7.8.14 Microwave devices

A cavity is one of many waveguide components. Microwave devices are the components used to generate, guide, control and measure high-frequency signals, usually quoted as $300\mathrm{MHz}$ to $300\mathrm{GHz}$ ($\lambda$ from $1\mathrm{m}$ to $1\mathrm{mm}$). Most waveguide hardware works between $1$ and $100\mathrm{GHz}$ (from WR-650 to WR-10). At these frequencies lumped elements ($R$, $L$, $C$) become inaccurate as soon as their size approaches the wavelength ($3\mathrm{cm}$ at $10\mathrm{GHz}$): the phase varies along them, they radiate, and their leads add inductance and capacitance. Transmission lines and waveguides are used instead as distributed structures, and the behaviour of a component is described by power waves and scattering parameters.

Figure 7.8.16 shows a transmitter chain built from the components of this section. The isolator protects the source from reflections, the coupler samples the power for a meter, and the cavity filter removes out-of-band signals.

Common waveguide components

• Bends, twists and transitions route the power with minimal reflection.
• Irises and posts introduce inductive or capacitive discontinuities, used for matching and as the couplings of cavity filters.
• Directional couplers sample a small fraction of the power travelling in one direction.
• Isolators and circulators protect sources from reflected power.
• Cavity resonators are the high-$Q$ frequency-selective elements.

An iris (Figure 7.8.17) excites evanescent higher modes (Section 7.3) that store energy next to it. Vertical strips and posts store mainly magnetic energy and act as a shunt inductance. Strips that narrow the gap between the broad walls store electric energy and act as a shunt capacitance. Two irises ${\lambda }_g/2$ apart form a cavity, which is how most waveguide filters are built.

Junctions and non-reciprocal components

Figure 7.8.18 shows the symbols of the most common multiport components.

• Tees. In an E-plane (series) tee a wave fed into arm 3 leaves arms 1 and 2 in opposite phase. In an H-plane (shunt) tee it leaves them in phase. A junction with both side arms is the magic tee (Exercise 7.8.8).
• Directional coupler. Two guides share a wall with coupling holes. A forward wave in port 1 sends a small sample to port 3 and nothing to port 4 (Section 7.8.16).
• Isolator and circulator. These are non-reciprocal, built with a magnetised ferrite. The isolator passes $1\to 2$ and absorbs $2\to 1$. The circulator routes $1\to 2\to 3\to 1$.