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 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:
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).