___________________________________________________ Principles of Transmission Lines
TERMINATION IN CAPACITANCE
3-138. When a line is terminated in capacitance, the capacitor does not absorb
energy, but returns all of the energy to the circuit. This means there is 100
percent reflection. The current and voltage relationships are somewhat more
involved than in previous types of termination. For this explanation, assume
that the capacitive reactance is equal to the characteristic impedance (Z0) of the
line. Current and voltage are in phase when they arrive at the end of the line,
but in flowing through the capacitor and the characteristic impedance
connected in series, they shift in phase relationship. Current and voltage arrive
in phase and leave out of phase. This results in the standing-wave
configuration shown in figure 3-34, view E. The standing wave of voltage is
minimum at a distance of exactly 1/8λ from the end. If the capacitive reactance
is greater than Z0 (smaller capacitance), the termination looks more like an
open circuit; the voltage minimum moves away from the end. If the capacitive
reactance is smaller than Z0, the minimum moves toward the end.
3-139. When the line is terminated in an inductance, both the current and
voltage shift in phase as they arrive at the end of the line. When XL is equal
to Z0, the resulting standing waves are as shown in figure 3-34, view F. The
current minimum is located 1/8λ from the end of the line. When the inductive
reactance is increased, the standing waves appear closer to the end. When
the inductive reactance is decreased, the standing waves move away from the
end of the line.
TERMINATION IN A RESISTANCE NOT EQUAL TO THE CHARACTERISTIC
3-140. Whenever the termination is not equal to Z0, reflections occur on the
line. For example, if the terminating element contains resistance, it absorbs
some energy, but if the resistive element does not equal the Z0 of the line,
some of the energy is reflected. The amount of voltage reflected may be found
by using the equation:
ER = Ei
ER = the reflected voltage
Ei = the incident voltage
Z0= the characteristic impedance of the line
3-141. If you try different values of RL in the preceding equation, you will
find that the reflected voltage is equal to the incident voltage only when RL
equals 0 or is infinitely large. When RL equals Z0, no reflected voltage occurs.
When RL is greater than Z0, ER is positive, but less than Ei. As RL increases
and approaches an infinite value, ER increases and approaches Ei in value.
When RL is smaller than Z0, ER has a negative value. This means that the
reflected voltage is of opposite polarity to the incident wave at the
termination of the line. As RL approaches zero, ER approaches Ei in value.
The smaller the value of ER, the smaller is the peak amplitude of the
standing waves and the higher are the minimum values.