|
| |
Back
Figure 3-22.—Instantaneous voltages along a transmission line | Up
Neets Module 10-Introduction to Wave Propagation, Transmission Lines, and Antennas | Next
Figure 3-23.—Dc applied to an equivalent transmission line |
3-24
3. The instantaneous voltages (oscilloscope displays) are the same in all cases except that a phase
difference exists in the displays seen at different points along the line. The phase changes
continually with respect to the generator until the change is 360 degrees over a certain length of
line.
4. All parts of a sine wave pass every point along the line. A plot of the readings of an ac meter
(which reads the effective value of the voltage over a given time) taken at different points along
the line shows that the voltage is constant at all points. This is shown in view C of figure 3-22.
5. Since the line is terminated with a resistance equal to Z
0, the energy arriving at the end of the
line is absorbed by the resistance.
VELOCITY OF WAVE PROPAGATION
If a voltage is initially applied to the sending end of a line, that same voltage will appear later some
distance from the sending end. This is true regardless of any change in voltage, whether the change is a
jump from zero to some value or a drop from some value to zero. The voltage change will be conducted
down the line at a constant rate.
Recall that the inductance of a line delays the charging of the line capacitance. The velocity of
propagation is therefore related to the values of L and C. If the inductance and capacitance of the rf line
are known, the time required for any waveform to travel the length of the line can be determined. To see
how this works, observe the following relationship:
Q = IT
This formula shows that the total charge or quantity is equal to the current multiplied by the time the
current flows. Also:
Q = CE
This formula shows that the total charge on a capacitor is equal to the capacitance multiplied by the
voltage across the capacitor.
If the switch in figure 3-23 is closed for a given time, the quantity (Q) of electricity leaving the
battery can be computed by using the equation Q = IT. The electricity leaves the battery and goes into the
line, where a charge is built up on the capacitors. The amount of this charge is computed by using the
equation Q = CE.
|
