Passive microwave circuits - Metal waveguides

There are many different types of waveguide, some metallic, others dielectric. In all cases, these structures remain invariant when translated along an axis that constitutes the direction of propagation of the active power (figure 1 ).

The medium 1 in which the waves propagate is always a dielectric medium. In metallic waveguides, this medium is bounded by a dielectric-conductor interface (medium 2, metallic), whereas in dielectric waveguides, this medium is bounded by a dielectric-dielectric interface (medium 2, dielectric).

Dielectric guides are rarely used in passive microwave circuits. This is because :

• the circular-section dielectric guide is mainly used for telecommunications at optical frequencies, hence the name optical fiber (see Optical fibers for telecommunications [E 7 110] in the Telecoms treatise);

• planar dielectric guides are used for millimeter-wave integrated circuits, where they are ideal for integrating active and passive components. They are also used for passive circuits, but at optical frequencies [1] [2].

Here, we'll be looking at metallic waveguides, which are widely used as passive circuits for centimetric and millimetric waves. The most commonly used structures are rectangular or circular in cross-section. They can operate either in fundamental mode (standard guides) or in higher modes (oversized guides). We'll also look at a few other types of metal guide, including central-rib guides, as well as U-shaped, H-shaped and grooved guides, which can be used above 100 GHz.

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