Waveguide Flanges

Waveguide flanges are used to enable different items of waveguide to be connected together. They are the equivalent of connectors for coax.

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Waveguide flanges are used to enable waveguide to be joined to other lengths of waveguide or to equipment that uses a waveguide interface.

Waveguide flanges have the characteristic metal interface that locates and enables the two interfaces to be tightly bolted together.

Waveguide flanges come in a variety of formats. These have been standardised to enable waveguide from different manufacturers to be joined perfectly well, provided they both conform to the same waveguide standard.

Waveguide flange designations & terminology

There are a number of different designations and abbreviation used to describe the different waveguide flange types. Several of these abbreviations for waveguide flanges are summarised in the table below:

Waveguide Flange 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 important aspect of any waveguide flange is the leakage that can occur at the joint. This results from the fact that as the joint are formed from a metal to metal joint and metal contact, any imperfections in the waveguide flange surface or dirt can result in an imperfect contact.

There are two ways that have been adopted to overcome this:

Use of an 'O' ring: Many waveguide flanges incorporate a grove cut in either surface so that a gasket can be added
Use of a thin metal gasket: Another method of reducing the leakage is to place a thin metal shim or gasket between the two surfaces. The metal used in this is slightly compressible enabling any imperfections in the surface to be taken up.

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 -130dB, 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.

Waveguide flanges are machined to high tolerances. As such they perform well and even though they are costly, they perform well an enable a system to be bolted together from individual components with relative ease.