DC loop resistance consists of the combined DC resistance of both the
inner and outer conductors. It is
listed as ohms per 1 000 feet, calculated at 680 F
and broadband systems operate on a 60 volt power supply scheme although there
in full-service networks utilizing various 90 volt powering schemes.
Both the inner and outer conductors
coaxial cables are used to transport this power.
As this distance can become quite long, the resistance of both
contribute to voltage drop. Based
on Ohm's Law, it a known current is passed through a circuit with
known resistance, the end result constitutes a drop in voltage. In
reviewing Ohm's Law, we find that:
1 = current in amperes
E = voltage in volts
R = resistance in ohms
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
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
60 volts 75
Figure 2: Ohm's Law
(with load resistance)
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:
= 1 x R
0.85 x 4 = 3.40
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.
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.
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
Cable Size Cable Size
3: DC Resistance
Cable Loss &
Attenuation can be defined as the reduction of a signal as it travels through a specified medium, usually expressed in decibels.
losses in a coaxial cable are comprised of dielectric loss and resistive loss,
as displayed in Figure 1.
Figure 1: Coaxial
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
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
2 illustrates the skin effect.
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
The ratio of
cable attenuation at two discrete
is approximately equal to the square root
the ratio of the two frequencies
square root of frequency loss factor).
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
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.
equation for the speed of light comes from basic physics, as follows:
is the velocity of propagation
is the magnetic permeability of free space
the dielectric constant of free space
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.
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 can best be defined as a function of the ratio
of the inner conductor diameter to the outer conductor diameter, respectively.
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.
illustrates the formula to calculate the characteristic impedance of coaxial
cable manufactured with a material dielectric.
impedance in ohms
= diameter of outer conductor
= diameter of inner conductor
Figure 1: Characteristic Impedance formula
Figure 2 illustrates the characteristic impedance of coaxial cable.
Figure 2: Coaxial cable characteristic Impedance
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
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.
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
Figure 3: Coaxial cable characteristics
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.
equipment described in the following procedure
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
Analyzer - HP8753 or equivalent
ohm Power Splitter - HP 11 667A or equivalent.
Jumper Cables - Type N
Lab Connector to Cable
Lab "N" to "F"
50to75ohm Matching Pads.HP 11852Borequivalent
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.
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
Connector must fit securely on the cable for best results.
If the connector fits loosely it should be repaired or replaced.
Set the "POWER" switch to "ON".
Let equipment warm-up 20 minutes.
Set "Start Frequency" to 5 MHz.
Set "Stop Frequency" to 1 000 MHz.
Set "Measurement" to A/R.
Set "Format" to Log Magnitude.
Set "Reference Line" to appropriate level.
Set "Sweep Time" to 1 0 seconds.
Calibration type - 75 ohm Type N.
Connect as shown in Figure 1.
if the sample under test meets the manufacturers specification.
Use the recorded length and the calculation in Section 7.0 for
cable loss is measured at a temperature
than 680 F (200 C) it must be converted for verifica-
The attenuation will increase or decrease approximately
% 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)
Structural Return Loss
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
3.03 (2) 75 ohm Jumper Cables - Type N
Variable Bridge - Wide Band EngineeringA56 orequivalent.
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
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.
1) Calibration Type - 75 ohm N.
2) Calibrate Variable Bridge Port.
3) Calibrate "Open".
4) Calibrate "Short".
5) Calibrate "75 ohm load".
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.
Adjustment of the resistance dial will effect the height of the trace.
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
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
8.01 Structural Return Loss (SRL) can be calculated
by the following formula.
TDR Measurements for 75 ohm Coaxial Cables
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
3.01 Tektronix 1 502C Metallic Time Domain
3.02 Tektronix 1502C Operator Manual
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
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
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