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DISCONTINUITY IN FEED LINES 365

One more outcome is worthwhile to be mentioned. Evidently, the presence of discontinuity in
regular slotline that disturbs quasi-TEM mode field structure should be the source of active
radiation too. Curiously, the highest portion of the radiation is emitted through the bottom
surface of the substrate as it follows from Figure 7.2.1c. That is why the ground plane on this
surface lessens the radiation radically but makes the line production more complicated and
expensive.
7.2.3 Waveguide Discontinuities

Table 7.2 illustrates the underlying discontinuities in WR, and they are common in wide variety
of hollow waveguides. Mostly, the equivalent circuits of all these discontinuities were built by
examining the nearby E- and H-field energy distributions. We classify discontinuities as
inductors while in this area > , capacitors if > , and parallel or series resonance

circuits when = . Meanwhile, the frequency dependence lets identify the resonance

circuit as parallel if at frequencies bellow resonance the circuit impedance is inductive and
capacitive above. Generally, the EM field structure in WR with discontinuities and their
equivalent circuits are more complicated than in lines with TEM or quasi-TEM modes. As such,
the data in Table 7.2 should be considered as the first order approximation. WR-90 (or RG-
52/U) was chosen as the base for the following discussion. This WR outer dimensions a x b and
wall thickness t are 25.4x12.7x1.27 mm (or 1x0.5x0.05 inches), the dominant TE10-mode cutoff
frequency is 6.56 GHz, and regular WR of infinite length is free of higher modes
while 6.56 GHz < < 13.02 GHz. Typically, the recommended operational bandwidth is 8.2
GHz – 12.40 GHz to avoid the cutoff proximity of dominant and first high TE20-mode. We have
discussed such single mode issue in Chapter 6.

Figure 7.2.2 Inductive iris: a) H- and E-field energy distribution, b) Smith chart

The family of the simplest discontinuities is the so-called irises (sometimes diaphragms) or the
relatively thin and highly conductive metal plates covering the cross section of WR partially.
The shape of iris windows could be arbitrary as well as the iris orientation and restricted mainly
to available manufacturing technology and cost production. The most commons are the flat
metal sheets placed at right angle to the longitudinal WR axis (see images in Table 7.2). We
kept the irises thickness in numerical simulations around 2 mm thereby much less than the
operation wavelengths. The red vector indicates the E-field orientation of dominant mode TE10.
Inductive irises. Let us start from an inductive iris shown in column 1 of Table 7.2. Figure