0195136047.pdf

15.1 WAVES, TRANSMISSION LINES, WAVEGUIDES, AND ANTENNA FUNDAMENTALS 671

used to connect some television antennas. The coaxial cable of Figure 15.1.1(b) is the most widely

used of the many possible cable-type transmission lines. For printed-circuit and integrated-circuit

applications, transmission lines sketched in Figures 15.1.1(c) through (f) are commonly employed.

At higher frequencies, when power levels are large and attenuation in transmission lines is

significant, connections between system components are often made throughwaveguides, which

are usually hollow, closed, rigid conductor configurations (much like water pipes) through which

waves propagate. The most common waveguides are eitherrectangularorcircularin cross

section, as depicted in Figure 15.1.2, but other shapes and flexible varieties are also possible.

Coaxial transmission lines commonly operate in what is called thetransverse electric

magnetic(TEM)mode, in which both the electric and the magnetic fields are perpendicular

Conductor

Conducting strip

Conductors

d

h

d

Dielectric εr

Dielectric εr

Dielectric εr Dielectric εr

Dielectric

εr

(a)

(c)

(b)

W

W

h

2 h

Hot lead

(e)

W

Conducting strip

Dielectric εr h

Free space ε 0

(d)

W

h

(f)

S W

Ground Hot lead Ground

S

D

Figure 15.1.1Transmission lines (cross sections of some common types).(a)Two-wire line.(b)Coaxial

line (cable).(c)Parallel strip line.(d)Microstrip line.(e)Strip line.(f)Coplanar waveguide.

b

a

(a)

a

(b)

Figure 15.1.2Waveguides.(a)Rectangular.(b)Circular.