What are the common waveguide flange sizes for WR-90 and WR-75 waveguides?

Waveguide Flange Standards for WR-90 and WR-75

For rectangular waveguides, the standard flange sizes are directly tied to the waveguide’s WR (Waveguide Rectangular) designation, which specifies the internal dimensions. The most common flange for a WR-90 waveguide (operating in the X-band) is the UG-39/U flange (or CPR-137G), and for a WR-75 waveguide (operating in the lower Ku-band), it is the UG-385/U flange (or CPR-137F). These standardized flanges ensure mechanical compatibility and reliable electrical connections between components from different manufacturers, which is critical for maintaining signal integrity in systems like radar and satellite communications. When you’re sourcing components, getting the right waveguide flange sizes is the first step to a successful assembly.

To understand why these specific flanges are used, we need to look at the history of waveguide standardization. Following World War II, there was a pressing need for interoperability in military and aerospace electronics. This led to the development of the UG (Universal Gauge) numbering system by organizations like the U.S. military. The system was designed so that a flange’s UG number corresponds to a specific WR number, defining not just the opening but also the bolt hole pattern, flange thickness, and overall dimensions. This prevents you from accidentally trying to mate a WR-90 component with a WR-75 flange—they simply won’t bolt together.

Detailed Specifications for WR-90 (UG-39/U) Flanges

The WR-90 waveguide is one of the most common types, with internal dimensions of 0.900 inches by 0.400 inches (22.86 mm by 10.16 mm). It is designed for operation in the X-band frequency range of 8.2 to 12.4 GHz. The UG-39/U flange is its perfect mechanical counterpart. Let’s break down its critical physical specifications.

The flange features a square shape with nominal outside dimensions of 1.872 inches (47.55 mm) on each side. The thickness is typically 0.125 inches (3.18 mm). The most important aspect is the bolt hole pattern. It uses a four-bolt configuration with holes sized for #4-40 UNC screws. The distance between the centers of these bolt holes, known as the bolt circle diameter (BCD), is precisely 1.562 inches (39.67 mm). This precise spacing ensures that when two flanges are brought together, the waveguide apertures align perfectly, creating a continuous electrical path with minimal signal reflection or leakage.

The following table provides a high-density data summary for the WR-90 and its standard flange.

Detailed Specifications for WR-75 (UG-385/U) Flanges

Moving to a higher frequency, the WR-75 waveguide has smaller internal dimensions of 0.750 inches by 0.375 inches (19.05 mm by 9.525 mm). It covers the Ku-band frequency range from 12.4 to 18.0 GHz. As the frequency increases, the wavelength decreases, requiring smaller waveguide cross-sections to support the fundamental propagation mode. Consequently, the associated UG-385/U flange is also smaller than the UG-39/U.

The UG-385/U flange has nominal outside dimensions of 1.622 inches by 1.622 inches (41.20 mm by 41.20 mm). It maintains the same four-bolt pattern but uses smaller #2-56 UNC screws. The bolt circle diameter is correspondingly smaller at 1.312 inches (33.32 mm). This reduction in size is necessary to maintain structural integrity and precise alignment for the higher-frequency signals, where even minor misalignments can cause significant performance degradation in the form of higher VSWR (Voltage Standing Wave Ratio) and insertion loss.

The table below provides a direct comparison with the WR-90 specifications.

Flange Types and Their Impact on Performance

It’s not enough to just know the UG number; you also need to specify the flange type, as this directly affects the electrical performance of the connection. The two primary types you’ll encounter are Cover Flanges and Choke Flanges.

Cover Flanges are the most basic type. They have a flat, machined surface that mates directly with an identical flange. A tight seal is achieved by bolting the two flat surfaces together. While simple and cost-effective, cover-to-cover connections can be susceptible to leakage at higher frequencies if the surfaces are not perfectly flat or if the bolts are not torqued evenly. They are often used in laboratory settings or within environmentally controlled enclosures.

Choke Flanges are more sophisticated and are essential for demanding applications. A choke flange has a precision-machined groove and a radial channel on its face that acts as a quarter-wave transformer. This design creates a short circuit at the outer edge of the flange for any leaking fields, effectively “choking” off the signal and preventing radiation leakage. This provides a much more reliable connection that is less sensitive to bolt torque or minor surface imperfections. Choke flanges are almost always used in outdoor or harsh environments, such as on radar antennas mounted on ships or aircraft.

Material and Manufacturing Considerations

The choice of material for a waveguide flange is a critical decision that balances electrical performance, environmental durability, weight, and cost. Aluminum is the most common choice for the waveguide and flange itself. It’s lightweight, has excellent conductivity, is easy to machine, and is relatively low-cost. For many commercial and aerospace applications, aluminum is the default material.

However, in environments where corrosion is a concern—such as marine applications—or where higher strength is required, brass or stainless steel may be used. While stainless steel has lower electrical conductivity, it can be plated with a highly conductive material like silver or gold to ensure low surface resistance. The plating is a crucial step; it ensures a low-loss contact surface between the mating flanges. The precision of the machining process is non-negotiable. Any deviation in the flatness of the mating surface or the alignment of the bolt holes can lead to gaps, which cause impedance discontinuities, signal reflections, and power loss.

Practical Installation and Alignment Tips

Getting the right flange is only half the battle; installing it correctly is what ensures peak system performance. The first rule is to always use the correct hardware. Using a #4-40 screw on a UG-385/U flange meant for #2-56 screws will damage the bolt holes and compromise the connection. Similarly, proper torque is vital. Under-torquing can lead to a loose connection and arcing at high power levels, while over-torquing can warp the flange. A torque screwdriver, calibrated to the specifications for the specific screw size (often around 5-8 inch-pounds for smaller screws), is a necessary tool for any professional installation.

Alignment is another critical factor. Before tightening the bolts, ensure the waveguide runs are physically supported to prevent strain on the flange faces. The flanges should be brought together evenly. A best practice is to tighten the bolts in a crisscross pattern, similar to tightening a car wheel, in multiple passes, gradually increasing the torque to the final value. This ensures even pressure across the mating surface, creating a uniform seal. For the highest frequency applications, visual inspection of the flange faces for nicks, scratches, or dust is recommended before connection.

Understanding these nuances—from the standard UG numbers and their dimensions to the practicalities of installation—is what separates a functional microwave system from a high-performance one. The integrity of every connection point, defined by its flange, directly impacts the overall efficiency and reliability of the entire assembly.