Waveguide vs. Coaxial Line: Key Differences Explained

Both waveguides and coaxial lines serve the purpose of carrying electromagnetic waves, but they operate with different frequencies and principles. This article delves into the functions, modes, advantages, and disadvantages of each, highlighting the key differences between them.

Coaxial Line

Figure depicts typical coaxial line. As shown it consists of center conductor and outer conductor.

As illustrated above, a typical coaxial line consists of a center conductor and an outer conductor. The fundamental, or dominant, mode wave in a coaxial line is Transverse ElectroMagnetic (TEM).

Since the dominant mode is TEM, there isn’t a cutoff frequency in a coaxial line cable for that mode. However, as the frequency increases, the wavelength becomes comparable to the dimensions of the coaxial cable. This can lead to the propagation of higher-order, non-TEM modes. These modes are generally undesirable as they increase attenuation and steal power from the dominant mode. The lowest non-TEM wave starts propagating from a wavelength given by the following equation:

λ = π/(a+b)

Waveguide

Figure depicts waveguide

Waveguides come in two primary structures: circular and rectangular. As shown in the figure, a waveguide comprises a single metallic wall that acts as a conductor. Crucially, there is no center conductor in a waveguide. Consequently, TEM waves cannot propagate through it, meaning there’s no conduction current. Energy transfer in a waveguide occurs using Transverse Electric (TE) or Transverse Magnetic (TM) modes.

For a deeper understanding of TE and TM modes, refer to resources explaining the difference between TE and TM waves. You can also explore the basics of TEM and quasi-TEM modes.

Key Differences: Waveguide vs. Coaxial Line

Here’s a breakdown of the major differences between waveguides and coaxial lines:

• Manufacturing: Waveguides, lacking a central conductor and typically being air-filled, are often easier to manufacture.
• Loss: Waveguides, being air-filled, generally exhibit less loss compared to coaxial lines. In a waveguide, minimal power is lost through radiation, and dielectric loss is negligible.
• Power Handling: Waveguides can handle significantly higher power levels than coaxial cables. This is primarily because the air dielectric within the waveguide has a high breakdown voltage (around 30 kV/cm).
• Size and Weight: The metallic outer wall of a waveguide makes it bulkier, heavier, and often more expensive than a coaxial line. Coaxial lines are smaller and lighter, making them suitable for a wider range of microwave applications.
• Wall Conductivity: The wall of a waveguide isn’t perfectly conducting, leading to some power loss as heat within the wall.
• Bandwidth: Waveguides have a narrower bandwidth compared to coaxial lines, which are used for broadband applications.
• Uniformity: Waveguides require a uniform cross-section to prevent mode conversion or the generation of higher-order modes. This isn’t as critical in coaxial lines.