Terminology » Waveguide » Rectangular vs. Circular Waveguides: Key Differences

Rectangular vs. Circular Waveguides: Key Differences

waveguide microwave rf circular rectangular

Waveguides are crucial in RF and microwave communication, efficiently guiding electromagnetic waves. Rectangular and circular waveguides offer unique advantages for specific applications. This guide explores the differences between them in terms of structure, operating frequency, mode propagation, and applications, helping you make an informed choice. Both waveguide types are metal hollow structures designed to guide EM waves. Their shapes define them as either rectangular or circular. Both behave much like a High Pass Filter and are passive microwave devices.

Rectangular Waveguide

A rectangular waveguide is a type of transmission line with a rectangular cross-section, widely used in RF and microwave systems. It supports electromagnetic wave propagation with minimal loss, typically in the dominant $TE_{10}$ mode. Rectangular waveguides are popular due to their simple design, ease of manufacturing, and high efficiency at specific frequencies. They are commonly employed in radar, satellite communication, and industrial heating applications, where precise signal control and low attenuation are critical.

Rectangular waveguide

Figure depicting a Rectangular waveguide with broad and narrow dimensions.

The cutoff wavelength equation for a rectangular waveguide is defined below:

waveguide cutoff wavelength equation

Where:

$m$ = number of half-waves along the broad side dimension
$n$ = number of half-waves along the shorter side

The following table lists cutoff wavelengths and cutoff frequencies for common modes in a rectangular waveguide.

Mode	Cutoff Wavelength ( $\lambda_c$ )	Cutoff Frequency ( $f_c$ )
$TE_{10}$	$2*a$ , $a$ =broad dimension	$(\frac{1}{\sqrt{\mu\epsilon}}) * (\frac{1}{2a})$
$TE_{11}$ , $TM_{11}$	$\frac{2ab}{\sqrt{a^2 + b^2}}$	$(\frac{1}{\sqrt{\mu\epsilon}}) * (\frac{\sqrt{a^2 + b^2}}{2ab})$
$TE_{20}$	$a$	$(\frac{1}{\sqrt{\mu\epsilon}}) * (\frac{1}{a})$
$TE_{01}$	$2*b$	$(\frac{1}{\sqrt{\mu\epsilon}}) * (\frac{1}{2b})$

Circular Waveguide

This waveguide features a cylindrical cross-section, offering unique advantages such as rotational symmetry and support for multiple polarizations. It operates predominantly in the $TE_{11}$ mode and is often chosen for applications requiring high power handling or specific mode propagation characteristics. Circular waveguides are widely used in radar systems, rotating joints, and industrial applications where smooth internal surfaces help minimize signal losses. However, their complex manufacturing process and mode degeneracy can be challenging for certain designs.

Circular waveguide

Figure depicting a Circular waveguide.

The cutoff frequency equation for a circular waveguide $f_c$ is:

$f_c = \frac{1.8412 * c}{2 * \pi * a}$

Where:

$c$ is the speed of light within the waveguide
$a$ is the radius of the circular cross-section

The dominant mode in a rectangular waveguide is $TE_{10}$ , and in a circular waveguide, it is $TE_{11}$ . Rectangular to circular waveguide transitions convert the dominant $TE_{10}$ mode of a rectangular waveguide to the $TE_{11}$ dominant mode of a circular waveguide and vice versa.

The following table shows cutoff wavelengths and frequencies for various common modes in a circular waveguide.

Mode	Cutoff Wavelength	Cutoff Frequency
$TE_{11}$	$1.706d$ , $d$ =diameter of the waveguide	$(\frac{1}{\sqrt{\mu\epsilon}}) * (\frac{1}{1.706d})$
$TM_{01}$	$1.306d$	$(\frac{1}{\sqrt{\mu\epsilon}}) * (\frac{1}{1.306d})$
$TE_{21}$	$1.028d$	$(\frac{1}{\sqrt{\mu\epsilon}}) * (\frac{1}{1.028d})$
$TM_{11}$	$0.820d$	$(\frac{1}{\sqrt{\mu\epsilon}}) * (\frac{1}{0.820d})$

Difference Between Rectangular and Circular Waveguides

Aspect	Rectangular Waveguide	Circular Waveguide
Shape	Rectangular cross-section	Circular cross-section
Mode of Propagation	Dominant mode is $TE_{10}$	Dominant mode is $TE_{11}$
Cutoff Frequency	Lower cutoff frequency	Higher cutoff frequency
Power Handling	Suitable for lower to moderate power	Better for high power applications
Losses	Higher loss due to sharper corners	Lower loss due to smoother internal surface
Manufacturing	Easier and less expensive	More complex and costlier
Mode Degeneracy	Modes are distinct with no degeneracy	Higher likelihood of mode degeneracy
Field Distribution	Fields are concentrated at edges	Fields are evenly distributed
Rotational Symmetry	No rotational symmetry	Rotationally symmetric
Polarization	Supports single polarization per mode	Supports multiple polarizations

Conclusion

Understanding the differences between rectangular and circular waveguides is essential for selecting the right one for your RF application. While rectangular waveguides are widely used due to their simplicity and versatility, circular waveguides are advantageous in applications requiring specific mode propagation or rotational symmetry. Evaluate your requirements to determine the most suitable option for your system.