Practical_Antenna_Handbook_0071639586
480 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s / 4 / 4 Antenna radiator element (Cavity) Microwave window Tuning disk Waveguide or coaxial cable fitting Tuning adjustment screw Figure 20.24 Cavity antenna. Related to the horn is the cavity antenna of Fig. 20.24. Here a quarter-wavelength radiating element extends from the waveguide (or transmission line connector) into a resonant cavity. The radiator element is placed a quarter-wavelength into a resonant cavity, spaced a quarter-wavelength from the rear wall of the cavity. A tuning disk provides a limited tuning range for the antenna by altering cavity dimensions. Gains to about 6 dB are possible with this arrangement. Dipole element a R a I 0111057 FIG 18-25 Reflector Antennas At microwave frequencies, it becomes possible to use reflector antennas because of the short wavelengths involved. Reflectors are theoretically possible at lower frequencies, but the extremely large physical dimensions that the longer wavelengths require make them impractical. In Fig. 20.25 we see the corner reflector antenna, used primarily in the high-UHF and lowmicrowave region. A dipole element located at the “focal point” of the corner reflector receives (in phase) reflected wavefronts from many regions of the reflector surface. Either solid metallic reflector surfaces or wire mesh Figure 20.25 Corner reflector.
C h a p t e r 2 0 : M i c r o w a v e W a v e g u i d e s a n d A n t e n n a s 481 A. Paraboloid B. Truncated paraboloid (surface search) C. Truncated paraboloid (height finding) D. Orange-peel paraboloid E. Cylindrical paraboloid F. Corner reflector Figure 20.26 Reflector antennas. may be used. When mesh is used, however, the holes in the mesh must be l/12 or smaller. An assortment of several other reflector shapes, most of which are found in radar applications, can be seen in Fig. 20.26. Parabolic “Dish” Antennas The parabolic reflector antenna is one of the most widespread of microwave antennas, and one that normally comes to mind when thinking of microwave systems. It derives its operation from the field of optics—possible in part because microwaves are in a transition region between ordinary radio waves and infrared/visible light. The dish antenna has a paraboloid shape, as defined by Fig. 20.27. In this figure, the dish surface is positioned such that the center is at the origin (0,0) of an x-y coordinate system. For purposes 0111057 of defining FIG 18-27 the surface, we place a second vertical axis called the directrix (y′) a distance behind the surface equal to the focal length (u). The paraboloid surface follows the function y 2 = 4uX, and has the property that a line from the focal point F to any point on the surface is the same length as a line from that same point to the directrix. (In other words, MN = MF.) If a radiator element is placed at the focal point F, it will illuminate the reflector surface, causing wavefronts to be propagated away from the surface in phase. Similarly, wavefronts intercepted by the reflector surface are reflected to the focal point. The gain of a parabolic antenna is a function of several factors, including dish diameter, feed illumination, and surface accuracy. The dish diameter D should be large compared with its depth. Surface accuracy refers to the degree of surface irregularities. For commercial antennas, 1 ⁄8-wavelength surface accuracy is usually sufficient, although on certain radar antennas the surface accuracy specification must be tighter.
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480 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s<br />
/ 4<br />
/ 4<br />
<strong>Antenna</strong> radiator<br />
element<br />
(Cavity)<br />
<br />
Microwave<br />
window<br />
Tuning disk<br />
Waveguide or<br />
coaxial cable<br />
fitting<br />
Tuning adjustment<br />
screw<br />
Figure 20.24 Cavity antenna.<br />
Related to the horn is the cavity antenna of Fig. 20.24. Here a quarter-wavelength<br />
radiating element extends from the waveguide (or transmission line connector) into a<br />
resonant cavity. The radiator element is placed a quarter-wavelength into a resonant<br />
cavity, spaced a quarter-wavelength from the rear wall of the cavity. A tuning disk provides<br />
a limited tuning range for the antenna by altering cavity dimensions. Gains to<br />
about 6 dB are possible with this arrangement.<br />
Dipole<br />
element<br />
a R<br />
a I<br />
<br />
<br />
0111057 FIG 18-25<br />
Reflector <strong>Antenna</strong>s<br />
At microwave frequencies, it becomes<br />
possible to use reflector antennas because<br />
of the short wavelengths involved.<br />
Reflectors are theoretically<br />
possible at lower frequencies, but the<br />
extremely large physical dimensions<br />
that the longer wavelengths require<br />
make them impractical. In Fig. 20.25<br />
we see the corner reflector antenna, used<br />
primarily in the high-UHF and lowmicrowave<br />
region. A dipole element<br />
located at the “focal point” of the corner<br />
reflector receives (in phase) reflected<br />
wavefronts from many regions<br />
of the reflector surface. Either solid<br />
metallic reflector surfaces or wire mesh<br />
Figure 20.25 Corner reflector.
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