MEASURING THE ATTENUATION OF A LINE

The ohmic resistance R′ and the conductance G′ are responsible within the line of the energy dissipation in the form of heat. This occurs within both the conductor and the dielectric material. These losses, which determine the attenuation characteristics of the line, are expressed in terms of “attenuation constant”, represented with the symbol “a”, and can be calculated through the following formula:

av = 20 . Log (V1/V2)

Where:

V1 = amplitude of the signal at the input of the line.

V2 = amplitude of the signal at the output of the line.

av = voltage attenuation in dB.

The purpose of this test is to measure the attenuation for the different trunks of the transmission line.

Procedure

F10-3 shows the method for the measurement to be performed. One end of the line is closed on the rated load Rt, while on the other end the sine-wave signal generator is connected.

The purpose of the resistance Ri is to match the generator to the line.

The value of Ri is equal to the load resistance connected to the output of the line, i.e. 68 ohms. This is true when the signal generator used has a neglectable internal impedance when compared to this ohmic value. In the opposite case it will be necessary to externally provide a resistance Ri whose value is such that, when added to the internal impedance of the generator, the result is 68 ohms.

The test is performed by setting the generator for an output signal of 3 V rms and a frequency of 50 kHz.

By using either the oscilloscope or the multimeter, the amplitude are measured that can be detected at the input of the line and at 25, 50, 75 and 100 meters.

The results are gathered in tabular form t be processed in the following.

The attenuations in the points where the measure has been performed are calculated through the following formula:

av = 20 . Log (V1/Vn)

Where:

Vn = amplitude of the signal measured at the distances of 25, 50, 75 and 100 m.

F10-3: Measuring the attenuation.