---
title: "Short note: TEM"
subject: "Electromagnetism"
chapter: 6
key: "semester_3/electromagnetism:short-note-tem"
source: https://www.bctengineer.com/q/semester_3/electromagnetism/short-note-tem
site: BCT Engineering
---

# Short note: TEM

Electromagnetism · Chapter 6: Wave guides

- Asked in: 2068 Baisakh, 3 marks
- Video: [Lecture 10b---TEM vs TE/TM](https://www.youtube.com/watch?v=4bJUyxgUNnQ) — James Nagel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electromagnetism/short-note-tem
- Whole chapter: https://www.bctengineer.com/md/semester_3/electromagnetism/chapter-6

## Question

Write a short note on TEM.

## Short answer

TEM waves have E_z = H_z = 0 with E, H, and propagation mutually perpendicular. They have no cutoff, propagate from DC with phase velocity v = 1/√(με) (non-dispersive), and require at least two conductors; a hollow waveguide cannot support them.

## Answer

A **TEM (transverse electromagnetic) wave** is a wave in which both the electric and the magnetic field lie entirely in the plane transverse to the direction of propagation: E_z = 0 and H_z = 0. E, H and the direction of travel are mutually perpendicular.

Properties:
- No cut-off frequency: it can propagate from DC upward.
- Phase velocity v = 1/√(με), independent of frequency, so it is non-dispersive.
- Intrinsic (wave) impedance η = √(μ/ε) = E/H (377 Ω in free space); on a line, the wave appears as voltage and current waves with Z₀ = √(L/C).
- The transverse field distribution is the same as a static field with the same boundary conditions.

Occurrence:
- Uniform plane waves in free space or a lossless dielectric.
- Waves on two-conductor transmission lines such as coaxial cable, parallel-wire lines and stripline (microstrip, with its mixed air-dielectric cross-section, supports only a quasi-TEM wave).

A TEM mode needs at least two conductors, so a hollow single-conductor waveguide cannot support it and carries TE or TM modes instead.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which statement defines a TEM wave?
   A. E_z = 0 and H_z = 0
   B. E_z = 0 only
   C. H_z = 0 only
   D. E and H both along the direction of travel
   Answer: A. In TEM both fields lie in the transverse plane, with E, H and propagation mutually perpendicular. (Key point: TEM definition)

2. What is true of a TEM wave's cut-off and phase velocity?
   A. It has a cut-off, and v depends on frequency
   B. It has no cut-off and v = 1/√(με), independent of frequency
   C. It has a cut-off and is non-dispersive
   D. It needs f above a cut-off and v > c
   Answer: B. TEM propagates from DC upward and is non-dispersive. (Key point: TEM cut-off and dispersion)

3. Which structure supports TEM waves?
   A. Hollow rectangular waveguide
   B. Hollow circular waveguide
   C. Coaxial cable
   D. Any single closed metal tube
   Answer: C. TEM needs at least two conductors, as in coax or parallel-wire lines. (Key point: Where TEM exists)

4. What is the intrinsic impedance of a TEM wave in free space?
   A. √(L/C)
   B. 377 Ω = √(μ/ε)
   C. 73 Ω
   D. 50 Ω
   Answer: B. η = √(μ/ε) = E/H, which is 377 Ω in free space. (Key point: Wave impedance)