, as shown in:  In addition to phase front curvature, reflections from ground and nearby objects are possible sources of degradation of test antenna illumination.

Antenna II LN09_Antenna Measurements 3 /10 Antenna Measurements  Experimental investigations have a number of drawbacks:  1. For pattern measurements, distance r>2D 2 /λ is too long even for outside range.  2. It also becomes difficult to keep unwanted reflections from surrounding objects.  3. In many cases, it may be impractical to move antenna from operating environment to measuring site.  4. For some antennas such as phased arrays, time required to measure may be enormous.  5. Outside measuring systems provide an uncontrolled environment, and they do not possess an all-weather capability.  6. Enclosed measuring systems usually cannot accommodate large antenna systems such as ships, aircraft.  7. Measurement techniques are expensive.  Some of above shortcomings can be overcome by using special techniques such as:  Indoor measurements.  Far-field pattern prediction from near-field measurements.  Scale model measurements.  Automated commercial equipment specifically designed for antenna measurements.  Utilizing computer assisted techniques.

Antenna II LN09_Antenna Measurements 4 /10 Antenna Measurements  Because of accelerated progress made in aerospace/defense related systems, more accurate measurement methods were necessary.  Accurate measurement methods are:  Tapered anechoic chambers.  Compact and extrapolation ranges.  Near-field probing techniques.  Improved polarization techniques.  Swept-frequency measurements.  Automated test systems.  A more extensive and exhaustive treatment of these and other topics can be found in:  IEEE Standard Test Procedures for Antennas [7].  A summarized journal paper [8].  A book on microwave antenna measurements [6].

Antenna II LN09_Antenna Measurements 5 /10 Antenna Measurements  Reflection Ranges:  There are two basic types of antenna ranges:  The reflection ranges.  The free-space ranges.  Reflection ranges for a judiciously design, can create a constructive interference in region of test antenna which is referred to as the “quiet zone.”  This is accomplished by designing ranges so that specular reflections from ground as shown:  Usually it is desirable for illuminating field to have a small and symmetric amplitude taper.  This can be achieved by adjusting transmitting antenna height while maintaining constant that of receiving antenna.  They are used for systems operating in the UHF to 16GHz frequency region.  Free-space ranges, designing to suppress contributions from surrounding environment, include as:  Elevated ranges.  Slant ranges.  Anechoic chambers.  Compact ranges.  Near-field ranges.

Antenna II LN09_Antenna Measurements 6 /10 Rectangular Tapered Antenna Measurements  Anechoic Chambers:  Anechoic chamber is an alternative to outdoor testing.  To provide a controlled environment, indoor anechoic chambers have been developed.  In general, as operating frequency is lowered, thickness of RF absorbing material must be increased to maintain a given level of reflectivity performance.  There are two basic types of anechoic chamber designs:  Rectangular chamber.  Tapered chamber.  Design of each is based on geometrical optics techniques.  Each attempts to reduce or to minimize specular reflections.  The rectangular chamber is designed to simulate free- space conditions and maximize volume of quiet zone.  Its design takes into account pattern and location of source, frequency and isotropic antenna.  Reflected energy is minimized by the use of high quality RF absorbers.  Despite use of RF absorbing material, significant specular reflections can occur.

Antenna II LN09_Antenna Measurements 7 /10 Antenna Measurements  Tapered anechoic chambers take form of a pyramidal horn.  They begin with a tapered chamber which leads to a rectangular configuration at test region.  Source is usually placed near apex so that reflections from side walls occur near source antenna.  For such paths, phase difference between paths are very small by properly locating source antenna near apex.  Thus direct and reflected rays near test antenna provide a relatively smooth amplitude illumination taper.  This can be illustrated by ray-tracing techniques.  By increasing f 0, it becomes increasingly difficult to place source sufficiently close to apex that phase difference between direct and secularly reflected rays can be maintained below an acceptable level.  For such applications, reflections from walls of chamber are suppressed by using high gain source antennas whose radiation toward walls is minimal.  In addition, source is moved away from apex, and it is placed closer to end of tapering section so as to simulate a rectangular chamber.

Antenna II LN09_Antenna Measurements 8 /10 Antenna Measurements  Compact Range antennas:  Microwave antenna measurements require a uniform plane wave.  Requirement of an ideal plane wave illumination can be achieved by utilizing a compact range.  A Compact Antenna Test Range (CATR) is a collimating device which generates nearly planar wave fronts in a very short distance.  Distance is typically 10–20m compared to the 2D 2 /λ.  Some attempts have been made to use dielectric lenses as collimators [15].  Name CATR refers to one or more curved metal reflectors which perform collimating function.  CATR are very large reflector antennas designed to optimize planar characteristics.  CATR are designated according to their analogous reflector antenna configurations: parabolic, Cassegrain, Gregorian.  Major drawbacks of compact ranges are :  Aperture blockage.  Direct radiation from source to test antenna.  Diffractions from edges of reflector.  Feed support.  Depolarization coupling between two antennas.  Wall reflections

Antenna II LN09_Antenna Measurements 9 /10 Antenna Measurements  CATR Performance: Amplitude and phase ripple in quiet-zone fields produced by a CATR caused by phasor sum of reflected and diffracted rays from reflector

Antenna II LN09_Antenna Measurements 10 /10 Antenna Measurements Two common CATR reflector edge treatments that are used to reduce diffracted fields in quiet zone.  CATR Performance: