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484 Antennas
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The characteristic impedance (Zo) of a waveguide varies with frequency. In this sense, it differs from coaxial or parallel-wire lines, whose Zo values are generally independent of the frequency. A properly installed and well-maintained waveguide is an exceptional transmission line because dry air has practically no loss, even at UHF and microwave frequencies. But it is important that the interior of a waveguide be kept free from dirt, dust, insects, spiderwebs, and condensation. Even a small obstruction can seriously degrade the performance and cause the waveguide to become lossy. The main limitation of a waveguide, from a practical standpoint, is inflexibility. You can t run a waveguide from one point to another in a haphazard fashion, as you can do with coaxial cable. Bends or turns in a waveguide present a particular problem, because they must be made gradually. Installing a waveguide for a UHF or microwave antenna is a little like putting in the conduit for a new electrical circuit in a home or business. It s a significant construction project! Waveguides are impractical for use at frequencies below approximately 300 MHz, because the required cross-sectional dimensions become prohibitively large.
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Dish A dish antenna must be correctly shaped and precisely aligned. The most efficient shape, especially at the shortest wavelengths, is a paraboloidal reflector. However, a spherical reflector, having
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27-11 Dish antennas with
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conventional feed (A) and Cassegrain feed (B).
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Antennas for Ultrahigh and Microwave Frequencies 485
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the shape of a section of a sphere, can also work well. The feed system consists of a coaxial line or waveguide from the receiver and/or transmitter, and a horn or helical driven element at the focal point of the reflector. Conventional dish feed is shown in Fig. 27-11A. Cassegrain dish feed is shown in Fig. 27-11B. The larger the diameter of the reflector in wavelengths, the greater the gain, the f/b ratio, and the f/s ratio, and the narrower the main lobe. A dish antenna must be at least several wavelengths in diameter for proper operation. The reflecting element can be sheet metal, a screen, or a wire mesh. If a screen or mesh is used, the spacing between the wires must be a small fraction of a wavelength.
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Helical A helical antenna is a circularly polarized, high-gain, unidirectional antenna. A typical helical antenna is shown in Fig. 27-12. The reflector diameter should be at least 0.8 at the lowest operating frequency. The radius of the helix should be approximately 0.17 at the center of the intended operating frequency range. The longitudinal spacing between helix turns should be approximately 0.25 in the center of the operating frequency range. The overall length of the helix should be at least at the lowest operating frequency. A helical antenna can provide about 15 dBd forward gain. Helical antennas are sometimes used in space communications systems. Corner Reflector A corner reflector, employed with a /2 dipole driven element, is illustrated in Fig. 27-13. This provides some gain over a /2 dipole by itself. The reflector is made of wire mesh, screen, or sheet metal. The flare angle of the reflecting element is approximately 90 . Corner reflectors are widely used in terrestrial communications at UHF and microwave frequencies. Several /2 dipoles can be fed in phase and placed along a common axis with a single, elongated reflector, forming a collinear corner reflector array. Horn The horn antenna is shaped like a squared-off trumpet or trombone horn. It provides a unidirectional radiation and response pattern, with the favored direction coincident with the opening of the horn. The feed line is a waveguide that joins the antenna at the narrowest point (throat) of the horn.
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27-12 A helical antenna
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with a flat reflector.
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486 Antennas
27-13 A corner reflector
with a dipole as the driven element.
Horns are sometimes used all by themselves, but they are also used to feed large dish antennas. This optimizes the f/s ratio by minimizing extraneous radiation and response that occurs if a dipole is used as the driven element for the dish.
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