Rectangular Waveguide Dimensions and the WR Series

Key Takeaways

• The standardization of waveguide dimensions is important, as it enables the use and integration of waveguides from different manufacturers in a common application or system.

• Common waveguide standards include the IEEE, IEC, EIA, and MIL standards.

• WR90 rectangular waveguide dimensions are most suitable for microwave link bands of frequencies 10GHz, 10.5 GHz, or 11 GHz. 

Rectangular waveguides and waveguide interfaces are common in high-frequency circuits when the signal to be transmitted is in the range of 100GHz and above. Rectangular waveguides and interfaces such as flanges make a reliable transmission medium for RF signals. The rectangular waveguide dimensions and their interface dimensions play an important role in achieving reliable signal transmission. The cut-off frequency and modes of propagation are dependent on waveguide dimensions. Similarly, according to EIA standards, waveguide flanges are chosen based on rectangular waveguide dimensions.

In this article, we will discuss the importance of waveguide dimensions, particularly in rectangular waveguides.

Waveguide Dimensions 

Waveguide dimensions are a significant factor in the performance of high-frequency signal transmission systems. It is important to note that depending on the signal frequency bands, the waveguide dimensions change. To cope with this, there are several waveguide shapes (such as rectangular, circular, parallel plate, etc.) and sizes available. 

Standardization of Waveguide Dimensions

The standardization of waveguide dimensions enables the use and integration of waveguides from different manufacturers in a common application or system. Standardization also allows the user to incorporate the waveguides of known properties in the system design. 

Determining Waveguide Properties

Waveguide dimensions determine waveguide properties. Waveguide parameters such as modes of propagation, cut-off frequency, and dominant modes are dependent on waveguide dimensions. The standardization of waveguide dimensions makes the designer choose the waveguide that matches the design specification from the standard array of waveguides. 

Waveguide Standards 

To connect the shape and size of a waveguide and to generalize the waveguide dimensions globally, waveguide standards are published. Current waveguide standards are set by agencies such as IEEE, MIL, EIA, IEC, etc. All these standards focus on the waveguide dimensions corresponding to particular shapes and the interfaces to be used along with each standard waveguide and frequency band. 

The two main waveguide standards are IEEE standards and IEC standards. The other standards referred to by manufacturers are the Electronic Industries Alliance Standards (EIA) and the United States Military (MIL) standards.

In every waveguide standard, alphanumeric letters are used to designate the waveguide size. Generally, the designation consists of two letters indicating the type of waveguide followed by numerals denoting the waveguide width. The letters commonly used in designators correspond to the series of standards pertaining to a particular waveguide shape. The commonly used series of waveguide standards are WG and WR, which represent the waveguide transmission line and waveguide rectangular, respectively. 

Rectangular Waveguide Dimensions 

Rectangular waveguide dimensions need to satisfy certain conditions to function as intended. The fundamental condition for any waveguide design, including rectangular waveguides, is satisfying the minimum cross-section. The cross-section of a waveguide should be large enough to accommodate one complete wavelength of the signal. If the cross-section is less than the wavelength, then there is no propagation of the electromagnetic signal. The waveguide width is crucial in determining the lower cut-off frequency and the ideal dimension equals the half wavelength of the lower cut-off frequency.

WR Series

The waveguide standard WR series gives the dimension and other parameters, such as cut-off frequency and propagation modes, related to rectangular waveguides. WR standards provide information concerning frequency bands and the inner dimensions of rectangular waveguides. The equivalent double rigid rectangular waveguide, referred to as the WG equivalent for any standard rectangular waveguide, can be selected with the help of WR series standards. Let’s familiarize ourselves with the WR series and its parameters by using an example: the rectangular waveguide WR90, based on EIA standards. 

WR90 Dimensions

The  WR90 is a standard rectangular waveguide made of quality aluminum, which is both lightweight and sturdy. It is useful for applications in the frequency range between 8.2GHz to 12.4 GHz (X-band of frequency). Here are a few facts to note:

• The inner dimensions of the WR90 rectangular waveguide are 0.9 and 0.4 inches, where the longest dimension corresponds to the length, denoted by ‘a’ in the rectangular waveguide schematics. 
• The internal dimension digits follow the designator WR in the standard rectangular waveguide series, hence the name WR90. 
• The breadth of the rectangular waveguide, denoted by ‘b’, (0.4 inches in the case of WR90) is always less than the length. 
• The rectangular waveguide dimensions define the cut-off frequency at the lowest order mode, which is 6.557GHz for the WR90 waveguide. The cut-off frequency of the next mode for WR90 is equal to 13.114 GHz. 

By knowing these facts about WR90, it is easy to check whether the standard rectangular waveguide is suitable for the given application. WR90 rectangular waveguide dimensions are most suitable for microwave link bands of frequencies 10GHz, 10.5 GHz, or 11 GHz. In case the WR90 waveguide is not available, two equivalent waveguides can be used: the RCSC standard (British Military standards) WG16 and the IEC standard R100.

The WR series covers rectangular waveguide dimensions including the frequency band, cut-off frequencies, and the equivalent IEC and RCSC waveguides. Apart from the standard waveguide structure, there are several variants of the WR series waveguides available with horns, adapters, and other standard interfaces. Designers can use Cadence software to design rectangular waveguides with or without horns, adapters, or other interfaces.

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