Antenna Measurements

1. Antenna Measurements

2. Antenna Measurements • Many antennas, because of complex configuration & excitation, cannot be investigated analytically. • Experimentalresults are often needed to validate theoretical data. • It is usually most convenient to perform antenna measurements with test antenna in its receivingmode. • By reciprocity, receiving mode characteristics are identical to TXmode. • Ideal condition for measuring is incidence of a plane waves havinguniform 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, 2D2/λ, then maximum phase error of incident field from an ideal plane wave is about 22.5o, as shown in: • In addition to phase front curvature, reflections from ground and nearby objects are possible sources of degradation of test antenna illumination.

3. Antenna Measurements • Experimental investigations have a number of drawbacks: • 1. For pattern measurements, distance r>2D2/λis 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.

4. 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].

5. 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 16GHzfrequency region. • Free-space ranges are designed to suppress contributions from surrounding environment and include: • Elevated ranges. • Slant ranges. • Anechoic chambers. • Compact ranges. • Near-field ranges.

6. 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. Tapered Rectangular

7. Antenna Measurements • Tapered anechoic chamberstake 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 f0, 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.

8. Antenna Measurements • Compact Rangeantennas: • 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 2D2/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

9. Antenna Measurements • CATR Performance: Amplitude and phase ripple in quiet-zone fields produced by aCATR caused by phasor sum of reflected and diffracted rays from reflector

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