TEM Wave vs. Quasi-TEM Wave: Key Differences Explained

This page compares TEM (Transverse Electromagnetic) waves and quasi-TEM waves, outlining the key differences between them.

TEM Wave

A TEM wave, short for Transverse Electromagnetic wave, is a mode where the electric field (E) and magnetic field (H) are perpendicular to each other and perpendicular to the direction of propagation. Think of it as all the action happening sideways to where the wave is going.

This mode typically exists in structures that have two excited conductors surrounded by a dielectric material.

Example: Coaxial Cable

A classic example is a coaxial cable.

In a coax cable:

• The electric field (E) is in the radial direction (pointing outwards from the center conductor).
• The magnetic field (H) forms concentric circles around the center conductor.
• The wave propagates along the length of the cable.

Quasi-TEM Wave

The term “quasi” means “resembling.” So, a quasi-TEM wave resembles a TEM wave but isn’t quite the same thing. These modes are commonly found in microstrip lines.

Microstrip Line Scenario

In a microstrip, you have a conductor on top, a dielectric substrate in the middle, and a ground plane on the bottom. The wave propagates both through the air above the top conductor and through the dielectric substrate.

Because air and the dielectric substrate have different permittivities (and therefore, different resistivities), the wave travels at different speeds in these two regions. This difference in speed is what makes it a quasi-TEM mode rather than a pure TEM mode.