How do I select the correct waveguide flange type for my frequency band and application?
Waveguide Flange Guide
The waveguide flange is the mechanical interface that joins two waveguide sections. Unlike coaxial connectors where the center conductor mates directly, waveguide flanges join the open apertures of two waveguide sections. The RF performance of the junction depends on the alignment accuracy, the flatness of the mating surfaces, and the contact quality at the waveguide walls.
Flat (cover) flanges rely on direct metal-to-metal contact at the waveguide aperture boundary. They require good surface finish, proper bolt torque, and accurate alignment to achieve low loss and return loss. Flat-to-flat connections are the most common in permanent installations.
Choke flanges use a quarter-wave groove machined into the flange face that creates a virtual short circuit at the waveguide wall, even without direct metal contact. This design is more tolerant of surface imperfections and slight misalignment, making it preferred for connections that are frequently made and broken. The choke design also reduces PIM because the metal-to-metal contact is moved away from the high-current region.
Frequently Asked Questions
What happens with misaligned flanges?
Misalignment excites higher-order modes and increases reflection. Axial offset of 0.1 mm in WR-90 degrades return loss by about 3 dB. Angular misalignment of 0.1° has minimal effect. Use locating pins (dowel pins) in the flange holes to ensure repeatable alignment.
Can I mix different flange types?
A flat flange can connect to a choke flange of the same waveguide size. This is the standard configuration for many systems. Two flat flanges can connect if both surfaces are clean and flat. Two choke flanges should not mate because the choke grooves create unpredictable gaps.
What about mmWave flanges?
At mmWave frequencies (WR-10 and smaller), precision flanges with alignment pins and pin-keyed orientation are essential. The IEEE 1785 standard defines precision flange interfaces with specified flatness (λ/20) and alignment (±25 μm). These precision interfaces enable measurement-grade repeatability to 110 GHz and beyond.