. (7.67) Because of the load, V+ and V can no
longer be arbitrarily chosen but must satisfy
the boundary conditions. These reflection and
transmission coefficients have a number of
interesting propert
relations among the voltage, electric field,
current, and charge motion. 7.1.2 Kirchhoffs
Laws There are two Kirchhoff Laws that can be
used to analyze the current flow in a circuit:
The sum of curre
relationships will hold as long as the
frequencies are low enough for the size of the
circuit to be much smaller than the
electromagnetic wavelength. Above this there
is a tricky regime in which the e
rewritten suggestively as r = 1 (x 2 + y 2 ) (1
x) 2 + y 2 r(1 x) 2 + x 2 1 + r + y 2 = 1 1 + r x 2
2x r 1 + r + r 1 + r + y 2 = 1 1 + r x 2 2x r 1 + r
+ r1+r2+y2=11+r+ r1+r2r1+r x
r 1 + r 2 + y 2 =
vanishes, the only way the transverse
components can be non-zero is for the
denominator k 2 c = 2+k 2 to also vanish.
This means that = ik = i = i/c,
therefore TEM waves travel at the speed of
light i
rectangular slabs that confine modes in both
directions and for circular dielectric
waveguides, although the imposition of these
boundary conditions becomes a more difficult
calculation [Yariv, 1991].
the current flowing through the inductance V
= Ldz I t . (7.52) Current flows from high to
low potential, so an increasing potential drop
across the inductor has the opposite sign from
the decreasing
load impedance ZL. The load might be a
resistor, or it could be another transmission
line. For a resistor the impedance is associated
with energy dissipated by ohmic heating, and
for a transmission li
t2 1 v 2 2V t2 , (7.56) where v 1 LC .
(7.57) This is a wave equation for the voltage
in the transmission line. It is solved by an
arbitrary distribution traveling with a velocity
v V (z, t) = f(z vt)
(7.46) 7.2.3 Wave Solutions Now consider the
little differential length of the transmission
line dz shown in Figure 7.5, with parallel
capacitance C dz and series inductance L dz. If
there is an incre
varying electric field associated with the
capacitor plates storing or releasing charge
rather than from real charge passing through
the capacitor. If the applied voltage is V = e
it, then the current
arriving at the T. 7 Circuits, Transmission Lines,
and Waveguides Electric and magnetic fields
contain energy, which can propagate. These
are the ingredients needed for
communications; in this chapter
iEy (7.82) which can be rearranged as Ex =
1 k 2 c Ez x + i Hz y Hx = 1 k 2 c i
Ez x Hz y Ey = 1 k 2 c Ez x + i
Hz y Hy = 1 k 2 c i Ez x + Hz y .
(7.83) If the axial components Ez, Hz are found
from
Similarly, the current across the load is IL(t) =
I+(t) + I(t) . (7.64) The current across the
termination must equal the current in the
transmission line immediately before the
termination: VL ZL = V
the charge density is q, the total charge in
this slab is Q = qdxA and it feels a net force
F~ = QE~ . Because a current is flowing, charge
is moving relative to this force and so work is
being done.
frequencies can no longer be solved for
analytically, for a coaxial cable a rough
approximation for the TM modes is to ask that
the wavelength be a multiple of radial spacing
c 2 n (ro ri) n = 1, 2, 3
radius 1/c located at (1, 1/c) given by the
complex part of the input impedance c. The
intersection of these two circles relates the
input impedance to the reflection coefficient,
conveniently found g
common type. 7.3.1 Governing Equations Start
with the wave form of Maxwells equations
without any sources: 2E~ = 2E~ t2
2H~ = 2H~ t2 . (7.77) We are looking
for waves that travel along the axis of the
have been considering conduction in the lowfrequency limit; in this section we will use
Maxwells equations to look at how AC fields
penetrate conductors and are guided by them
at higher frequencies. 7
than the cut-off wavelength c , is pure real
and so the mode decays exponentially. When
is greater than the cut-off frequency for a
mode, is pure imaginary and the mode
propagates. These modes are la
infinitely wide. w h I -I Figure 7.9. Transmission
line for Problem 7.4. (7.5) The most common
coaxial cable, RG58/U, has a dielectric with a
relative permittivity of 2.26, an inner radius of
0.406 mm
the same. Since the field of a solenoid of
radius r and length l with n turns/meter is H =
nI, the inductance is L = I = N I Z B~ dA~ = nl
I nIr2 = n2 lr2 . (7.24) 7.2 Wires and
Transmission Lines 85
Voltage Standing-Wave Ratio (VSWR) VSWR
Vmax Vmin = 1 + |R| 1 |R| (7.69) or |R| =
VSWR 1 VSWR + 1 . (7.70) The VSWR is one
of the most important measurements in an RF
system, used to ensure that impe
related to the capacitance and inductance per
unit length. In a real cable, different
frequencies are damped at 7.2 Wires and
Transmission Lines 91 different rates, changing
the pulse shape as it trav
twisted pair of conductors surrounded by a
grounded shield. Why the twist? Why the
shield? (7.2) Salt water has a conductivity 4
S/m. What is the skin depth at 104 Hz? (7.3)
Integrate Poyntings vector
positive charge. A charge q such as an electron
in a wire feels a force F~ = qE~ , and so
according to these definitions electrons flow
from low to high potentials (Figure 7.1). The
current ~I, in amp
transmission line, consider the coaxial cable in
Figure 7.4). Other important transmission line
geometries include parallel wires or strips
(Problem 7.4), and a strip above a ground
plane (called a st
reflection at all! This is why cables carrying
high-frequency signals are terminated with
resistors that match the cables characteristic
impedance. Such terminations are particularly
important to elim
boundary condition for a perfect conductor.
The part of the field that does leak into the
conductor causes a current to flow, and this
current leads to resistive dissipation, therefore
in making this
drawn down produces a polarizationpreserving fiber that retains the polarization
of the light [Galtarossa et al., 1994]. Weve
also assumed that the medium is linear, but
the intense fields in the smal