Page 389 - Maxwell House

DISCONTINUITY IN FEED LINES 369

The inductive posts are broadly used as the elements of microwave bandpass filters and
matching networks due to their simplicity, reliability and a great range of impedance values that
can be realized. For example, we can shift the post from the WR center to its sidewall. Thereby,
the post hits the area where E-field intensity of the incident TE10-mode is reduced. As a result,
the EM field disturbance drops as well as the lumped inductor value. The value of inductive
impedance sharply increases if the set of several thin posts commonly called diaphragm forms
the structure like the fence along WR or in its cross section.

Capacitive post in column 2 of Table 7.2 narrows the air opening between the top and bottom
WR walls thereby increasing the intensity of E-fields in the two resulting air gaps. The effect
is quite predictable without any numerical simulation: > in the air gaps and the induced

impedance is capacitive. Loosely speaking, the capacitive post can be considered as a plate
capacitor with tiny surface proportional to the post diameter. If so, the production of such
discontinuities with substantial impedance might be problematic requiring installation of quite
thick metal post with diameter up to one-third of WR height and even more.

Resonance posts. The inductive post can be converted into a resonance one by reducing its
height as shown in column 4 of Table 7.2. If so, the strong E-fields is generated around the
sharp tip of the post (edge effect) and in the air gap between the post tip and top WR wall. The
images in Figure 7.2.5a - 7.2.5c demonstrate this phenomenon at 9.6957 GHz. The post height

E-field E-field
Reactive Part
h INDUCTANCE
12.4 GHz
a) 9.6957 GHz

H-field
7 GHz
Reactive Part
b) c) CAPACITANCE d)

Figure 7.2.5 Resonance post: a) E-field energy distribution, b) H-field energy
distribution, c) E-field force line image, d) Smith chart demonstrating the series
resonance at 9.6957 GHz
and diameter is 8.16 mm and 1 mm, respectively, while the WR-90 height is 10.16 mm. The
electrons flowing from the bottom WR wall induce the conductivity current along the post. This
moving up the stream of electrons does not have typically enough energy to escape from the
metal post. As a result, part of them is accumulated on the sharp tip thereby exerting the strong
E-field and displacement current all around the tip and in the air gap as depicted in Figure
7.2.5c. Loosely speaking, another and a major portion of moving up electrons reach the post
end and run down thereby forming the standing wave distribution of electric current over the
post that Figures 7.2.5b, c undoubtedly illustrates. It, therefore, follows that H-field energy
stores mainly around the post bottom section while the peak of E-field energy shifts to the post
tip. Translating this effect into the lumped element circuit, we can come to the conclusion that
the lower section of the post is the equivalent of inductor while the top one is a capacitor.
Evidently, they are attached in series since the displacement current in the air gap is the