Rectangular Waveguide
Dimensions Chart
Complete reference of all standard rectangular waveguide sizes from WR-2300 to WR-0.65 (1.7 THz). Includes inner dimensions (inches and millimeters), cutoff frequencies, recommended operating bands, flange designations, and power handling. Covers microwave, millimeter-wave, sub-millimeter, and terahertz waveguides. Used by RF and microwave engineers worldwide.
Standard Rectangular Waveguide Sizes
All standard rectangular waveguide designations from WR-2300 (L-band, 0.32 GHz) through WR-0.65 (1.7 THz). Includes EIA, MIL-DTL-85, and sub-millimeter/THz research sizes. Click any column header to sort.
| Band ▲ | WR Size ▲ | a × b (in) ▲ | a × b (mm) ▲ | Freq Range (GHz) ▲ | fc TE10 (GHz) ▲ | λc (mm) ▲ | CW Power (kW) | Atten. (dB/ft) | Flange Type |
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What Is a Rectangular Waveguide?
A rectangular waveguide is a hollow metallic tube with a rectangular cross-section used to transmit electromagnetic waves at microwave and millimeter-wave frequencies. Unlike coaxial cable, waveguide has no center conductor. Instead, the electromagnetic field propagates through the hollow interior, guided by the conductive walls. This structure offers significantly lower loss than coaxial cable at frequencies above approximately 3 GHz, making it the preferred transmission medium for high-frequency, high-power applications.
Rectangular waveguides are used in radar systems, satellite earth stations, particle accelerators, industrial heating, radio astronomy, medical equipment, and military communications. They are manufactured from aluminum, brass, or copper, and are often electroplated with silver or gold to reduce conductor loss.
What Do the WR Numbers Mean?
WR stands for "Waveguide, Rectangular." The number following WR represents the broad wall inner dimension in hundredths of an inch. For example, WR-90 has a broad wall dimension of 0.900 inches (90 hundredths of an inch), and WR-28 has a broad wall of 0.280 inches. This naming convention was established by the Electronic Industries Alliance (EIA) and is used worldwide as the standard designation system for rectangular waveguides.
The narrow wall dimension (b) is typically half the broad wall dimension (a/2), though this ratio varies slightly for some sizes. The WR number alone is sufficient to identify the complete waveguide cross-section, operating frequency range, and compatible flange types.
How Is Cutoff Frequency Calculated?
The cutoff frequency for the dominant TE10 mode in a rectangular waveguide is determined entirely by the broad wall dimension:
fc = c / (2a)
Where:
c = speed of light = 299,792,458 m/s
a = broad wall inner dimension (meters)
Example (WR-90):
a = 0.900 in = 22.86 mm = 0.02286 m
fc = 299,792,458 / (2 × 0.02286) = 6,557 MHz = 6.557 GHz
Below the cutoff frequency, the TE10 mode cannot propagate and the waveguide acts as a high-pass filter with exponential attenuation. The recommended operating range is between approximately 125% and 189% of the cutoff frequency to maintain single-mode propagation and avoid excessive loss near cutoff.
What Is the Recommended Operating Range?
The recommended operating frequency range for a rectangular waveguide is defined by two boundaries:
- Lower limit (~125% of fc): Operating too close to the TE10 cutoff causes high attenuation, increased group delay variation, and degraded VSWR performance.
- Upper limit (~189% of fc): The TE20 mode begins to propagate at twice the TE10 cutoff frequency. Operating above this causes multimode propagation, resulting in signal distortion and unpredictable behavior.
Waveguide Modes Explained
Electromagnetic waves in a rectangular waveguide propagate in specific spatial patterns called modes. These are classified as TE (Transverse Electric) or TM (Transverse Magnetic) modes, identified by subscripts m and n:
- TE10 (dominant mode): The lowest-order mode with the lowest cutoff frequency. Single half-wave variation across the broad wall, uniform along the narrow wall. This is the mode used in virtually all standard waveguide applications.
- TE20: First higher-order mode. Cutoff at twice the TE10 frequency. Sets the upper frequency limit for single-mode operation.
- TE01: Cutoff depends on the narrow wall dimension. Usually higher than TE20 for standard aspect ratios.
- TM11: The lowest TM mode. Requires both field components to vary across a and b dimensions.
Flange Types: Cover vs. Choke
Waveguide flanges provide the mechanical and electrical connection between waveguide sections. The two primary types are:
- Cover (flat) flanges: Have a flat mating surface with bolt holes surrounding the waveguide aperture. They provide a simple, reliable connection and are the most common type. Two cover flanges can mate together, but require precise alignment and clean, flat surfaces for good RF performance.
- Choke flanges: Feature a circular groove (quarter-wave choke joint) machined into the mating surface. This groove creates an RF short circuit at the flange interface, significantly reducing leakage and contact resistance. A choke flange is designed to mate with a cover flange for optimal performance.
For standard connections, one cover flange and one choke flange are used together. For applications where both flanges must be identical (such as rotating joints), two cover flanges are used with proper torque specifications.
Common Flange Designations
- UG-135/U, UG-136/U: Cover and Choke flanges for WR-90 (X-band)
- UG-595/U, UG-596A/U: Cover and Choke flanges for WR-42 (K-band)
- UG-599/U, UG-600/U: Cover and Choke flanges for WR-28 (Ka-band)
- UG-383/U: Cover flange for WR-22 (Q-band)
- UG-385/U: Cover flange for WR-19 (U-band)
- UG-387/U: Cover flange for WR-10, WR-8, WR-7, and WR-5 (mmWave)
Waveguide Material and Plating
- Aluminum: Lightweight, low cost. Used for most commercial and military applications. Often chromate-coated for corrosion resistance.
- Brass: Heavier but more dimensionally stable. Used for precision components and test equipment.
- Copper (OFHC): Lowest conductor loss. Used for high-performance laboratory and cryogenic applications.
- Silver plating: Reduces conductor loss by 5-10% compared to bare brass. Standard for precision waveguide components.
- Gold plating: Provides corrosion resistance with minimal loss increase. Common in aerospace and defense applications.
Sub-Millimeter and Terahertz Waveguides
Below WR-2.2 (330 GHz), waveguide enters the sub-millimeter and terahertz (THz) regime. These sizes present extreme manufacturing challenges because the aperture dimensions shrink to fractions of a millimeter:
- WR-1.5 (500 to 750 GHz): Broad wall of just 0.381 mm (0.015 inches). Used in radio astronomy receivers, atmospheric remote sensing, and molecular spectroscopy.
- WR-1.0 (750 GHz to 1.1 THz): Broad wall of 0.254 mm. Approaching the terahertz gap. Used in advanced scientific instrumentation and imaging systems.
- WR-0.65 (1.1 to 1.7 THz): Broad wall of just 0.165 mm. The smallest standard waveguide designation. Used in cutting-edge THz spectroscopy and quantum-cascade laser coupling research.
At these scales, traditional CNC machining cannot achieve the required tolerances. Manufacturing typically uses Deep Reactive Ion Etching (DRIE) of silicon wafers or advanced electroforming techniques. Virginia Diodes Inc. (VDI) is one of the primary suppliers of components at these frequencies, using their WM (Waveguide Micro) flange system.
Surface roughness becomes a dominant loss mechanism at THz frequencies because surface imperfections that are negligible at microwave frequencies become comparable to the wavelength. Gold plating on silicon substrates is the standard approach for minimizing conductor loss in sub-mm waveguide.
Frequently Asked Questions
What do the WR numbers mean for waveguides?
WR stands for "Waveguide, Rectangular." The number represents the broad wall inner dimension in hundredths of an inch. WR-90 = 0.900 inches, WR-28 = 0.280 inches, WR-10 = 0.100 inches. This EIA naming convention is the worldwide standard for identifying rectangular waveguide sizes.
How do I choose the right waveguide size?
Select the waveguide size whose recommended operating frequency range covers your system's operating band. For example, if your system operates at 28 GHz, WR-28 (26.5 to 40.0 GHz) is the correct choice. Always ensure your entire signal bandwidth falls within the waveguide's single-mode operating range.
Why not use coaxial cable instead of waveguide?
Coaxial cable loss increases with frequency due to skin effect in the center conductor and dielectric absorption. Above approximately 18 GHz, waveguide provides significantly lower insertion loss per unit length. At 40 GHz, for example, WR-28 waveguide has roughly 0.15 to 0.22 dB/ft of attenuation compared to 3+ dB/ft for typical semi-rigid coax. Waveguide also handles much higher power levels without breakdown.
What is the power handling capacity of waveguide?
Power handling depends on the waveguide size, material, pressurization, and frequency. Larger waveguides handle more power because the cross-sectional area is greater, keeping the field strength below the breakdown threshold. WR-90 at X-band can handle approximately 990 kW CW at sea level in dry air. WR-28 at Ka-band handles approximately 95 to 145 kW. Power capacity decreases at higher altitudes and in the presence of moisture.
Can I connect different WR sizes together?
Different WR sizes can be connected using waveguide transitions (tapers or step transitions). These transitions gradually change the cross-section from one WR size to another over a controlled length, maintaining low VSWR. They are commonly used when connecting test equipment to antenna feeds or when integrating components from different frequency-band subsystems.
What is the difference between WR-28 and WR-28 (MIL) specifications?
Standard EIA WR-28 and MIL-DTL-85/3 WR-28 share the same inner dimensions (0.280 x 0.140 inches). The MIL specification adds requirements for wall thickness tolerances, material certification, plating standards, dimensional inspection, and environmental testing. MIL-spec waveguide is required for defense and aerospace applications where reliability and traceability are critical.