What is the relationship between waveguide dimensions and operating frequency band?
Waveguide Sizing
The WR designation system simplifies waveguide identification. WR-XX means the broad wall dimension a = XX hundredths of an inch. WR-90 has a = 0.900 inches (22.86 mm). The complete set of standard waveguide sizes covers frequencies from 320 MHz (WR-2300) to 325 GHz (WR-3). Each size provides approximately 40% bandwidth within the single-mode operating range.
The narrow dimension (b) determines the maximum operating power (through the field strength limit) and the impedance of the waveguide. Standard aspect ratio is a/b ≈ 2.0, which balances power handling and mode separation. Reduced-height waveguide (a/b > 2) provides a lower profile at the cost of reduced power handling and higher attenuation.
At millimeter wave frequencies, the waveguide dimensions become very small. WR-10 (75-110 GHz) has inside dimensions of 2.54 × 1.27 mm. WR-3 (220-325 GHz) has inside dimensions of 0.864 × 0.432 mm. These tiny waveguides are fabricated by precision CNC machining, wire EDM, or LIGA (electroforming). The tight tolerances required at these sizes make components expensive.
Frequently Asked Questions
What waveguide do I need for my frequency?
Select the standard waveguide whose operating band includes your frequency. For 10 GHz: WR-90 (8.2-12.4 GHz). For 24 GHz: WR-42 (18-26.5 GHz). For 77 GHz: WR-12 (60-90 GHz) or WR-10 (75-110 GHz). For frequencies at the edge of a band, choose the band that places your frequency near the center for best performance.
Can I use non-standard dimensions?
Yes, but you lose compatibility with standard flanges et components. Non-standard waveguide is used for specific applications: oversized waveguide for reduced loss (at the cost of multi-mode risk), reduced-width for higher cutoff, or double-ridged for extended bandwidth.
What is the IEC waveguide designation?
IEC uses R-designation (R-XX) corresponding to WR sizes. R-100 = WR-90, R-220 = WR-42. The R number is the cutoff frequency of the TE10 mode in hundreds of MHz. Some regions also use EIA or IEC band designations (Ku, K, Ka, V, W, D, G bands).