MEASURING THE CHARACTERISTICS OF A LINE

The first test proposed consists in experimentally measuring the characteristics (R′, L′, G′, C′, Z₀ and γ) for the transmission line included in DL 2597.

Required components, instruments and accessories

1. Coaxial cable (100 meters) with intermediate sockets.
2. Connecting cables and chords.
3. Universal measuring bridge.
4. Digital multimeter.

Procedure

F10-2 synthetically shows the modalities for the measurement to be performed. Both the inductance and the ohmic resistance of the line are measured by short-circuiting an end of the line and connecting the measuring instruments to the other end. Instead, the capacitance and the conductance are measured by operating on the open line.

The resistance R and the conductance G are measured with an ohmmeter. For the conductance to be measured an ohmmeter is required which is able to perform resistance measurements with a range greater than 100 Mhoms.

For the measurement of both the inductance L and the capacitance C a measuring bridge is required.

The results of these measurements will be values of R, L, C and G referred to the cable length that, in out case, is of 100 meters.

In accordance with the knowledge acquired from the theoretical course, the characteristic impedance Z₀ can be calculated from the measured values through the following formula:

In the great majority of applications the lines are used in working frequency ranges whose terms of series resistance R′ and parallel conductance G′ normally produce a neglectable effect on the characteristic impedance Z₀. The latter, therefore, is essentially determined only by the capacitive and inductive terms. Therefore, assuming ωL′ to be much smaller than R′ and ωC′ to be much smaller than G′, the previous formula is reduced to:

Use the above formula to calculate, from the measured values, the characteristic impedance of the line.

F10-1: Equivalent circuit of a coaxial line.

F10-2a: Measuring the inductance (L) and resistance (R).

F10-2b: Measuring the capacitance (C) and the conductance (G)