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RG-59/U Main transmission line
18-21 Stacking VHF antennas
made of RG-58/U coaxial cable This cable is then fed with RG-59/U coax from the receiver There is nothing magical about scanner receivers that require any form of antenna that is significantly different from other VHF/UHF antennas Although the designs might be optimized for VHF or UHF, these antennas are basically the same as others shown in this book As a matter of fact, almost any antennas, from any chapter, can be used by at least some scanner operators
CHAPTER
Microwave waveguides and antennas
THE MICROWAVE PORTION OF THE RADIO SPECTRUM COVERS FREQUENCIES FROM ABOUT
900 MHz to 300 GHz, with wavelengths in free-space ranging from 33 cm down to 1 mm Transmission lines can be used at frequencies from dc to about 50 or 60 GHz, although, above 5 GHz, only short runs are practical, because attenuation increases dramatically as frequency increases There are three types of losses in conventional transmission lines: ohmic, dielectric, and radiation The ohmic losses are caused by the current flowing in the resistance of the conductors making up the transmission lines Because of the skin effect, which increases resistance at higher frequencies, these losses tend to increase in the microwave region Dielectric losses are caused by the electric field acting on the molecules of the insulator and thereby causing heating through molecular agitation Radiation losses represent loss of energy as an electromagnetic wave propagates away from the surface of the transmission line conductor Losses on long runs of coaxial transmission line (the type most commonly used) cause concern even as low as the 400-MHz region Also, because of the increased losses, power handling capability decreases at high frequencies Therefore, at higher microwave frequencies, or where long runs make coax attenuation losses unacceptable, or where high power levels would overheat the coax, waveguides are used in lieu of transmission lines What is a waveguide Consider the light pipe analogy depicted in Fig 19-1 A flashlight serves as our RF source, which (given that light is also an electromagnetic wave) is not altogether unreasonable In Fig 19-1A the source radiates into free space and spreads out as a function of distance The intensity per unit area, at the destination (a wall), falls off as a function of distance (D) according to the inverse square law (1/D2) But now consider the transmission scheme in Fig 19-1B The light wave still propagates over distance D, but is now confined to the interior of a mirrored pipe Almost all of the energy (less small losses) coupled to the input end is delivered to the output end, where the intensity is practically undiminished Although not
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370 Microwave waveguides and antennas
Large diffused beam
"Light pipe"
Small intense beam 19-1 Waveguide analogy to light pipe
perfect, the light pipe analogy neatly summarizes, on a simple level, the operation of microwave waveguides Thus, we can consider the waveguide as an RF pipe without seeming too serenely detached from reality Similarly, fiber-optic technology is waveguidelike at optical (IR and visible) wavelengths In fact, the analogy between fiber optics and waveguide can withstand more rigorous comparison than the simplistic light pipe analogy The internal walls of the waveguide are not mirrored surfaces, as in our optical analogy, but are, rather, electrical conductors Most waveguides are made of aluminum, brass, or copper In order to reduce ohmic losses, some waveguides have their internal surfaces electroplated with either gold or silver, both of which have lower resistivities than the other metals mentioned above Waveguides are hollow metal pipes, and can have either circular or rectangular cross sections (although the rectangular are, by far, the most common) Figure 19-2 shows an end view of the rectangular waveguide The dimension a is the wider dimension, and b is the narrower These letters are considered the standard form of notation for waveguide dimensions, and will be used in the equations developed in this chapter
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