Microwave Concepts, Devices, and Antennas, Schemes and Mind Maps of Microwave Engineering and Acoustics

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Principles of Electronic Principles of Electronic

Communication Systems Communication Systems

Third Edition

Louis E. Frenzel, Jr.

Chapter 16 Chapter 16

Microwave Communication

16-1: Microwave Concepts 16-1: Microwave Concepts

 Microwaves are the ultrahigh, superhigh, and

extremely high frequencies directly above the lower

frequency ranges where most radio communication

now takes place and below the optical frequencies

that cover infrared, visible, and ultraviolet light.

16-1: Microwave Concepts 16-1: Microwave Concepts

Microwave Frequencies and Bands

 (^) The practical microwave region is generally considered to extend from 1 to 30 GHz, although frequencies could include up to 300 GHz.  (^) Microwave signals in the 1- to 30-GHz have wavelengths of 30 cm to 1 cm.  (^) The microwave frequency spectrum is divided up into groups of frequencies, or bands.  (^) Frequencies above 40 GHz are referred to as millimeter (mm) waves and those above 300 GHz are in the submillimeter band.

16-1: Microwave Concepts 16-1: Microwave Concepts

Benefits of Microwaves

 (^) Moving into higher frequency ranges has helped to solve the problem of spectrum crowding.  (^) Today, most new communication services are assigned to the microwave region.  (^) At higher frequencies there is a greater bandwidth available for the transmission of information.  (^) Wide bandwidths make it possible to use various multiplexing techniques to transmit more information.  (^) Transmission of high-speed binary information requires wide bandwidths and these are easily transmitted on microwave frequencies.

16-1: Microwave Concepts 16-1: Microwave Concepts

Disadvantages of Microwaves

 (^) The higher the frequency, the more difficult it becomes to analyze electronic circuits.  (^) At microwave frequencies, conventional components become difficult to implement.  (^) Microwave signals, like light waves, travel in perfectly straight lines. Therefore, communication distance is limited to line-of-sight range.  (^) Microwave signals penetrate the ionosphere, so multiple-hop communication is not possible.

16-1: Microwave Concepts 16-1: Microwave Concepts

Microwave Communication Systems: Transmitters

 (^) Like any other transmitter, a microwave transmitter starts with a carrier generator and a series of amplifiers.  (^) It also includes a modulator followed by more stages of power amplification.  (^) The final power amplifier applies the signal to the transmission line and antenna.  (^) A transmitter arrangement could have a mixer used to up-convert an initial carrier signal with or without modulation to the final microwave frequency.

16-1: Microwave Concepts 16-1: Microwave Concepts

Figure 16-3: Microwave transmitters. ( a ) Microwave transmitter using frequency multipliers to reach the microwave frequency. The shaded stages operate in the microwave region.

16-1: Microwave Concepts 16-1: Microwave Concepts

Microwave Communication Systems: Receivers

 (^) Microwave receivers, like low-frequency receivers, are the superheterodyne type.  (^) Their front ends are made up of microwave components.  (^) Most receivers use double conversion.

16-1: Microwave Concepts 16-1: Microwave Concepts

Microwave Communication Systems: Receivers

 (^) The antenna is connected to a tuned circuit, which could be a cavity resonator or microstrip or stripline tuned circuit.  (^) The signal is then applied to a special RF amplifier known as a low-noise amplifier (LNA).  (^) Another tuned circuit connects the amplified input signal to the mixer.  (^) The local oscillator signal is applied to the mixer.  (^) The mixer output is usually in the UHF or VHF range.  (^) The remainder of the receiver is typical of other superheterodynes.

16-1: Microwave Concepts 16-1: Microwave Concepts

Microwave Communication Systems: Transmission

Lines

 (^) Coaxial cable, most commonly used in lower-frequency communication has very high attenuation at microwave frequencies and conventional cable is unsuitable for carrying microwave signals.  (^) Special microwave coaxial cable that can be used on bands L, S, and C is made of hard tubing. This low-loss coaxial cable is known as hard line cable.  (^) At higher microwave frequencies, a special hollow rectangular or circular pipe called waveguide is used for the transmission line.

16-1: Microwave Concepts 16-1: Microwave Concepts

Microwave Communication Systems: Antennas

 (^) At low microwave frequencies, standard antenna types, including the simple dipole and one-quarter wavelength vertical antenna, are still used.  (^) At these frequencies antennas are very small; for example, a half-wave dipole at 2 GHz is about 3 in.  (^) At higher microwave frequencies, special antennas are generally used.

16-2: Microwave Lines and 16-2: Microwave Lines and Devices Devices

Microstrip Tuned Circuits

 (^) At higher frequencies, standard techniques for implementing lumped components such as coils and capacitors are not possible.  (^) At microwave frequencies, transmission lines, specifically microstrip, are used.  (^) Microstrip is preferred for reactive circuits at the higher frequencies because it is simpler and less expensive than stripline.  (^) Stripline is used where shielding is necessary.

16-2: Microwave Lines and 16-2: Microwave Lines and Devices Devices Figure 16-6: Microstrip transmission line used for reactive circuits. ( a ) Perspective view. ( b ) Edge or end view. ( c ) Side view (open line). ( d ) Side view (shorted line).