Transmission Line Effect

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.

Theory

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 energy.

A signal propagating down a transmission line is generally suffered from attenuation or reflection. The voltage and current amplitudes of the signal vary at different positions of the line and different time. Characteristic impedance, Zo, is the impedance of an infinitely long transmission line. On a transmission line of infinite length, no reflected wave is present. The characteristic impedance is defined as Zo = Vo/ Io.

The apparatus shown in Fig. 1, a VSWR meter is used to measure the voltage standing wave ratio on a slotted line. Slotted line is a transmission line configuration that allows the sampling of the electric field amplitude of a standing wave on a terminated line.

Figure 1

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...