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ANTENG Ch17SAL032006 - ECE 5318/6352 Antenna Engineering Dr...

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Unformatted text preview: ECE 5318/6352 Antenna Engineering Dr. Stuart Long Chapter 17 Measurements 1 ANTENNA MEASUREMENTS Scale models Sometimes original antenna in original environment is difficult to measure. Use scaling to make physical dimensions smaller (usual case) 2 ANTENNA MEASUREMENTS (CONT) Scaling modifications 1.) linear dimensions of model are 1/n times that of full scale antenna 2.) operating frequency is n times that of original frequency 3.) conductivities are n times that of original antenna 4.) permittivity and permeability ( and ) stay the same 3 ANTENNA MEASUREMENTS (CONT) Scaling modifications 1 l'= l n f '= n f ' = n '= '= usually n > 1 ALL CHARACTERISTICS OF FIELDS WILL BE PRESERVED radiation pattern; gain; directivity; input impedance; efficiency 4 ANTENNA MEASUREMENTS (CONT) Practical Solutions 1.) conductivities not scaled just "good" for both cases 2.) take care with and dependence on frequency 3.) must reproduce environment (ground planes, guy wires, etc.) 5 ANTENNA MEASUREMENTS (CONT) FAR FIELD RADIATION PATTERN in general field E(, ) is a function of angles. The magnitude, but not the shape of the pattern, depends on the distance from the antenna (R) in the far field 6 ANTENNA MEASUREMENTS (CONT) FAR FIELD RADIATION PATTERN far-field region not sharply defined specifies path difference from center of antenna to edge of antenna R R D2 2D 2 R < 8 R < 16 R < 32 (R < fraction of ) 4D 2 R 7 ANTENNA MEASUREMENTS (CONT) FAR FIELD RADIATION PATTERN use spherical coordinate system hold one angle constant and vary the other usually orient polarization along spherical unit vector e.g. measure E ( = 0 , ) E ( , = 0 ) E ( = 0 , ) E ( , = 0 ) 8 ANTENNA MEASUREMENTS (CONT) FAR FIELD RADIATION PATTERN record field strength or power as function of angle linear or polar plot 3-D pattern or series of 2-D "cuts" 9 ANTENNA MEASUREMENTS (CONT) Spherical coordinate system geometry 10 ANTENNA MEASUREMENTS (CONT) Linearly polarized field patterns ____E - plane Co - pol ____H - plane Co - pol ____E - plane Cross - pol ____H - plane Cross - pol 11 ANTENNA MEASUREMENTS (CONT) Practical Antenna Range transmitting horn antenna under test transmitter can rotate receiving antenna in two planes and rotate transmitter for polarization measurements receiver 12 ANTENNA MEASUREMENTS (CONT) Practical Antenna Range orientation of transmitting antenna determines polarization rotation of test antenna determines "cut" 13 ANTENNA MEASUREMENTS (CONT) Practical Antenna Range circular polarization rotating linear pattern Fig. 17-26 rotate transmitting horn more rapidly than test unit produces plot which shows axial ratio of the circular polarization as a function of angle 14 ANTENNA MEASUREMENTS (CONT) Pattern of a circularly polarized test antenna taken with rotating, linearly polarized, source antenna 15 ANTENNA MEASUREMENTS (CONT) Outdoor Antenna Range occupy large area; weather dependent, have ground reflections 16 ANTENNA MEASUREMENTS (CONT) Anechoic Chamber (an-echoic no echos) enclosure with microwave absorbing material lining walls, floor, ceiling re c ta n gu lar d ed re er pe ap tta source 17 ANTENNA MEASUREMENTS (CONT) CATR Compact Antenna Test Range 18 ANTENNA MEASUREMENTS (CONT) Modifications to CATR serrated-edge rolled-edge reflectors edge treatment use to reduce the diffracted fields in the quiet zone 19 ANTENNA MEASUREMENTS (CONT) Modifications to CATR 20 ANTENNA MEASUREMENTS (CONT) NEAR FIELD RANGE measure field strength in near field region on a planar, cylindrical, or spherical surface use Fourier Transform techniques to calculate far field pattern, computer intensive; not in real time 21 ANTENNA MEASUREMENTS (CONT) GAIN (DIRECTIVITY) usual technique is by comparison to a known standard gain horn substitute for horn with antenna under test 22 ANTENNA MEASUREMENTS (CONT) INPUT IMPEDANCE Slotted line measure standing wave pattern (VSWR and position of min.) calculate Zin at each frequency Automatic Network Analyzer - automated measurement of reflection coefficient and other scattering matrix parameters displays Zin on Smith Chart or other format as a function of frequency 23 ANTENNA MEASUREMENTS (CONT) IMPEDANCE Resistance - Reactance R X f 24 ANTENNA MEASUREMENTS (CONT) REFLECTION COEFFICIENT 0 dB f1 B W f2 f |S11| -10 dB -20 dB 25 ANTENNA MEASUREMENTS (CONT) VSWR 4 B W VSWR 2 0 f1 f2 f 26 ANTENNA MEASUREMENTS (CONT) IMPEDANCE f1 - - - - = 0.33 (VSWR = 2.0) f2 27 ...
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