Transmission Line Trainer ST2266 Operating.pdf

Transmission Line Trainer ST2266

Operating Manual Ver 1.1

An ISO 9001 : 2000 company

94-101, Electronic Complex Pardeshipura, Indore- 452010, India Tel : 91-731- 2570301/02, 4211100 Fax: 91- 731- 2555643 email : [email protected] Website : www.scientech.bz Toll free : 1800-103-5050

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ST2266

Scientech Technologies Pvt. Ltd. 2

ST2266


Transmission Line Trainer ST2266

Table of Contents

1. Features 4

2. Technical Specifications 5 3. Basic Principles of Transmission Line 6 4. Types of Transmission Line 6 5. Equivalent Circuit Representation of a Transmission Line 6 6. Losses in Transmission Line 7 7. Characteristic Impedance of a Transmission Line 7 8. Basic Properties of the Coaxial Cable (used in the trainer) 7 9. Instruments required for experimentation 7

• Experiment 1 8 Measuring the characteristics of a line

• Experiment 2 11 Measuring the attenuation of a line

• Experiment 3 14 Measuring the Input Impedance of the Line

• Experiment 4 17 Phase displacement between the current & voltage at input of line

• Experiment 5 19 Frequency characteristic of the line

• Experiment 6 21 Study of Stationary Waves

• Experiment 7 24 Signal Phase shift along the line

• Experiment 8 27 Fault localization within the line

• Experiment 9 30 Line under Pulsed condition

10. Warranty 34

11. List of Accessories 34

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RoHS Compliance

Scientech Products are RoHS Complied. RoHS Directive concerns with the restrictive use of Hazardous substances (Pb, Cd, Cr, Hg, Br compounds) in electric and electronic equipments. Scientech products are “Lead Free” and “Environment Friendly”. It is mandatory that service engineers use lead free solder wire and use the soldering irons upto (25 W) that reach a temperature of 450°C at the tip as the melting temperature of the unleaded solder is higher than the leaded solder.

Features

• Self contained easy to operate trainer

• On board line, terminating resistances and test generators

• Functional blocks indicated on board mimic

• Built in power supply

• Operating manual & workbook provided

• Compact size

Technical Specifications

Transmission Line : Coaxial cable 100m (25m × 4)

Impedance matching resistances : 0 − 100 Ohms. 2 Nos.

Test Generators : 1. Sine Wave 40 KHz − 400 KHz (High Range) 2. Square Wave 400 KHz − 4 MHz (Low Range)

Power Supply : 230 V +/− 10%, 50 Hz Power Consumption : 3 VA (Approximately) Interconnections : 4 mm Banana sockets

Test Points : 10

Dimensions (mm) : W 450 × H 113 × D 280

Weight : 5.5 Kg. (Approximately)

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Basic Principles of Transmission Line

Transmission lines are a means of conveying signals or power from one point to another. From such a broad definition, any system of wires can be considered as forming one or more transmission lines. However, if the properties of these lines must be taken into account, the lines might as well be arranged in some simple, constant pattern. This will make the properties much easier to calculate, and it will also make them constant for any type of transmission line. Thus all practical transmission lines are arranged in some uniform pattern; this simplifies calculations, reduces costs and increases convenience.

Types of Transmission Line One of the simplest forms of a transmission line is the open-wire line or the twisted pair. Since the two conductors of this type of line have same relationship with respect to ground, it is a Balanced Line. But this type of line has very poor shielding properties and has a tendency to radiate. See figure 1. Coaxial lines are the more popular of the two in RF communication. A coaxial line consists of a central conductor and an outer conductor with the outer conductor referred to as shield normally grounded. Due to the outer conductor normally grounded the two conductors do not have similar relationship with respect to ground and that is why a coaxial line is an unbalanced line. However, due to shielding, coaxial lines have extremely low radiation loss.

Figure 1

Equivalent Circuit Representation of A Transmission Line

Since each conductor has a certain length and diameter, it must have resistance and inductance, since there are two wires close to each other, there must be capacitance between them. Finally, the wires are separated by a medium called the dielectric, which cannot be perfect in its insulation; the current leakage through it can be represented by a shunt conductance. The resulting equivalent circuit is as shown in figure 1. Note that all the quantities shown are proportional to the length of the line, and unless measured and quoted per unit length, they are meaning less. See figure 2.

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General equivalent circuit of transmission line Figure 2

Losses in Transmission Line

The three major sources of losses in RF transmission lines are : 1. Copper losses

2. Dielectric losses 3. Radiation losses

Characteristic Impedance of a Transmission Line

Characteristic impedance of a transmission line is its input impedance if it was infinitely long. Refer to the transmission line equivalent circuit of figure 2. It can be proved with simple mathematics that the characteristic impedance of this line is given by :

Z = √(R + jω) L) / (G + jω C)

Where, R = Distributed resistance per unit length L = Distributed inductance per unit length G = Distributed Shunt conductance per unit length C = Distributed Shunt capacitance per unit length. In a loss less transmission line R = 0, G = 0

Therefore,

Characteristic Impedance (Z0) =√ L / C The input impedance of a finite line terminated in its characteristic impedance is equal to its characteristic impedance only. See figure 3.

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Figure 3 Basic Properties of the Coaxial Cable

(Used in the Trainer) Type : RG 174

Length : 100 meters

Series Inductance : 28µH (Frequency 1 KHz, 100 m) approximately

Parallel capacitance : 11µF (Frequency 1 KHz, 100 m) approximately

Conductance : 0.4 µ mhos

Impedance : 50 Ω approximately

Instruments Required for Experimentation

1. 20 MHz Dual Trace Oscilloscope ST201.

2. 3½ Digits Digital Multimeter. 3. L C R Q Meter or Universal L C R bridge.

Important points to note : 1. The coaxial line used in the trainer is placed in the coiled form of 25 meters

each. This is done for space saving. In practice the lines are straight. The coil form has caused some deterioration. For the convenience of the students the basic properties given above are of coiled form.

2. The attenuation of RG 174 cable is 40dB / 100 m at 200 MHz. But due to coiled form it will show 3dB/ 100 m at 3.6 MHz.

3. In place of internal sine wave test generator you may use other generator of higher frequency.

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Experiment 1 Objective : Measuring the characteristics of a line Characteristic of a shielded line : The coaxial lines used for the transmission of electromagnetic waves consist of an external conductor of cylindrical shape, with an inner conductor arranged along the axis of the former. The two conductors are separated by dielectric material of suitable features. One of the advantages of this kind of lines is that these lines are intrinsically self-shielding, due to the geometry of the arrangement of the two conductors. Moreover the shielding features of the coaxial lines improve when the frequency increases.

From the electric point of view, a coaxial line can be considered as a cascade of line trunks. Each one of them can be represented as being composed of resistive, inductive and capacitive circuit elements of concentrated kind, as shown in the figure 1. R = ohmic resistance for unit length (100m in this trainer) L = inductance for unit length G = conductance for unit length C = capacitance for unit length The transmission characteristics of a line are described in terms of propagation. constant γ and of characteristic impedance Z0. These parameters are typical values for each single line. The same is true for the capacitance, the inductance, the resistance and the conductance for length unit. In the telecommunications field, these values are generally expressed per meter or kilometer, for practical reasons. In this case, the symbol used to indicate these magnitudes are the common symbols.

This experiment measures the characteristic parameters such as R, L, C, G, Z0 and r for the transmission line included in this trainer.

Procedure : 1. Figure 4 shows the modalities for the measurement to be performed.

2. Make connections as in figure 5. 3. Both the inductance and the ohmic resistance of the line are measured in series

by short-circuiting end of the line and connecting the measuring instruments to the start of the line. The capacitance and the conductance are measured in parallel by operating on the open line.

4. The resistance R and the conductance G can be measured with an ohmmeter or DMM. For the conductance to be measured an ohmmeter is required which is able to perform resistance measurements with a range greater than 100 MΩ.

5. For the measurement of series inductance L and the parallel capacitance C, a LCR meter or measuring bridge is required. The results of these measurements give values of R, L, C and G referred to the cable length that, in our case, is of 100 meters.

Z0 can be measured by using the following formula :

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Z0 = √L /C Compare the readings obtained by you with those specified previously on page no. 7.

Alternate : If the LCRQ meter is not available this experiment can be performed as explained in experiment no. 2.

Figure 4

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Figure 5

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Experiment 2 Objective : Measuring the input impedance of the line The input impedance of the line depends on features like the ohmic resistance, the conductance, the inductance and the capacitance. It is also related to the resistance that loads the line at the opposite end and to both the frequency and voltage of the signal. The purpose of the first part of the test is to measure the input impedance of the line under different load conditions : 1. Line terminated with matched load 2. Open line 3. Short-circuited line. In the second part of the test we will measure the phase displacement between the input voltage and current, under the 3 conditions of line termination. When the modulus and the phase displacement are known the impedance vector is fully identified. See figure 6.

Figure 6 Procedure : 1. Adjust Ri and RL for 18Ω and 68Ω respectively with the help of DMM. 2. Make the connections as shown in figure 7.

3. A 1Ω resistance in series between the generator and the transmission line as shown, in figure 12 allows to measure the value of input current.

4. Set the input at 0.4p-p and freq 100 KHz of sine-wave (both measurement on CRO).

5. Take readings of Vin and Vm (across 1Ω) on oscilloscope. 6. Calculate the input impedance according to the following formula :

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Zin = Vin / I = Vin / Vm × 1Ω 7. Change the frequency to 1MHz and note the values of Vin and Vm at this

frequency.

8. Note down these results. The input impedance at 100 KHz is around 80 Ω and at 1 MHz is around 50Ω.

Repeat the experiment with shorted and open line and use the following formulas to compute the impedance when line is open circuited Zoc and when short circuited Zsc.

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Figure 7

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Experiment 3 Objective : Measuring the attenuation of line The ohmic resistance R & the conductance G are responsible for energy disspation in the form of heat. These losses, which determine the attenuation characteristics, are expressed in terms of “attenuation” “a” and can be calculated by :

a = 20 log (V2 / V1) Where V1 = amplitude of signal at input

V2 = amplitude of signal at output a = attenuation for given length

In this experiment we will measure the attenuation for the different trunks of transmission line available on the trainer. See figure 8

Figure 8 Concept of matched line : Though the concept of match line is not treated in detail in this manual but the subject is certainly known to the students from the theoretical course. We have already found out the characteristic impedance of the line as 50Ω from the previous experiment. The short-circuited resistance of the line when measured with Digital Multimeter is shown to be 18Ω. Therefore; the total effective resistance of the line is 68Ω. For optimum power transfer we should have the source resistance and terminating resistance also as 68Ω. Assuming generator resistance Rg as 50Ω we must connect 18Ω Ri in series with the generator to match the line. For this purpose, it is recommended that the student must set Ri to 18 Ω using DMM and RL to 68Ω initially and this setting should be utilized for all the experiments wherever terminated line experiment is done.

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Procedure : 1. Adjust Ri and RL for 18 Ω and 68 Ω respectively with the help of DMM. 2. Make connections as shown in figure 9. 3. Set the sine-wave frequency to approximately 100 KHz and level to 0.4 V. 4. Oscilloscope CH 1 shows applied input CH 2 shows outputs. 5. Measure signal level at Input, and at 25, 50, 75, and 100 m lengths. 6. Tabulate as under :

Length (m) V1 (input) V2 (output)

25

50

75

100

7. Now, calculate the attenuations in dB at various lengths by the formula given below : a = 20 Log V2 / V1

8. The attenuation is approximately -2 dB at 100 m.

9. Try the same with open-ended line and short-ended line.

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Figure 9

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Experiment 4 Objective : Phase displacement between the current & voltage at input of line. The phase displacement between the current & voltage at input of line, under the different load conditions viz. matched line, open line and short-circuited line. See figure 10.

Figure 10

Procedure : 1. Adjust Ri and RL for 18 Ω and 68 Ω respectively with the help of DMM. 2. Make the connections as shown in figure 12. 3. A1Ω resistance in series between the generator and the transmission line as

shown, in figure 12 allows measuring the value of input current. 4. Set sine wave frequency to 100 KHz (use CRO). 5. Set the oscilloscope to XY mode. 6. Output across 1Ω is connected to Y and input signal is connected to X. 7. Observe suitable Lissajous pattern on CRO by adjusting V / div of each channel. 8. The Lissajous pattern allows measuring the phase displacement between the two

signals through the ratio of the semi axis of the ellipse. The phase displacement is approximately 15° at 100 KHz.

Figure 11

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Figure 12

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Experiment 5 Objective : Frequency characteristic of the line When the frequency of the input signal increases, the line attenuation due to both the ohmic resistance R and the conductance C progressively increases because of “Skin effect”. Starting from a given frequency onwards, the line attenuation increases. The cut off frequency of the line is defined as the frequency at which the attenuation reaches the level of -3dB compared to the low frequency level -3dB is approximately down to 70 %. The purpose of this test is to measure the cut off frequency for the coaxial line provided in ST2266. This measurement is performed with terminated line.

Figure 13

Procedure : 1. Adjust Ri and RL for 18 Ω and 68 Ω respectively with the help of DMM. 2. Make connections as shown in figure 14. 3. Set oscilloscope to 0.1 V / div for both channels. 4. Adjust the sine generator for an output of 0.2 Vp-p (2 div. deflection on CH1)

and at frequency 40 KHz. 5. At this point CH1 is reading 2 div. deflections and CH2 is reading 1.6 div. (This

is due to the fix attenuation of the line) 6. Now, vary the frequency of generator gradually keeping the input amplitude

constant (observe CH 1 and maintain 2 div deflection by adjusting AMP VAR control) till the waveform at the end of 100 m line falls to -3 dB (1.4 div of CH 2 on the oscilloscope).

7. Note, this frequency on the oscilloscope. This frequency is known as the cut off frequency. For the cable used in this trainer this frequency is approximately 3.5 MHz.

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Figure 14

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Experiment 6 Objective : Study of Stationary Waves A line that has not been terminated with a load equal to its characteristic impedance is subject to a reflection phenomenon of the power from the remote end. The amount of the reflected power depends on the amount of mismatch between the characteristic impedance of the line and the load impedance. In the extreme cases of short-circuited line (RL = 0) and open line (RL = ∞) a situation of total reflection occur for either the current wave or the voltage wave. The purpose of this test is to study the establishment of the stationary waves within the line. See figure 15.

Procedure :

1. Adjust Ri and RL for 18 Ω and 68 Ω respectively with the help of DMM. 2. Make connections as shown in figure 16

3. Set oscilloscope to 0.1 V / div for both channels. 4. Adjust the sine generator for an output of 0.2 Vp-p (2 div Deflection on CH 1)

and at frequency 100 KHz. 5. Observe the peak to peak voltages on CH 2 at 100 m and at intermediate sockets

at 75 m, 50 m & 25 m and 0m. 6. Tabulate results as under :

Distance Vp-p 0 m 25 m 50 m 75 m

100 m

7. Calculate the stationary wave ratio ‘s’ by the following formula : s = V max / V min

For 100 KHz‘s’ is approximately 1.25 8. The reflection coefficient ‘r’ of the line shows how much of the energy supplied

at the input is being reflected as a consequence of the load decoupling. The reflection coefficient is normally expressed in percentage and can be determined from the stationary wave ratio through the following formula :

r = (s - 1) / (s + 1) At 100 KHz ‘r’ is approximately 11 %

9. Repeat the same procedure for open line & short-circuited line.

10. Try the experiment with other frequencies to see the effect of frequency on 's'.

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Figure 15

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Figure 16

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Experiment 7 Objective : Signal phase shift along the line The propagation of the electromagnetic waves through the vacuum occurs at a speed approximately equal to the light speed. (3×108 m/s). In a line, the propagation speed is of course reduced by the characteristics of the line, like the capacity and the inductance. In particular, in the coaxial lines, the propagation speed of the electromagnetic waves is approximately between 60 & 80% of the light speed, as a function of the line characteristics. The objective of this test is to measure the phase displacement between the input signal and the output signal for the line in the trainer, whose length is 100 m. As a consequence the phase speed and the phase delay will be calculated related to the phenomenon of electromagnetic waves propagating along the line.

Figure 17 Procedure :

1. Adjust Ri and RL for 18 Ω and 68 Ω respectively with the help of DMM. 2. Make connections as shown in figure 18.

3. Set oscilloscope to 0.05 V / div CH 1. 4. Adjust the sine generator for an output of 0.2 Vp-p (4 div deflection on CH 1)

and frequency to 100 KHz. 5. Set the oscilloscope to XY mode.

6. Measure phase angle by ellipse formula.

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7. When the value of phase displacement ϕ has been determined through the measurement, the phase speed can be calculated on the basis of the following formula :

v = f × l × 360 / ϕ where,

f = frequency at which the measurement is made.

l = length of the line, v = phase speed. The phase speed can be also expressed in another way, by pointing out its relationship with the characteristic parameters of the line, on the basis of :

v = 1 /√ LC

8. The phase delay‘t’ is calculated from the ratio of the line length and phase speed.

t = l /v.

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Figure 18

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Experiment 8 Objective : Fault localization within the line The Localization of the faults within the line can be performed following different methods. The method shown here for performing this test is of special interest, being based upon the use of the phenomenon of the establishment of stationary waves. Let's assume that the line is broken at unknown point between two ends. If the line is connected to a signal generator, the wave will be reflected from the break point, and a stationary wave condition is established between the input and the breakpoint. The waves along the line have maximum and minimum points at regular intervals corresponding to ¼ of the wave - length of the input signal. For the fault to be pinpointed, it is necessary to determine, the frequency value at which a voltage minimum occurs at the input. This frequency is noted as f1. The same operation is repeated at the remote end, of broken cable, and obtaining f2 value. These values are substituted in the following formula :

l’ = [f2 / (f1 + f2)] × l Where, l = line length in meters l’ = distance in mts of the point of fault referred to the input of the line.

Procedure : 1. Make connections as shown in figure 20 a. Note that the line is broken at 50 m

length. 2. Set oscilloscope channel 1 to 0.1 V/ div.

3. Adjust the sine generator for output of 0.4 Vp-p (4 div deflection on CH 1) 4. Keep the frequency variable control at the minimum position.

5. Gradually increase the frequency and note the frequency at which the signal on CRO falls to minimum. This frequency is f1.

6. Repeat the test at the other end of the line as shown in figure 20 b and note the Frequency at which signal on CRO falls to minimum. This is f2.

7. Enter these values in the formula and calculate the distance of break point from the input.

For the fault generated at 50 m f1 and f2 are 900 KHz approximately

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Figure 19

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Figure 20a

20b

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Experiment 9 Objective : Line under pulsed condition. If the line is supplied with a pulsed signal and the line is not matched at the ends, the pulses sent into the line will be more or less reflected as a function of the mismatch. The reflected fraction of the pulse moves along the line in the opposite direction to the generator and when the characteristic impedance of the line is not matched to the impedance of the generator it is again reflected to the other end. The purpose of this test is to study the propagation of the pulse edges along the line, under different matching conditions viz. open line, short circuited line & matched line.

Figure 21 Procedure :

1. Adjust Ri and RL for 18 Ω and 68 Ω respectively with the help of DMM.

2. Make connections as shown in Figure 22. 3. Observe the input and output wave shapes and also amplitude levels on the

Oscilloscope. 4. Now make the load open and repeat the same procedure.

5. Again, repeat the experiment for short-circuited load. 6. Note the observations for all 3 conditions of load and compare them

.

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Figure 22

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The following experiments are additionally possible on transmission line trainer with Cable fault locator 9300 1. To locate the fault in the line at various distances i.e. 25 m, 50 m, 75 m, &

100 m.

2. Study of short circuited line on Cable fault locator. 3. Study of open circuited line on Cable fault locator.

4. Study of matched (terminated) line on Cable fault locator.

Procedure : 1. Connect the intermediate blocks of the trainer so as to get a 100 m transmission

line.

2. Keep the line open circuited at the far end. 3. Connect the input terminals of the line to the terminals of the Cable fault

locator. 4. Press the ‘On’ key of the Cable fault locator to turn it ‘On’.

5. After the initialization period set the propagation velocity factor (PVF) by means of the arrow keys when in PVF mode to 0.62.

6. Use the menu select key to select meters for the cursor value. 7. Select the minimum range where the fault pulse is clearly visible, in our case

select 300 m. 8. Change to locate mode and operate the cursor keys to position the cursor line at

the start of the break point of the fault pulse. The reflection has a upward polarity as shown in figure 23

Figure 23

9. The distance to the fault may now be read directly from the cursor value, which is at the bottom center of the LCD display.

10. Now short circuit the far end of the transmission line. 11. The reflection has a downward polarity now; place the cursor line at the start of

the break point of the fault pulse.

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Figure 24

12. The distance to the fault pulse may now be read directly from the cursor value. 13. Above experiments can be repeated with any other distance by simply

disconnecting the intermediate blocks and either opening or shorting the ends. 14. For matched loads, connect the source resistance and the load resistance equal to

68Ω and connect the Cable fault locator as usual. 15. Switch on the Cable fault locator and you will see that there is no reflected pulse

because the line is perfectly matched.

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Warranty 1. We guarantee the product against all manufacturing defects for 24 months from

the date of sale by us or through our dealers. Consumables like dry cell etc. are not covered under warranty.

2. The guarantee will become void, if

a) The product is not operated as per the instruction given in the operating manual.

b) The agreed payment terms and other conditions of sale are not followed.

c) The customer resells the instrument to another party. d) Any attempt is made to service and modify the instrument.

3. The non-working of the product is to be communicated to us immediately giving full details of the complaints and defects noticed specifically mentioning the type, serial number of the product and date of purchase etc.

4. The repair work will be carried out, provided the product is dispatched securely packed and insured. The transportation charges shall be borne by the customer.

List of Accessories 1. BNC to BNC Cable ................................................................................. 1 No.

2. BNC to Test Prod Cable .......................................................................... 1 No.

3. Patch Cord 16”........................................................................................14 Nos.

4. Mains Cord .............................................................................................. 1 No.

5. e-Manual.................................................................................................. 1 No. Updated 26-06-2008

Documents

Transmission Line Trainer ST2266 Operating.pdf