Transmission Line Effect
The goal of this experiment
Investigate the characteristics of a low-loss transmission line. The characteristic impedance of the transmission line under test is obtained by the experiment. The capacitance and inductance per unit length can also be found. The students are required to understand the physical meanings of voltage reflection coefficient, voltage standing wave ratio, etc.
Tools used in this experiment
1. A microwave signal generator,
2. A voltage standing wave ratio (VSWR) meter,
3. A slotted transmission line (its characteristics to be found), and
4. Purely resistive loads of various known values.
The electrical properties of a transmission line at a given frequency are completely characterized by its four distributed parameters, R, L, G, and C. Where:
R (W/m) is conductor loss.
L (H/m) is store magnetic energy.
G (S/m) is dielectric loss.
C (F/m) is store electric ...view middle of the document...
When measuring the VSWR using the VSWR meter, the probe is moved along the line and the positions of the voltage minima are recorded. The distance between two adjacent minima is l/2, and then the value of wavelength can be found. In principle, voltage maxima locations could be used as well as voltage minima positions, but that voltage minima are more sharply defined than voltage maxima and so usually result in greater accuracy. If the value of the resistive load is close to that of the characteristic impedance of the line, the difference between the values of voltage maxima and minima is so small that it is difficult to accurately determine the minima positions. Discard that load and choose another one.
1. Set up the apparatus as shown in Fig. 1. Select a suitable frequency of the microwave generator (frequencies between 2 to 5GHz are recommended). Record the selected frequency.
2. Connect one of the loads, ZL, to the slotted line.
3. Move the probe of the VSWR meter along the line. Record the positions of the voltage minima and the value of VSWR, S.
4. Calculate the distance between the minima positions and hence the wavelength, l.
5. Repeat from step 2 using another load until all the loads have been used.
6. Plot ZL against S on the graph as shown in Fig. 1.
7. Determine the characteristic impedance, Zo, from the slope of the curve on the graph.
8. Take the average of the values of l's obtained from step 4.
9. Calculate the values of L and C using the results from steps 7 and 8.
In general, a transmission line is a material medium or structure that forms a path for directing the transmission of energy from one place to another, such as electromagnetic waves or acoustic waves, as well as electric power transmission. In this experiment, we calculate the characteristic impedance (Zo) of a transmission line represent the ratio of the amplitude of a single voltage wave to its current wave in order to approve that most transmission lines have a reflected wave, and have some kind of low-loss in transmission process.