File:      7:1:Ca:1:2   Subject:    General Cable  Information

 

DC Loop Resistance

1.01        DC loop resistance consists of the combined DC resistance of both the inner and outer conductors.  It is

commonly listed as ohms per 1 000 feet, calculated at 680 F

1.02        Most CATV and broadband systems operate on a 60 volt power supply scheme although there is some

interest in full-service networks utilizing various 90 volt powering schemes.  Both the inner and outer conductors

of coaxial cables are used to transport this power.  As this distance can become quite long, the resistance of both

conductors contribute to voltage drop.  Based on Ohm's Law, it a known current is passed through a circuit with

a known resistance, the end result constitutes a drop in voltage.  In reviewing Ohm's Law, we find that:

                                E

                   I=    ---                                 1 = current in amperes

                                R                                             E = voltage in volts

                                                                                R = resistance in ohms

Using this formula, Figure 1 illustrates that when 60 volts is applied across a 75 ohm resistor, it will produce a current of 0.80 amperes.

 

Figure 1: Ohm’s Law applied

 

1.03        If 4 ohms of resistance is added to each interconnecting lead in the circuit (between the voltage source and power source) it is simple to calculate the voltage drop through the 1 leads will be. (Figure 2 illustrates this circuit.)

2 ohms

                                ----------------------

                                        60 volts             75 ohms

----------------------

2 ohms

Figure 2: Ohm's Law (with load resistance)

 

1.04        Again, using the Ohm's Law formula to calculate current, eigure 2 will now produce a current of 0.85 amperes.  Utilizing the voltage formula of Ohm's Law, we find that:

E =   1 x R

                                      0.85 x 4   =   3.40 volts

The voltage now applied to the 75 ohm load will be 3.4 volts less than the source voltage of 60 volts, or 56.6 volts.

1.05        The previous information offers the basics required for calculating voltage drop in a 60 volt telecommunications system.  A real system will have several loads and several cable lengths with a different DC loop resistance for each location.

1.06        In order to calculate system powering the DC loop resistance for the cable must be known.  Figure 3 illustrates the maximum DC resistance for various P-3, copper clad coaxial cables.  The resistance is listed in ohms/1000 ft., calculated at 68 F

 


DC Resistance     Cable Size      Cable Size

ohms/1000'           0.500              0.750

inner conductor      1.35                0.57

outer conductor      0.37                0.19

loop                      1.72               0.76

DC Resistance     Cable Size      Cable Size

ohms/1000'           0.875             1.000                                                                                        

inner conductor      0.42               0.32

outer conductor      0.13               0.08

loop                       0.55                       0.40

 

Figure 3: DC Resistance

 


   Cable Loss & Attenuation

1.01        Attenuation can be defined as the reduction of a signal as it travels through a specified medium, usually expressed in decibels.

1.02        The losses in a coaxial cable are comprised of dielectric loss and resistive loss, as displayed in Figure 1.

 

Figure 1: Coaxial cable losses

 

1.03     Dielectric loss is the loss of signal caused by the material between the two conductors, commonly referred to as “dielectric material".  Resistance loss is caused from resistance of both the inner and outer conductors. As the diameter of the inner (center) conductor is significantly smaller, its resistance will be higher and therefore will contribute most of the loss.                  

1.04   Center conductor loss at RF frequencies is also affected by a phenomenon known as "skin effect". At higher AC frequencies, such as RF, the skin effect becomes more pronounced.  The higher the frequency, the more electrons flow nearer the outer surface of the center conductor. The net result is higher transmission loss, since the cross section area which carries the electrons is reduced. This accounts for higher losses at higher AC frequencies. This is the square root of frequency loss factor experienced in CATV systems        

                                                                                                                                            

Figure 2 illustrates the skin effect.

 

 

 

 

1.05 In Dc applications , if the center conductor size is increased, it provides a larger area for electrons flow which reduces overall resistance. The same holds true for AC signals, but becomes a frequency sensitive factor due to the skin effect.

1.06 The ratio of cable attenuation at two discrete

frequencies is approximately equal to the square root 

 of the ratio of the two frequencies

(the square root of frequency loss factor).

 

An example of this is to calculate the approximate cable. The net loss at 55 MHz when the loss at 450 MHz is 20 dB,


Velocity of Propagation

 

1.01         The velocity of propagation is best defined as the velocity of signal transmission along a coaxial cable relative to the speed of light in free space.  Electromagnetic waves travel at a speed of 186,280 miles per second (or 3 x 108 meters per second) in a vacuum.  Light in a vacuum travels at the same rate, 186,280 miles per second.

 

1.02        The equation for the speed of light comes from basic physics, as follows:

 

 

 

 

c is the velocity of propagation

p is the magnetic permeability of free space

is the dielectric constant of free space

 

1.03        When electromagnetic waves are propagated through dielectric materials other than air, the net-egect is that the wave slows down or the velocity of propagation becomes reduced.  Once the signal travels slower, the traveling distance for a single cycle becomes less and the wavelength becomes shorter.

 

1.04        In a coaxial cable, it is known that the velocity of propagation is less than that of light due to the materials used for the dielectric.  When the velocity of propagation is listed for coaxial cable, it is listed as a percentage.  Modern types of coaxial cable can otter a velocity of propagation of up to 88%.  This means that the electromagnetic waves of energy will travel on the cable at a rate equal to 88% of the speed of light.  Older generations of P-1 cable, with a gas injected dielectric, would have a velocity of propagation on the order of 82%.

 

 

Characteristic Impedance

1.01        Characteristic impedance can best be defined as a function of the ratio of the inner conductor diameter to the outer conductor diameter, respectively.

1.02        In order to determine the characteristic impedance of coaxial cable the nature of the dielectric itself becomes a major factor.  The dielectric constant of the dielectric material must be known.  Depending on the dielectric material used, this number can range from 1.0 for coaxial cable using mostly air as dielectric material, to 2.3 for the cables using a solid type of poly material.  Most gas injected foam found in today's cable will have a dielectric constant of about 1.5.

1.03        Figure 1 illustrates the formula to calculate the characteristic impedance of coaxial cable manufactured with a material dielectric.

 

Z = impedance in ohms

D = diameter of outer conductor

d = diameter of inner conductor

K= dielectric constant


Figure 1: Characteristic Impedance formula

1.04        Below, Figure 2 illustrates the characteristic impedance of coaxial cable.

Figure 2: Coaxial cable characteristic Impedance

1.05        The characteristic impedance used for CATV system plant is 75 ohms.  This impedance matches that of a common receiving antenna and, therefore, allows maximum power transfer from the antenna throughout the system.

1.06        The characteristic impedance of free space (vacuum) is about 377 ohms.  Electromagnetic waves travel slower through materials.  RF electromagnetic waves travel through copper at 8/1 0 the rate of free space (8/1 0 of 377 ohms is equal to 300 ohms).  As 300 ohms is the characteristic impedance of a loop antenna, the characteristic impedance of a half-wave copper antenna is 1/4 that of the full wave.  An antenna built to receive 1/4 of a signals wavelength is going to be shorter, easier to mount to a tower or mast, and simpler to work with.  The remaining is simplified: 1/4 of 300 ohms is 75 ohms.

1.07        Utilizing an impedance of 75 ohms ensures maximum signal transfer in a CATV system.  As Figure 3 illustrates 30 ohms would be more efficient for power carrying capacity, or the use of 60 ohms would be ideal for breakdown voltage.

 

 

                       Figure 3: Coaxial cable characteristics


                                         

Attenuation Loss Measurement

 

1.01         The following is one method of measuring Attenuation in 75 ohm coaxial cable.  Attenuation is the difference be^ tween transmitted and received power.  This method is intended for use as a quality assurance inspection for incoming shipments of Coaxial Cable.

 

2.01 The equipment described in the following procedure

was not meant to be used in a hostile environment, such as outdoors on a job site.  Extra care must be exercised if the equipment is to be used in this manner.

 

3.01        Network Analyzer - HP8753 or equivalent

3.02    50  ohm Power Splitter - HP 11 667A or equivalent.

3.03   (2) 75 Jumper Cables - Type N               

3.04     75 Calibration Kit

3.05   (2) Lab Connector to Cable                               Gilbert Engineering                                                       

            GTC-xxx-N or equivalent.                                                                                                                          

3.06   (2) Lab "N" to "F"     Adapter                           Gilbert Engineering                                                       

          GTC-N-F-Adapter or equivalent.

3.07   (2) 50to75ohm Matching Pads.HP 11852Borequivalent

     

4.01 The sample under test should be a known length; use a 3dB length at the lowest frequency and no more than a 5OdB length at the highest frequency. Record the footage in this length for final calculation and verification of the loss in the sample under test.   

4.02        Cable end preparation - Cut the cable end square for best results.  A pair of cable cutters or a hack saw will do.  Remove 2 inches of jacket and any flooding compound on the outer conductor.

4.03        The Lab Connector must fit securely on the cable for best results.  If the connector fits loosely it should be repaired or replaced.

 

5.01 Equipment Interconnection

5.02 Settings

 

1)      Set the "POWER" switch to "ON".  Let equipment warm-up 20 minutes.

 

2)      Set "Start Frequency" to 5 MHz.

 

3)      Set "Stop Frequency" to 1 000 MHz.

 

4)      Set "Measurement" to A/R.

 

5)      Set "Format" to Log Magnitude.

 

6)      Set "Display".

 

7)      Set "Reference Line" to appropriate level.

 

8)      Set "Sweep Time" to 1 0 seconds.

 

5.03     Calibration

 

1)      Calibration type - 75 ohm Type N.

 

2)      Calibrate.

 

3)      Response "Thru".

 

4)      Connect as shown in Figure 1.

 

5)      Store Calibration.

 

6.01        Determine if the sample under test meets the manufacturers specification.  Use the recorded length and the calculation in Section 7.0 for verification.

 

7.01        When the cable loss is measured at a temperature

other than 680 F (200 C) it must be converted for verifica-

tion. The attenuation will increase or decrease approximately

0.1 % per IF (0.2% per 'C).  Attenuation is directly proportional to length and conversion to dB/1 00 feet is given by;

 

 

                                   Attenuation (dBI100 ft)

 

=100 X   Atten.(dBIlength)

 

Length(ft)

 


Structural Return Loss Measurement

1.01      The following is one method of measuring Structural Return Loss (SRL) in 75 ohm coaxial cable.  SRL is defined as the return loss characteristics of coaxial cable that are related to periodic discontinuities within the cable itself.  This method is intended for use as a Quality Assurance inspection for incoming shipments of Coaxial Cable.

2.01     The equipment described in the following procedure is not meant to be used in a hostile environment, such as outdoors on a job site.  Extra care must be exercised if the equipment is to be used in this manner.

 

3.01     Network Analyzer - H P8753 or equivalent.

3.02     50 ohm Power Splitter - HP 11 667A or equivalent.

3.03     (2) 75 ohm Jumper Cables - Type N

3.04     Variable Bridge - Wide Band EngineeringA56 orequivalent.

3.05     Variable Terminator - Wide Band Engineering A56175 or equivalent.

3.06     (2) 75 ohm Jumper Cables - Type N

3.07     75 ohm Calibration Kit - Type N

3.08     (2) 50 to 75 ohm Matching Pads

3.09     (2) Lab Connector to Cable - Gilbert Engineering

            GTC-xxx-N or equivalent.

3.10     (2) Lab "N" to "F" Adapter - Gilbert Engineering GTC-N-F-Adapter or     equivalent.

4.01     Interconnection Equipment

 


4.02     Equipment Set-up

 

          1)    Set the "POWER" switch to "ON".

          2)    Set "Start Frequency" to 5 MHz.

          3)    Set "Stop Frequency" to 1 000 MHz.

          4)    Set "Measurement" to A/R.

          5)    Set "Format" to Log Magnitude.

          6)    Set "Display".

          7)    Set "Reference Level".

          8)    Set "Sweep Time" to 1 0 seconds.

          9)    Set number of points to 1601.

4.03   Calibration

          1)    Calibration Type - 75 ohm N.

          2)    Calibrate Variable Bridge Port.

          3)    Calibrate "Open".

          4)    Calibrate "Short".

          5)    Calibrate "75 ohm load".

6)    Store Calibration.

4.04     Variable Bridge and Termination

            1)    Adjust the resistance and capacitance dials on the variable bridge and           terminator until the trace is flat and at its lowest point.

a.    Adjustment of the resistance dial will effect the height of the trace.

               b.    Adjustmentofthecapacitancedialwilleffectthe slope of the trace.

5.01     Cut the cable end square for best results.  A pair Of cable cutters or hack saw     will do.  Remove 2 inches ofjacket and any flooding compound from the outer           onductor.

5.02     The Lab Connector must fit securely on the cable for best results.  If the connector fits loosely it should be repaired or replaced.

6.01     Connect cable as shown in Figure 1. Terminate other end of cable.

6.02     Adjust Variable Bridge capacitance and resistance to flatten SRL response.

6.03     Set marker maximum to locate the minimum SRL.  The SRL of the cable under    test is this minimum value.  Record this value (dB) and the frequency (MHz).             The SRL minimum must be greater than the minimum SRL specified by the   manufacturer or the purchase Specification.

7.01     Determine if the sample under test meets the SRL specification.

8.01     Structural Return Loss (SRL) can be calculated by the following formula.

 

 

 

 

TDR Measurements for 75 ohm Coaxial Cables

1.01 The following method is one way of measuring length

and locating faults in coaxial cable using a Time Domain Reflectometer (TDR).  The TDR sends an electrical pulse down the cable, and detects any discontinuities.  In coaxial cable these discontinuities will be detected as impedance changes.  This method is intended for use as a Quality Assurance inspection after the cable has been installed.

2.01     Please read and follow manufacturer's safety and warnings found in the Operator Manual.  Referto Operator Manual for more concise information that may not be explained in this method.

3.01     Tektronix 1 502C Metallic Time Domain Reflectometer (TDR).

3.02     Tektronix 1502C Operator Manual

3.03     Distance Measurement Accuracy; 1.6 inches or ±1%

            of distance measured.

4.01     Attach 3 feet (or longer) of 75 ohm coaxial drop cable (jumper) to the TDR.  One end of the jumper must have a BNC connector in order to connect to the TDR.  The other end can be fitted with an "F" connector.  Fittings used on the cable under test will need to be adapted down to the "F."

4.02     Attach the appropriate adapter and test connector to the jumper for the size cable under test.  All connections between cable, adapters and connectors should be tight to prevent erratic responses in the pulse.

4.03     Turn power on.  The following settings will be observed on screen (1 avg, 500 m, (default) ft/div and the footage in the upper right corner will be set to 0.000 ft).

4.04     Adjust Vertical Scale to 6.67m. Adjust position control counter clock wise (CCW) to bring the reflected pulse down to the center line on the scope.

4.05     Adjust Velocity of Propagation (Vp) to Manufacturer's specification for the cable under test.  The Vp should not be changed from this point.  Any changes to the Vp will require restarting at Step 4.3.

4.06     Move the cursor (< >) position to the rising portion of the reflected pulse.  This is the end of the jumper and any connectors on the jumper.  You may use any metallic object too short across the connector to verify the end.

4.07     Turn Noise Filter to horizontal.  Set will appear just above the noise filter control.

4.08     Press store and return Noise Filter to 1 avg.  The connectors and jumper from the end back to the front panel will be calibrated.  The footage in the upper right corner should be at 0.000 ft.

4.09     Attach the test connector securely to the sample under test.  The DIST/DIV can be adjusted at this point to observe all or any part of the reflected pulse on screen.

5.01     With the Vertical Scale set at 6.67 m each horizontal line above or below the reference pulse is equal to a 1 ohm change.

5.02     A "rise" or "fall" in the reflected pulse is an impedance mismatch such as observed at the end of the jumper just before the cable under test.

5.03     A dramatic rise in the pulse indicates an open.  A dramatic drop in the pulse indicates a short.  Inductive and capacitive effects on the cable appear as bumps and dips in the pulse.  Capacitive faults will appear as a drop in the pulse indicating water in the cable.  Inductive faults appear as a rise in the pulse.

5.04     When an abnormality is found, set the cursor at the beginning of the fault and read the distance in the upper right hand corner.