Electromagnetic fields are formed as voltage and current waves travel down a transmission line. As propagation frequency increases a portion of the associated wavelength becomes comparable to the geometry of the transmission line dimensions. As the frequency increases, different types of propagation modes appear. The principal mode is the one which can carry energy at all frequencies. Higher modes are those that propagate only above a definite frequency range, and the point at which these frequencies start to propagate is called the cut-off frequency for that particular mode. An analogy is shown in Fig. 7.6.
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Figure 7.6(a) shows particles moving down a tube. In this case the particles have similar cross-sectional dimensions to the tube, and if the particles are given a constant velocity then a certain propagation will exist. This is analogous to single-mode propagation. If the diameter of the tube is increased or the particle size decreased, then the particles would propagate down the tube in different modes as shown in Fig. 7.6(b). This is analogous to high-order modes. If the particle size is further decreased the number of modes or propagation patterns increases. Hence certain patterns will occur when a definite change in size occurs either in the tube dimensions or in the propagating frequency.
A two-wire transmission line exhibits electric and magnetic fields as current flows along it. Both these fields are perpendicular to one another and also to the direction of propagation. Hence in the principal mode these waves are called transverse electromagnetic (TEM) waves.
If the frequency of transmission is increased so that the wavelength becomes similar to the cross-sectional dimensions of the transmission line higher-order modes can propagate. These higher-order modes will have at least one of their field components showing in the propagation direction, i.e. either the H-wave (magnetic mode) or the E-wave (electric mode). Depending on the mode involved it will be called transverse electric (TE) or transverse magnetic (TM).
The velocity of the TE and TM modes is different from that of the TEM mode. Two types of velocity are involved, namely group velocity (the velocity of the entire moving group) and phase velocity (including all path movements of the particles).
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