Basics of TEM, TE, and TM propagation

Energy can propagate through a medium or through what is currently believed to be a vacuum. Propagation modes vary depending upon the type of energy and the nature of the medium.

Consider sound: It can travel through any material, solid, liquid or gas. In air, sound consists of waves, discrete like a single click, or continuous, like a sustained musical tone. The waves are alternately more compressed and more rarified regions of air that radiate in concentric shells, wherever there is air. The transitions between positive and negative peaks may be smooth as in a sine wave, or abrupt as in a square wave, and there are infinite variations.

To humans, the differing waveforms are perceived as changes in sound, so we are able to recognize and differentiate between flute and saxophone, violin and French horn. Frequency variations are perceived as changes in pitch. Amplitude translates to volume. Both frequency and volume are perceived in accordance with a logarithmic scale. The volume, but not the pitch, varies with the cube of the distance.

In regard to energy conveyed through a body of water, the situation is radically different. There are two reasons. For one thing, water is far less compressible than air. The other reason is that there is a boundary condition, where water ends and air above it begins. It is here that wave action can be observed. As waves crest, the water rises above the interface, strongly opposed by gravity since water is much heavier than air. These waves become larger and the energy more visible as they approach the ocean shore, because there is less volume of water below the surface to convey all that energy.

Electromagnetic energy is also transmitted in waves. Like sound, it may propagate equally in all directions, the wave energy radiating in concentric shells corresponding to amplitude. Or it may be directional, focused to form a tight beam.

Light has a dual nature, particle or wave. It is a form of electromagnetic radiation and has associated with it electric and magnetic fields. As in other forms of electromagnetic radiation, both fields are oscillating. Depending on the transmission mode, the axis of oscillation in electromagnetic transmission may have different orientations to the direction of travel.

In the Transverse Electric and Magnetic (TEM) mode, both the electric field and the magnetic field (which are always perpendicular to one another in free space) are transverse to the direction of travel. If you wonder how this is possible, bear in mind that it is happening in three-dimensional space.

In the Transverse Electric (TE) mode, the electric field is transverse to the direction of propagation while the magnetic field is normal to the direction of propagation.

In the Transverse Magnetic (TM) mode, the magnetic field is transverse to the direction of propagation while the electric field is normal to the direction of propagation.

Hollow, metallic waveguides are compatible with TE and TM modes only. This is in contrast to coaxial cable. When it is functioning as intended, the TEM mode of propagation is supported. Coaxial cable involves two conductors, the inner pin and the outer grounded shield. For this reason, capacitive losses preclude transmission at the highest frequencies. In a waveguide, including optical fiber, the outer shell or cladding functions only as a reflecting surface, so propagation takes place in one direction only, which is why there are no capacitive losses.