Page 377 - Maxwell House

DISCONTINUITY IN FEED LINES 357

Figure 7.1.4 Coaxial step-up discontinuity: a) Illustration, b) E-filed energy distribution, c)
H-field energy distribution, d) Smith chart showing capacitance impedance between 0 and
10GHz, e) Equivalent circuit
We know that the increase in the center conductor diameter reduces the H-field intensity (check
the expression (1.66) in Chapter 1). E-field simultaneously surges as the gap between inner and
outer conductor decreases. Therefore, we can expect that such discontinuity is associated with
the lumped capacitor. The numerical simulation based on CST model fully confirms this
physically based thinking. According to image in Figure 7.1.4c, there is no visible concentration
of H-field energy in proximity of the step-up, i.e. > nearby (see Figure 7.1.4e). Both E-

and H-field energy surges in the front of discontinuity due to the incident wave and reflected
wave deposit their energy there.
7.1.3 Open-Ended Coaxial Line
A schematic of the open end of the coaxial line is depicted in Figure 7.1.5d. The classical circuit
theory teaches us that the end impedance of an open circuit transmission line must be in infinite,
the current at the open end is zero, the voltage is doubled, and the reflection coefficient is equal

Figure 7.1.5 Open-ended coaxial line section: a) E-field pattern, b) E-field energy
distribution, c) Equivalent circuit, d) Surface electric current distribution, e) Radiation
pattern, f) Smith chart showing matching performance between ~0 and 10GHz

to one. If so, the ideal standing wave is formed in line. That is not the case now since EM wave
in coaxial line cannot abruptly stop its propagation at the end of the line. Unmistakably, the part