Waveguides

A waveguide is a hollow metallic tube that confines and guides electromagnetic waves. Unlike coaxial line (which supports TEM mode down to DC), waveguides have a cutoff frequency below which propagation does not occur. Above cutoff, waveguides offer very low loss, making them the preferred transmission medium at millimetre-wave frequencies and for high-power radar systems.

Modes

Rectangular waveguide. Cutoff frequency fc = c/(2a) for the dominant TE₁₀ mode.

Rectangular waveguides support TE (transverse electric) and TM (transverse magnetic) modes. The dominant mode TE₁₀ has the lowest cutoff frequency and is used in virtually all single-mode waveguide applications. The cutoff frequency of the TE₁₀ mode in a rectangular waveguide of width \(a\) is:

TE₁₀ mode: E-field is vertical, sinusoidal in x, zero at walls. f_c = c/(2a) — only this mode propagates near cutoff.

\(f_c = \dfrac{c}{2a}\)

Standard Waveguide Bands

WR-90 X-band: 8.2–12.4 GHz
WR-42 K-band: 18–26.5 GHz
WR-28 Ka-band: 26.5–40 GHz
WR-10 W-band: 75–110 GHz

Attenuation

Waveguide losses are due to finite conductivity of the walls. Attenuation is minimum at approximately 1.8× the cutoff frequency and rises toward both the cutoff and at very high frequencies. Rectangular waveguide attenuation is typically 0.01–0.5 dB/m, much lower than coax at the same frequency.

Applications

Waveguides are used in high-power radar systems, satellite ground station feeds, millimetre-wave communication links, and as the connecting structure between antenna feeds and LNAs in satellite receivers (where minimum loss before the LNA is critical for noise figure).

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