Waveguide flanges

- details of RF waveguide flanges, their use, performance, and standards used when describing waveguide flanges.

In order that sections of waveguide may be joined, waveguide flanges are used. These waveguide flanges come in a variety of standard formats, enabling the right types of waveguide flange to be used for the given application, but also being standardised, flanges from different manufacturers can mate together, provided they conform to the same style or standard.

Waveguide flange designations and terminology

There are a number of different designations for waveguide flange types and also different abbreviations that apply to them. These are summarised in the table below:

Waveguide Flange
Designation or Terminology
Waveguide Flange Designation or Terminology Details and Information
Choke UG style waveguide flanges with an o-ring groove and a choke cavity.
CMR CMR waveguide flanges are the miniature version of the Connector Pressurized Rectangular (CPR) style flanges.
CPRF Connector Pressurized Rectangular (CPR) refers to a range of commercial rectangular waveguide flanges. CPRF is flat CPR flange.
CPRG Connector Pressurized Rectangular (CPR) refers to a range of commercial rectangular waveguide flanges. CPRG is Grooved CPR flange.
Cover or Plate Square, flat UG style waveguide flanges
UG UG is the military standard MIL-DTL-3922 for a range of waveguide flange types

Waveguide flange leakage

One aspect of waveguide flanges that is of particular importance is the leakage that occurs across the joint. As the joints across the waveguide flanges are metal to metal contact, and they may not be completely flat and perfect some leakage will always occur.

In order to improve the waveguide flange leakage caused by imperfect metal surfaces joining together, many waveguide flanges incorporate a grove cut in either surface so that a gasket can be added.

The measurement of the actual leakage from a waveguide flange is very difficult. To attain a level of consistency across measurements a standard procedure with defined test equipment and a given environment need to be adopted.However it is found that in general measurements made of the fields made using probes show a sharp peak around the edge of the waveguide flange connection. Levels are typically around -130dBc, which indicates a low level of leakage. To achieve this, the waveguide flange surfaces must be clean and bolts must be tightened to the required torque level. Good RF gaskets also ensure these levels are maintained or improved upon.

Waveguide flange insertion loss

As is likely to be anticipated there will always be some loss, even if small, caused by the introduction of a joint, including the flange.

The waveguide flange insertion loss will arise mainly from two main factors:

  • Loss arising from leakage:   The leakage through the joint between two waveguide flanges is normally small, but in some instances a poor joint may give rise to measurable levels of loss due to leakage.
  • Loss arising from flange resistance:   If the two waveguide flanges are not bolted together tightly enough, there will be resistance between the flanges. As the waveguide relies on the conduction in the surface of the waveguide for its transmission, the resistance between the two waveguide flanges is critical. Additionally the resistance of the waveguide surface is crucial because of the skin effect which is very pronounced at these frequencies. Accordingly the resistance of the waveguide flanges is particularly important in the region closes to the cavity.

Normally losses are low, but precautions must be taken when using waveguide flanges to ensure that the joints are well made - the surfaces should be clean and free from oxide and small particles. Also gaskets should be used with the waveguide flanges if appropriate.

Waveguide flange resistance and bolt torque

To ensure that a waveguide flange does not leak and also provides a low level of loss across the join, the force between the two adjacent waveguide flange faces must be sufficient to prevent leakage. In turn this means that the bolts must be torqued to the recommended specification.

It is generally accepted that there must be a force of 1000 lb / linear inch of waveguide flange connection to give a satisfactory seal for high power applications. Also for low power applications, this will provide for lower levels of loss.

By Ian Poole

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