Studying of coaxial cables with unknown length

A mutual capacitance of coax can be written as

, Fpkm (2.5)

Here ε is an equivalent dielectric constant of insulation;

D – Inner diameter of outer conductor;

d – Diameter of inner conductor.

Coaxial cable surge impedance is

, Ohm. (2.6)

Considering that shorting coefficient is then we get:

. (2.7)

Thus to study coax with unknown precise length, we could indirectly determine the shorting coefficient via the formula (2.7) and measure the surge impedance preliminarily.

It is easiest to measure surge impedance of coax by comparison method. To do this the measured cable section is connected to a Р5-10 instrument. Variable active load resistance is connected at the end of circuit chain. If the resistance is not matched with the cable surge impedance then the pulse-response characteristic will have a break, which corresponds to a reflection of the pulse from the unmatched load. Therefore to determine a value of the surge impedance of the line being measured it is necessary to choose a load resistance so that the break disappears (or reduces to a minimum); then the required surge impedance will be equal to the value of load resistance. It is worth remembering that in case a load resistance is less than the impedance characteristic’s break is directed downwards and in the opposite case - upwards.

Thus, having determined the surge impedance we may evaluate the shorting coefficient for this cable and other parameters as well.

 

Key questions

3.1 What is the principle of line’s obstacles determination by using a pulse method?

3.2 How do you determine a distance to the obstacle’s location in the transmission line?

3.3 How do you determine a wavelength shorting coefficient’s value?

3.4 What quantity characterizes surge impedance’s obstacle of transmission line?

3.5 Write down the expressions for reflection and shorting coefficients of electromagnetic wave.

3.6 Give expressions for C, , v _ph determination using ε.

Home work

In process of preparation for the laboratory work it is necessary to:

4.1 Study the recommended literature.

4.2 Familiarize yourself with a pulse method of place and nature of line’s fault determination (p. 2.1).

4.3 Learn a methodology of measurement for:

– a distance to an obstacle (a fault) of a transmission line;

– a wavelength shorting coefficient for a line with a known length;

– a surge impedance for transmission line with an unknown length.

4.4 Prepare recitations to key question.

4.5 Prepare the report.

Laboratory task

5.1 Familiarize yourself with the laboratory stand.

5.2 Turn on a Р5-10 device and set it ready for work.

5.3 Connect the device to an artificial extension line and carry out the following actions:

а) run a pulse-response characteristic of an operable line which includes different obstacles;

b) determine the distance to certain points of transmission line (for aerial lines ξ = 1.05);

c) determine locations and nature of line’s faults according to the task assigned by instructor.

5.4 Measure a surge impedance of coax’ sections according to a comparison method and evaluate the shorting coefficient.

5.5 Having evaluated the shorting coefficient, measure each cable section’s length.

EQUIPMENT

Laboratory stands of aerial lines several sections of coaxial cables with different surge impedance values.

A Р5-10 device or its modern analogue.

Resistance set.

Report content

7.1 A drawing of pulse-response characteristics of operable and inoperable aerial lines.

7.2 Results of measurements of a distance to certain TL stations and places of faults determined according to the pulse characteristics.

7.3 Results of measurements of a surge impedance Z_sim and a length L of the cable sections, determination of a shorting coefficient ξ.

RECOMMENDED LITERATURE

8.1 Гроднев И. И., Курбатов Н. Д. Линии связи. - М.: Связь, 1980. – С. 414.

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

Р5-10 work start

9.1.1 Before turning on the device it is necessary to check that the device is switched to corresponding current and voltage.

9.1.2 Set the knobs to the base point:

«Gain» – to the extreme left position;

«Distance» – 0;

«Set of reading» – to the extreme left position;

«Power» – to the lowest position (turned off).

9.1.3 Check if the device is grounded.

9.1.4 Switch on power by the knob «Power». The indicator will light up on the front panel. In 0.5…2 minutes a sweep trace will appear.

9.1.5 Regulate brightness, focus and position of sweep trace on the display by the knobs «☼», «8» and «↕». The sweep trace must be in the center of the screen.