Waveguide
Understanding Waveguides
Waveguide has been a cornerstone of microwave engineering since the 1930s. Despite being larger and less flexible than coaxial cable, waveguide remains the preferred transmission medium whenever loss, power handling, or signal purity is critical. At millimeter-wave frequencies above 40 GHz, waveguide becomes the only practical guided transmission medium.
Rectangular Waveguide
The most common type. The broad wall dimension (a) determines the cutoff frequency of the dominant TE10 mode: fc = c/(2a). The waveguide operates in a frequency band between the TE10 cutoff and the TE20 cutoff (which occurs at 2x the TE10 cutoff). This single-mode operating range typically spans about 40% bandwidth.
Circular Waveguide
Used in rotary joints, antenna feeds, and long-distance overmoded transmission. The dominant mode is TE11. Circular waveguide can carry dual-polarized signals, making it useful in satellite feed systems and orthomode transducers (OMTs).
Waveguide Advantages
- Lowest loss: No center conductor means no dielectric loss and reduced ohmic loss. At 94 GHz, WR-10 waveguide has about 2 dB/m loss vs. 15+ dB/m for coaxial cable.
- Highest power handling: No center conductor eliminates the voltage breakdown limit of coax. Waveguide can handle megawatts of peak power.
- Excellent shielding: The continuous metallic enclosure provides over 100 dB of shielding effectiveness.
- Phase stability: Rigid construction provides excellent phase stability vs. temperature and vibration.
Waveguide Components
A complete waveguide system includes straight sections, bends (E-plane and H-plane), twists, transitions (to coax or other waveguide sizes), couplers, filters, circulators, and terminations. Each component must maintain the field pattern of the propagating mode while achieving its specific function.
fc = c / (2a)
where a = broad wall dimension, c = speed of light
Wavelength in waveguide:
λg = λ0 / √(1 - (fc/f)²)
Wave impedance (TE10):
Z_TE = η0 / √(1 - (fc/f)²)
where η0 = 377 Ω (free space impedance)
Example: WR-90 (a=22.86mm), fc = 6.557 GHz
Operating band: 8.2 - 12.4 GHz (X-band)
Common Waveguide Sizes
| WR Size | Band | Frequency (GHz) | a (inches) | b (inches) |
|---|---|---|---|---|
| WR-284 | S-Band | 2.60 - 3.95 | 2.840 | 1.340 |
| WR-137 | C-Band | 5.85 - 8.20 | 1.372 | 0.622 |
| WR-90 | X-Band | 8.20 - 12.40 | 0.900 | 0.400 |
| WR-42 | K/Ka-Band | 18.0 - 26.5 | 0.420 | 0.170 |
| WR-28 | Ka-Band | 26.5 - 40.0 | 0.280 | 0.140 |
| WR-10 | W-Band | 75 - 110 | 0.100 | 0.050 |
| WR-03 | G-Band | 220 - 330 | 0.034 | 0.017 |
Frequently Asked Questions
What is a waveguide used for?
Waveguide is used to transmit microwave and millimeter-wave signals with the lowest possible loss. Applications include radar feed networks, satellite communication systems, radio telescope receivers, and high-power transmitter output connections. At frequencies above 40 GHz, waveguide is the dominant transmission medium.
Why does waveguide have lower loss than coaxial cable?
Waveguide has no center conductor and no dielectric fill (in standard air-filled waveguide). This eliminates dielectric loss and reduces ohmic loss because current flows only on the interior walls. At 94 GHz, WR-10 waveguide has about 2 dB/m loss compared to over 15 dB/m for the best coaxial cables.
What is the cutoff frequency of a waveguide?
The cutoff frequency is the lowest frequency that can propagate through the waveguide. For the dominant TE10 mode in rectangular waveguide, fc = c/(2a) where a is the broad wall dimension. Below cutoff, the fields decay exponentially and no power is transmitted. Waveguide is operated in the band between the TE10 and TE20 cutoff frequencies.