CHAPTER 2 - Conveying Power at Radio Frequency
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2.7 Standing waves and resonance
Whenever there is a mismatch of impedance between transmission line and load, reflections
will occur. If the incident signal is a continuous AC waveform, these reflections will mix with more of
the oncoming incident waveform to produce stationary waveforms called standing waves.
Figure 2.7-1 shows how a triangle-shaped incident waveform turns into a mirror-image
reflection upon reaching the line's unterminated end. The transmission line in this illustrative
sequence is shown as a single, dotted line rather than a pair of wires, for simplicity. The incident
wave is shown travelling from left to right, while the reflected wave travels from right to left.
After the incident signal hits the end of the line, the overall voltage observed on the line will
be the sum of incident and reflected voltages. Figure 2.7-1 shows the step by step transients of the
total voltage along the line up until the reflected voltage reaches the source end. After this transient
time, what we observe on the line is the standing wave shown in Figure 2.7-2. This is representative
of the steady-state voltage along the line, i.e. the sum of incident and reflected voltage waves. This
standing wave oscillates in instantaneous magnitude, but does not propagate down the cable's
length like the incident or reflected waveforms causing it. Note that the "zero" points of the standing
wave, where the incident and reflected waves cancel each other, never change position (Figure
2.7-2).
In the next section we will examine these phenomena in more detail for a range of terminations.
Conquer Radio Frequency
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