Page 388 - Maxwell House

368 Chapter 7

line that is typically under normal atmospheric conditions. The standard approach is to employ
the welded to metal WR walls iris like shown in in column 3 of Table 7.2 and then replace the
iris air window with a dielectric plate of high mechanical strength, high thermal conductivity,
high electrical breakdown voltage, low dielectric loss, joinability and weldability to iris metal
parts. Sapphire and some other similar materials are the best. The same suspended resonance
iris is shown in columns 4 and 5 of Table 7.2 because it demonstrates according to Figure 7.2.3d
two resonances of a different kind: full reflection at frequency 7.59 GHz (like the series
resonance contour connected in parallel) and total transmission at frequency 12.08 GHz (like
the parallel resonance contour connected in parallel).

Inductive posts. The metal or in some cases dielectric posts (also called rods or probes) are
among the most used discontinuities in waveguides. They could be easily installed using quite
simple and low-cost fabrication technology. The metal post of partial height like shown in
column 4 of Table 7.2 is frequently used as a tuning screw that penetrates waveguide metal
wall, can be adjusted in its tallness by a screwdriver and then secured with jam nuts. Let us
consider first an inductive post that is typically maintained through the holes in WR broad walls
and welded to them. Looking back at Figure 6.6.6 in Chapter 6, we see that the longitudinal
component (z-component) of electric current flowing on the inside surface of the bottom wall
can reach the top wall only transforming into displacement current proportional to E-field
(green arrows). Remind that it follows from Maxwell’s equations and particularly from the net
current continuity equation (1.64) of Chapter 1. Meanwhile, the metal post opens the conductive
path for electrons from the bottom wall to move up or down along the post. As a result, the post
behaves like a radiating electric dipole creating an infinite set of partial waves as Figure 6.7.4b
demonstrates. It means that the dipole generates two TE10-modes, the first reflects back and the
second passes the post. Along with them, the energy of excited but not propagating TEm0-modes
(m > 1) stores in the form of reactive energy. The same way as in the case of an inductive iris,
the whole set of modes organizes itself such way that the E-field zero boundary condition on
the perfectly conductive post surface is satisfied. As such, the accumulation of the E-field
energy nearby the post is relatively low while the electric current on the post surface exerts
strong H-field (red arrows) around the post. Therefore, in the post vicinity > as Figure

7.2.4a-c demonstrates and the lumped equivalent circuit is an inductor connected in parallel due
to the longitudinal current splitting through the post. Note that red color indicates the highest
energy concentration while the green corresponds to the drop of about 20 dB in energy intensity.

Figure 7.2.4 Top view on post in WR and EM field energy distribution nearby and its
impedance: a) E-field energy, b) H-field energy, c) Smith chart showing post impedance
variation