Page 387 - Maxwell House

DISCONTINUITY IN FEED LINES 367

Meanwhile, the E-field vector is tangential to the iris PEC (perfect electric conductor) surface
and must vanish on it according to the boundary condition. As such, the electric energy
concentration around the iris is quite low, > and, as expected, the equivalent circuit is

a lump inductor at all frequencies (see Smith chart in Figure 7.2.2b). All shown fields are
normalized to their peak and demonstrated on log scale between -5 dB and -40 dB. The two
tangential components and of incident TE10-mode generate the vertical component of

the surface electric current . In its turn, this current reradiates exerting the numerous TEm0-
modes. Although each of these extra modes neither satisfies the zero boundary condition on
iris, they do it as a group forming the composite near-field structure. Note that the incident,
reflected and going through TE10-mode are contained within this group.

Capacitive irises. The same way, we can identify the iris in column 2 of Table 7.2 as a primarily
lumped capacitor. Noticeably, E-field energy accumulates in the gap between the iris body and
top WR wall as depicted in Figure 7.2.3a. Although > , the concentration of H-field as

well the component of surface current is quite substantial near the sharp iris edges, i.e.

just slightly overpasses . It explains why the value of lump capacitive impedance shown in

Figure 7.2.3b is relatively low.

Figure 7.2.3 Capacitive and resonance iris: a) H- and E-field energy distribution around the
capacitive iris, b) Impedance variation of capacitive iris, c) Impedance variation of resonance
iris, d) Impedance variation of suspended resonance iris

Resonance irises. Now, we know what will happen if we comprise the capacitive and inductive
irises at the same WR cross section as shown in column 3 of Table 7.2. It just means that the
lumped capacitor and inductor are connected in parallel to some resistive impedance
proportional to WR characteristic impedance (typically in the range of hundred Ohms) that
restricts the quality Q and bandpass of such resonance circuit. Smith chart in Figure 7.2.3c
illustrates the impedance deviation of resonance iris over the frequency. The impedance is
inductive at frequencies below the resonance, crosses the chart origin (the point of no reflection)
at 9.238 GHz and then becomes capacitive. In other words, the iris behavior resembles a
standard parallel resonance contour. The obvious application of such irises is quite compact
bandpass filters where one thin sheet iris replaces the whole resonance circuit like cavity
resonator. Another common application area is ultra-high power vacuum tube like magnetrons,
klystrons, and gyrotrons. The latter is capable of generating a hundred megawatts of microwave
power and widely used in ground and space communication systems, radars, nuclear science,
plasma and material heating, EM warfare systems, etc. What is not obvious is how to preserve
the deep vacuum inside the tube and at the same time transfer EM power to the outside feed