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Introduction to Microwave Systems

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Introduction to Microwave Systems c14IntroToMicrowaveSystems Pozar September 26, 2011 18:40 Chapter Fourteen Introduction to Microwave Systems A microwave system consists of passive and active microwave components arranged to perform a useful function. Probably the two most important examples are microwave com- munication systems and microwave radar systems, but there are many others. In this chapter we will discuss the basic operation of several types of microwave systems to give a general overview of the application of microwave technology, and to show how the subjects of earlier chapters fit into the overall scheme of complete microwave systems. An important component in any radar or wireless communication system is the antenna, so we will first discuss some of the basic properties of antennas. Then we treat wireless commu- nication, radar, and radiometry systems as important applications of RF and microwave tech- nology. We also briefly discuss propagation effects, biological effects, and other miscellaneous applications. All of these topics are of sufficient depth that many books have been written for each. Our purpose here is to introduce these topics as a way of placing the earlier material in this book in the larger context of practical system applications. The interested reader is referred to the references at the end of the chapter for more complete treatments. 14.1 SYSTEM ASPECTS OF ANTENNAS In this section we describe some of the basic characteristics of antennas that will be needed for our study of microwave communications, radar, and remote sensing systems. We are interested here not in the detailed electromagnetic theory of antenna operation, but rather in the systems aspect of the operation of an antenna in terms of its radiation patterns, di- rectivity, gain, efficiency, and noise characteristics. References [1] and [2] can be reviewed for a more in-depth treatment of the fascinating subject of antenna theory and design. Figure 14.1 shows some of the different types of antennas that have been developed for commercial wireless systems. 658 c14IntroToMicrowaveSystems Pozar September 26, 2011 18:40 14.1 System Aspects of Antennas 659 FIGURE 14.1 Photograph of various millimeter wave antennas. Clockwise from top: a high-gain 38 GHz reflector antenna with radome, a prime-focus parabolic antenna, a corru- gated conical horn antenna, a 38 GHz planar microstrip array, a pyramidal horn antenna with a Gunn diode module, and a multibeam reflector antenna. A transmitting antenna can be viewed as a device that converts a guided electromag- netic wave on a transmission line into a plane wave propagating in free space. Thus, one side of an antenna appears as an electrical circuit element, while the other side provides an interface with a propagating plane wave. Antennas are inherently bidirectional, in that they can be used for both transmit and receive functions. Figure 14.2 illustrates the basic FIGURE 14.2 Basic operation of transmitting and receiving antennas. c14IntroToMicrowaveSystems Pozar September 26, 2011 18:40 660 Chapter 14: Introduction to Microwave Systems operation of transmitting and receiving antennas. The transmitter can be modeled as a Thevenin source consisting of a voltage generator and series impedance, delivering a power Pt to the transmitting antenna. A transmitting antenna radiates a spherical wave that, at large distances, approximates a plane wave over a localized area. A receiving antenna in- tercepts a portion of an incident plane wave, and delivers a receive power Pr to the receiver load impedance. A wide variety of antennas have been developed for different applications, as summa- rized in the following categories: r Wire antennas include dipoles, monopoles, loops, sleeve dipoles, Yagi–Uda arrays, and related structures. Wire antennas generally have low gains, and are most often used at lower frequencies (HF to UHF). They have the advantages of light weight, r low cost, and simple design. Aperture antennas include open-ended waveguides, rectangular or circular horns, reflectors, lenses, and reflectarrays. Aperture antennas are most commonly used at r microwave and millimeter wave frequencies, and have moderate to high gains. Printed antennas include printed slots, printed dipoles, and microstrip patch an- tennas. These antennas can be made with photolithographic methods, with both radiating elements and associated feed circuitry fabricated on dielectric substrates. Printed antennas are most often used at microwave and millimeter wave frequencies, r and can be easily arrayed for high gain. Array antennas consist of a regular arrangement of antenna elements with a feed network. Pattern characteristics such as beam pointing angle and sidelobe levels can be controlled by adjusting the amplitude and phase excitation of the array elements. An important type of array antenna is the phased array, in which variable-phase shifters are used to electronically scan the main beam of the antenna. Fields and Power Radiated by an Antenna While we do not require detailed solutions to Maxwell’s equations for our purposes, we do need to be familiar with the far-zone electromagnetic fields radiated by an antenna. Con- sider an antenna located at the origin of a spherical coordinate system. At large distances, where the localized near-zone fields are negligible, the radiated electric field of an arbitrary antenna can be expressed as − jk0r ¯ ˆ ˆ e E(r,θ,φ)= θ Fθ (θ, φ) + φFφ(θ, φ) V/m,(14.1) r where E¯ is the electric field vector, θˆ and φˆ are unit vectors in the spherical coordinate system, r is the radial distance from the origin, and k0 = 2π/λ is the free-space propaga- tion constant, with wavelength λ = c/f . Also defined in (14.1) are the pattern functions, Fθ (θ, φ) and Fφ(θ, φ). The interpretation of (14.1) is that this electric field propagates in − the radial direction with a phase variation of e jk0r and an amplitude variation with dis- tance of 1/r. The electric field may be polarized in either the θˆ or φˆ direction, but not in the radial direction, since this is a TEM wave. The magnetic fields associated with the electric field of (14.1) can be found from (1.76) as Eθ Hφ = , (14.2a) η0 −Eφ Hθ = , (14.2b) η0 c14IntroToMicrowaveSystems Pozar September 26, 2011 18:40 14.1 System Aspects of Antennas 661 where η0 = 377 , the wave impedance of free-space. Note that the magnetic field vector is also polarized only in the transverse directions. The Poynting vector for this wave is given by (1.90) as ∗ S¯ = E¯ × H¯ W/m2,(14.3) and the time-average Poynting vector is 1 1 ∗ S¯ = Re {S¯}= Re {E¯ × H¯ } W/m2.(14.4) avg 2 2 We mentioned earlier that at large distances the near fields of an antenna are negli- gible, and that the radiated electric field can be written as in (14.1). We can give a more precise meaning to this concept by defining the far-field distance as the distance where the spherical wave front radiated by an antenna becomes a close approximation to the ideal planar phase front of a plane wave. This approximation applies over the radiating aperture of the antenna, and so it depends on the maximum dimension of the antenna. If we call this maximum dimension D, then the far-field distance is defined as 2D2 R = ( . ) ff λ m. 14 5 This result is derived from the condition that the actual spherical wave front radiated by the antenna departs less than π/8 = 22.5◦ from a true plane wave front over the maximum extent of the antenna. For electrically small antennas, such as short dipoles and small loops, this result may give a far-field distance that is too small; in this case, a minimum value of Rff = 2λ should be used. EXAMPLE 14.1 FAR-FIELD DISTANCE OF AN ANTENNA A parabolic reflector antenna used for reception with the direct broadcast sys- tem (DBS) is 18 inches in diameter and operates at 12.4 GHz. Find the far-field distance for this antenna. Solution The operating wavelength at 12.4 GHz is c 3 × 108 λ = = = 2.42 cm. f 12.4 × 109 The far-field distance is found from (14.5), after converting 18 inches to 0.457 m: 2D2 2(0.457)2 R = = = 17.3m. ff λ 0.0242 The actual distance from a DBS satellite to Earth is about 36,000 km, so it is safe to say that the receive antenna is in the far-field of the transmitting antenna. ■ Next, define the radiation intensity of the radiated electromagnetic field as 2 2 ¯ r ˆ ∗ ˆ ˆ ∗ ˆ U(θ, φ) = r |S |= Re Eθ θ × Hφ φ + Eφφ × Hθ θ avg 2 2 r 2 2 1 2 2 = |Eθ | +|Eφ| = |Fθ | +|Fφ| W, (14.6) 2η0 2η0 c14IntroToMicrowaveSystems Pozar September 26, 2011 18:40 662 Chapter 14: Introduction to Microwave Systems where (14.1), (14.2), and (14.4) were used. The units of the radiation intensity are watts, or watts per unit solid angle, since the radial dependence has been removed. The radiation intensity gives the variation in radiated power versus position around the antenna. We can find the total power radiated by the antenna by integrating the Poynting vector over the surface of a sphere of radius r that encloses the antenna. This is equivalent to integrating the radiation intensity over a unit sphere: 2π π 2π π ¯ 2 Prad = Savg ·ˆrr sin θdθdφ = U(θ, φ) sin θdθdφ. (14.7) φ=0 θ=0 φ=0 θ=0 Antenna Pattern Characteristics The radiation pattern of an antenna is a plot of the magnitude of the far-zone field strength versus position around the antenna, at a fixed distance from the antenna.
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