Attenuation coefficient measurement in coaxial cables

An attenuation coefficient measurement in coaxial circuit can be carried out by a bridge method, a compensation method, a light scattering coefficient method, a voltage-ratio method (a two-voltmeter method) and other ones.

The simplest and most demonstrable method is the two-voltmeter one. It is widely used for work situations.

The circuit of attenuation change measurement is shown in Figure 2.1

According to the method a generator G is connected to the circuit input, its frequency can be changed gradually in the given range. An output resistance of the generator must be equal to a surge impedance of the measured line. At the circuit’s output a variable resistance R ₂ is connected, which must be purely active one because a surge impedance of coaxial cable at high frequencies has virtually active character.

Voltmeters V₁ and V₂ are connected to the sending and dead ends of the line in order to measure voltages U ₁ and U ₂ at these points.

Changing resistance R ₂ value it is possible to get R ₂ equal to the surge impedance Z_sim. In this case the line works in a running-wave mode. R ₂ value is selected for the measurement so that slight frequency deviation (±25%) doesn’t cause appreciable deviation of the output voltage U ₂ (it must stay stable). If the load resistance R ₂ is not matched with the surge impedance Z_sim then the frequency deviation will cause voltage U ₂ oscillation. In case of complete match () due to generator’s frequency increase the voltmeter’s V₂ reading will decrease a little bit without any periodic oscillations.

On measuring the voltage at the sending and the dead ends of line the attenuation coefficient a can be easily calculated as:

dBpkm (2.9)

Here l is length of the measured cable in meters.

Key questions

3.1 Give the definition of the secondary transmission parameters and show their frequency dependences.

3.2 Write down the expression of a wavelength shorting coefficient and give its definition.

3.3 How do you measure a value of shorting coefficient in transmission line?

3.4 What do a, | Z_sim |, b and v _ph depend on in high-frequency range?

3.5 Analyze the expression according to which measured parameter’s values are reduced to for the temperature of 20°С.

3.6 What are temperature coefficients ?

3.7 Principles of transmission line attenuation measurement by the two-voltmeter method.

Home work

For self-reading for the laboratory work it is necessary:

4.1 Learn the recommended literature.

4.2 Learn the methodology of a wavelength shorting coefficient measurement in a transmission line (p. 9.1).

4.3 Learn the principles of attenuation measurement according to a two-voltmeter method (p. 2.2).

4.4 Prepare recitations to the key questions.

4.5 Prepare the report.

Laboratory task

5.1 Familiarize yourself with the laboratory model.

5.2 Turn on instruments make them ready to work.

5.3 Measure according to a task a lector has given to you:

– a wavelength shorting coefficient value in a coaxial cable;

– U ₁and U ₂values at four frequencies in the same cable.

5.4 Calculate:

– the secondary transmission parameters according to x measured at four frequencies;

– an attenuation coefficient of the cable at the same frequencies using U ₁and U ₂ values obtained by two-voltmeter method.

5.5 Chart frequency characteristics of the secondary parameters according to the measurement results.

Equipment

6.1 Models of coaxial cables РК-75, РЖ-58 and РЖ-59 30 m long.

6.2 Р5-10 device or its present-day analogue.

6.3 Generators, voltmeters, resistances.

Report content

7.1 Results of shorting coefficient measurement for three cable models.

7.2 Results of the secondary transmission parameter calculation at four frequencies and frequency dependence diagrams.

7.3 Results of an attenuation coefficient measurement by a two-voltmeter method for the same cable at the same four frequencies and a frequency dependence diagrams.

7.4 Make the conclusion which includes the analysis of the obtained results.

RECOMMENDED LITERATURE

8.1 Гроднев И. И., Курбатов Н. Д. Линии связи. - М.: Связь, 1980. – С. 113–115.

8.2 Ефимов И. Е., Останькович Г. А. Радиочастотные линии передачи. - М.: Связь, 1977.

8.3 Измеритель неоднородностей линий Р5-10. Техническое описание и инструкция по эксплуатации. – 1980.

APPENDIX