The fundamental difference between cover flange and choke flange sizes lies in their physical dimensions and the functional tolerances they are designed to meet. While a cover flange (also known as a flat flange) is typically a simple, flat mating surface with bolt holes, a choke flange incorporates a precisely machined groove or channel on its face that acts as a resonant cavity. This groove is the “choke,” and its dimensions—depth and width—are critically tuned to a specific frequency to create a short circuit for any stray radio frequency (RF) energy attempting to leak out between the flanges. Consequently, for a given waveguide size like WR-75, the overall outer dimensions and bolt hole patterns might be standardized, but the face of a choke flange has additional, highly specific geometrical features that a cover flange lacks. This makes the choke flange more complex to manufacture and its size specifications far more detailed.
The primary reason for these dimensional differences boils down to their sealing performance. A cover flange relies on metal-to-metal contact to form the RF seal. The flatness of the mating surfaces is paramount. Any imperfection, warping, or surface roughness can lead to gaps, resulting in significant RF leakage, especially at higher frequencies. The size and tolerance of the flange face are therefore designed to ensure the best possible physical contact when bolted together with a specific torque. In contrast, a choke flange does not depend on a perfect metal-to-metal seal. Instead, the choke groove is designed to be a quarter-wavelength deep at the operating frequency. When RF energy tries to escape through the inevitable tiny gap between the flanges, it travels down the groove, reflects back, and arrives 180 degrees out of phase, effectively canceling itself out. This means the mechanical contact requirements are less stringent than for a cover flange, but the dimensional accuracy of the groove itself is absolutely critical.
Let’s break down the specific dimensional parameters that differ. For a standard UG flange series (like UG-385/U for a WR-75 waveguide), the cover and choke flanges will share the same overall footprint. This includes the A, B, C, and D dimensions which define the outer rectangle and the bolt hole pattern. This standardization is intentional, allowing a choke flange to mate directly with a cover flange. The critical differences are in the E and F dimensions, which describe the features on the flange face.
For a cover flange, the E dimension is simply the width of the raised flat land that makes contact. For a choke flange, the E dimension defines the width of the choke groove. The F dimension for a cover flange might be the height of this land, but for a choke flange, F is the depth of the groove. This depth is not arbitrary; it is calculated to be approximately one-quarter of the guide wavelength (λg/4) at the center frequency of the waveguide’s operating band. A slight miscalculation or machining error in this depth can shift the resonant frequency of the choke, rendering it ineffective and leading to high levels of leakage.
The following table illustrates a concrete example for a common waveguide size, WR-75 (which operates in the 10-15 GHz range), comparing the key dimensional differences between its standard cover flange (UG-385/U) and choke flange (UG-387/U). All dimensions are in millimeters.
| Dimension Parameter | Cover Flange (UG-385/U) | Choke Flange (UG-387/U) | Functional Significance |
|---|---|---|---|
| Overall Flange Size (A x B) | 31.80 x 31.80 mm | 31.80 x 31.80 mm | Standardized for interoperability. |
| Bolt Circle Diameter (C) | 28.60 mm | 28.60 mm | Standardized for bolt alignment. |
| Bolt Hole Size (D) | 3.50 mm | 3.50 mm | Accepts standard hardware. |
| Critical Face Dimension (E) | Width of contact land: ~2.0 mm | Width of choke groove: ~1.5 mm | Defines the sealing mechanism. |
| Critical Face Dimension (F) | Height of land: ~0.1 mm | Depth of choke groove: ~4.7 mm (λg/4 @ ~12.5 GHz) | Most critical dimension for choke performance. |
| Leakage Performance | ~ -50 dB (highly dependent on surface flatness and torque) | ~ -90 dB or better (less dependent on torque) | Choke flange offers superior RF sealing. |
As you can see from the table, the most dramatic difference is in the F dimension. The choke groove’s depth is a dominant feature, whereas the cover flange’s contact land is minimal. This dimensional difference directly translates to the leakage performance, with choke flanges typically offering 40 dB or better isolation compared to their cover flange counterparts. This makes them indispensable in high-power systems where even minor leakage can cause interference or heating, and in sensitive receiver systems where external signals leaking in can desensitize the front end.
The application space is another angle that dictates the choice and, by extension, highlights the importance of their size differences. Cover flanges are perfectly adequate for many benchtop testing scenarios, low-power applications, and systems operating at lower microwave frequencies (e.g., below 8 GHz) where leakage is less of a concern. They are simpler, lighter, and more cost-effective. However, when you step into the world of satellite communications (SATCOM), radar systems, or high-power terrestrial microwave links, the choke flange becomes the default choice. In a satellite uplink, for example, the high-power amplifier (HPA) might be transmitting kilowatts of power. A poorly sealed flange could leak enough energy to heat up adjacent components or even cause arcing, leading to system failure. The precise dimensions of the choke groove are what prevent this disaster.
Manufacturing and cost implications are directly tied to these size and tolerance differences. Producing a cover flange is relatively straightforward. It involves precision milling to achieve the flatness and the bolt hole locations. The tolerances are tight, but they are conventional machining tolerances. A choke flange, however, requires much more sophisticated machining. Cutting the narrow, deep groove with a precise depth and a good surface finish is challenging. It often requires specialized tooling and post-machining processes like plating to ensure the groove walls are smooth and conductive. This added complexity naturally increases the cost and lead time for choke flanges significantly compared to cover flanges. When selecting components for a bill of materials, an engineer must weigh the superior performance of the choke flange against its higher cost and determine if the application truly requires it.
Finally, it’s impossible to discuss these differences without considering the frequency of operation. The performance gap between cover and choke flanges widens as the frequency increases. At Ka-band (26-40 GHz) and above, wavelengths are so short that even a micron-level gap in a cover flange joint can act as a significant radiator. The quarter-wave choke design becomes not just beneficial but essential for containing the RF energy within the system. The dimensions of the choke groove scale directly with the wavelength. For a V-band waveguide (WR-15, 50-75 GHz), the groove depth (the F dimension) will be much smaller than for the WR-75 example above, requiring even greater machining precision. This is why a comprehensive understanding of waveguide flange sizes and types is non-negotiable for microwave engineers working on cutting-edge systems.