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

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

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.

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:

(

)

RL -ZO

ER = Ei

RL +ZO

Where:

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.

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