Content uploaded by David Witkowski
Author content
All content in this area was uploaded by David Witkowski on May 17, 2019
Content may be subject to copyright.
Release V1.1 January 14, 2013 www.anritsu.com Anritsu Company Document # 11410-00700-A
1
Abstract— Transmission Line and Antenna Testing has become a common test of RF network integrity over the past few years.
This relatively new testing methodology is a result of new test equipment evolutions and the need to fully understand the integrity
of RF networks after installation. While common today, in the past this component was ignored because the equipment necessary
to perform the tests was either laboratory grade or non-existent in the field. Recently, it has become evident that, while the
capabilities of testing the integrity of transmission line, connectors, and antennas is readily available in the field, the results and
conclusions of this testing are not consistent. Cable manufacturers and antenna manufacturers are often blamed for failed tests
while the hardware is proven to be fully compliant, and functional as designed. In an effort to improve these valuable tests and
establish consistency in the results and conclusions from the test, experts from the industry decided to work together to establish
some basic guidelines for the tests and Methods of Procedure (MOP). This consensus work was performed in a controlled
environment with the major leaders of the industry present. The recommendations and techniques presented were developed to
improve the integrity of the results, and represent effective conclusions that can be reached, regardless of the cable, connector, test
equipment, or antenna evaluated. The purpose of this paper is to develop and achieve consistent results.
I. INTRODUCTION
On October 2012, 25 individuals representing every
discipline from engineer, project manager, business manager,
and technician convened a critical workshop. The attendees
represented Antennas manufacturers, Cable manufacturers,
Filter and RF system manufacturers, Field Engineers, System
Technologists, Test equipment manufacturers, and Business
managers. The purpose of this workshop was to allow industry
leaders and manufacturers to discuss and develop a unified
understanding of Line Sweeping methodologies. A complete
list of the collaborating manufacturers in attendance is
included in Appendix 1.
Standardized testing has been a part of RF Communications
since the first conversation was transmitted. To ensure reliable
communication, the health of the components involved was
required to be tested and verified. Until recently this applied
to the communication hardware, but not the RF network. The
RF network refers to the hardware between the
transmitter/receiver and the antenna. The RF network
transports the modulated RF energy to the antenna or from the
antenna to the receiver. This network allows the antenna to
effectively radiate the energy or receive energy. The RF
network has, for the most part, been taken for granted and is
expected to be a drop-in component. Very little testing and
evaluation was performed in the past. However, the
expectation of a stable, predictable, and dependable
component was not warranted. Components, while reliably
designed and manufactured, were damaged when installed or
shipped or improperly installed. The simple testing performed
on site was inadequate to properly evaluate the effectiveness of
the hardware. Recently, with the evolution of new test
equipment, the ability to test the RF network components and
antennas to a degree that resembles the manufacturer’s testing
has led to new procedures and expectations. Field personnel
now expect to be able to reproduce the same data that the
manufacturers declare on the components. While this
expectation is achievable, discipline is required.
Return Loss vs. Frequency is the primary means of testing
RF networks because the effectiveness of an RF network
depends so greatly on impedance matching. Physics dictates
Evaluation of RF Network Testing
An Industry review of techniques and procedures
Bryan Corley
Principal Staff Engineer – Motorola Solutions Inc.
B.Corley@motorolasolutions.com
Don Huston
Strategic Account Mgr – Bird Technologies
DHuston@bird-technologies.com
Mike Schaefer
Product Line Manager – CommScope HELIAX® Products
Mike.Schaefer@commscope.com
David Witkowski
Senior Product Manager – Anritsu Company
David.Witkowski@anritsu.com
Release V1.1 January 14, 2013 www.anritsu.com Anritsu Company Document # 11410-00700-A
2
that maximum power is transferred from the point of origin to
the destination when the origin impedance, transmission
network impedance, and the destination impedance are
perfectly matched. Return Loss (RL) and/or VSWR are the
measure of the deviation from a theoretically ideal impedance
match. The higher the absolute value of the RL, the better the
match, resulting in better power transfer. Impedance
irregularities anywhere in the RF network will result in power
being reflected back to the source. This reflected power
reduces the amount of power transferred from a source to a
load. In order to maximize the radiated power from a system,
the RL of all of the components must be established and
verified. The impedance could vary at certain points due to
manufacturing variances or because of faults at certain points
in the network. Regardless of how or why these occur the
overall result is a reduction in transferred power.
Transmission line testing is critical in determining if there
are irregularities, and to locate where these irregularities occur.
Testing is also important in establishing a benchmark for
future measurements to find changes or deterioration of
components. More important than performing these tests is to
ensure testing is done consistently and competently. Without
standardized testing procedures, the opportunity for
inconsistencies exists.
II. TEST EQUIPMENT
Our goal, as RF system engineers, is to provide site designs
that will perform as the customer requires. Test equipment
allows the components of the system to be optimized and
verified to specific standards used in the design. When the
performance of a system equals the designed performance,
energy transfer from the source to the load will meet or exceed
design criteria and maximum coverage will result. Without the
ability to test the installed equipment the system coverage is a
leap of faith. This is how systems were designed and installed
in the 20th century but not in the 21st century. Best practices
say we should validate the operation against the design goals
empirically.
A. Test Equipment available
RF network test equipment migrated from the laboratory into
the field beginning in the late 80’s and 90’s as integrated
circuits capable of performing the required analysis and
computation became available. Test equipment that could
only be maintained and used by laboratory engineers is now
available to technicians and RF installation field personnel.
This precision test equipment is only now available because
the test equipment manufacturers have made it less
complicated to use, significantly smaller, and more rugged,
while not compromising the accuracy and detailed testing
possible. Tests now can be performed in the field that were
not available in large, high-priced laboratory grade equipment
ten years ago.
1) Wattmeter
The wattmeter has been used for years to evaluate the quality
of the RF network using transmit power. While providing a
very crude VSWR measurement, the wattmeter did not allow
testing of the receive network or assist in understanding where
problems occurred in the network.
The wattmeter simply measures the power in the forward
direction and the power in the reverse direction. Comparing
these two measurements allows the calculation of Voltage
Standing Wave Ratio (VSWR). VSWR can be converted
mathematically to Return Loss. While the VSWR measured
may be accurate for the position of the wattmeter in the
system, it does not allow an understanding of where a
mismatch is located, or how bad the mismatch may be. If the
mismatch occurs far down the transmission line, the full effect
of the mismatch may be masked by the loss of the transmission
line in between. A mismatch occurring in the antenna cannot
be distinguished from cable/connector problems. The use of a
wattmeter to determine RF network quality is about as
effective as trying to read a book in the dark.
2) Time Domain Reflectometer (TDR)
The original piece of equipment in this family was the Time
Domain Reflectometer (TDR), a device which inserted a DC
pulse into a system. The pulse traveled from the insertion
point to the antenna and was reflected back by any
irregularities, shorts or opens within the system. The speed of
the pulse is known as the speed of electromagnetic radiation,
the speed of light. Because of the “velocity factor” of the
cable, the pulse is slowed, but by a known amount, which is
included in the calculations. Using the return time and the
level of the returning signal, the device calculates the distance
to any faults in the system.
All of the RF specifications for a system are based on the
frequency of operation. A TDR sends a DC pulse through the
system which does not take the frequency specific
characteristics into account. A TDR's pulsed DC stimulus
reflects little energy at RF faults or impedance mismatches.
Furthermore, almost 100% of the TDR's source energy is
reflected by the antenna or any other in-line, frequency-
selective device e.g., frequency combiners, filters or quarter-
wave lightning arrestors. Due to the square wave nature of the
DC pulse, the TDR's spectral content is splattered across a
wide frequency range, but the amplitude is not consistent with
frequency and the spectral magnitude and the output pulses
tend to roll off rapidly at high frequencies. Typically, less than
2% of a TDR's pulsed energy is distributed in the RF
frequency ranges. For these reasons - and others - the use of
the TDR is deemed marginal for evaluating RF networks.
3) Frequency Domain Reflectometer (FDR)
A Frequency Domain Reflectometer (FDR) generates an RF
sweep that includes only the frequency range selected by the
operator, allowing frequency-selective characteristics to be
displayed clearly. Measurements include return loss (or
voltage standing wave ratio, VSWR) vs. frequency, and return
loss (or VSWR) vs. distance. Frequency Domain
Reflectometry (FDR) developed as embedded processors
became available to handle the higher data rate and complex
mathematics needed to perform this type of test. The FDR
injects RF energy of constant amplitude across the frequency
band of interest, and analyzes the returned signal to look at
each part of the RF system across the band.
Release V1.1 January 14, 2013 www.anritsu.com Anritsu Company Document # 11410-00700-A
3
The FDR does work similarly to the TDR in that they both
inject energy into a system and compare it to the energy
returned, but by using a constant amplitude sweep of
frequencies; the FDR is able to detect the reactance of
components instead of DC resistance or the presence of a short
or open. By doing this, it is capable of quickly giving the
operator a “snapshot” of how the entire system reacts to the RF
bands of interest. By applying mathematics to convert
frequency domain into the time domain, fault location is
possible.
The Bird Site Analyzer®, Agilent FieldFox® and Anritsu
Site Master® are just a few examples of Frequency Domain
Reflectometer devices available. All of these products display
Insertion or Cable Loss relative to frequency, VSWR relative
to frequency, Return Loss relative to frequency and Distance
to Fault measure in Return Loss or VSWR relative to distance.
Each of these measurements is helpful in evaluation and
maintaining a system. FDR capabilities are being integrated
into multipurpose testing equipment.
All test equipment, including frequency domain
reflectometers, have accuracy specifications published by their
respective manufacturers which should be understood and
considered by the end user of the equipment, and included in
determining the condition of feed line and other components in
transmit or receive systems.
III. ABSOLUTE VS. RELATIVE TESTING
Testing of an RF network can be performed in two
configurations – Absolute and Relative Testing. The degree of
information obtained and the accuracy of the information is the
primary difference between them. Absolute testing is
performed with a 50 ohm precision load as the termination of
the product or system being tested. Relative testing uses a
non-precision load such as the antenna to terminate the system.
1) Absolute Testing
Absolute testing relates to using laboratory precision
terminations and controlled testing techniques to perform tests.
Absolute testing is precision testing within a controlled testing
environment. Absolute testing emulates the tests performed by
the manufacturer and can be used to validate manufacturer
specifications.
The RF network is tested in a closed manner that factors out
external uncertainties. Absolute testing is never conducted
with the final antenna attached. Depending on which
measurement is being made, a known good and calibrated
Open, Short or Load termination must be inserted at the end of
the network under test. A calibration Standard (sometimes
called a “Cal Kit”) has three different terminations: a
Calibrated OPEN, a Calibrated SHORT, and a Calibrated 50
ohm LOAD. A calibrated load is different from other 50 ohm
loads used for line termination because it is an extremely pure
load that has not only been made from a precision resistor, but
also designed to have known consistent frequency, amplitude,
and phase characteristics. Likewise the calibrated open and
short are designed to respond to RF energy in a specific and
repeatable way. The quality of your measurements is only as
good as the quality of your calibration standard! Before any
absolute testing can be performed, the calibration standard
must be verified. A calibration standard that has been stored
in a tool box, or never calibrated, could be damaged, and
should not be used for absolute testing. The calibration
standard must be treated with the same respect and care any
piece of precision test equipment deserves. Understanding the
accuracy and repeatability of the calibration standard’s RF
response is crucial in absolute testing. It is critical that the
calibration standard be returned along with the test equipment
during the regular calibration cycle. This allows the
calibration lab to validate and verify each of the calibration
standards. To further ensure an accurate calibration standard
is used, it is recommended that a cross verification be
performed regularly. Cross verification refers to using a
second network analyzer or FDR to verify the calibration
standard in question. Remember, the calibration standard is a
piece of test equipment itself, and should be treated as such.
Since absolute testing is based on known matching
characteristics, the results can be used to compare with
manufacturer specifications. Performing absolute testing on an
existing system requires taking the system off the air and
inserting the appropriate termination at the top of the tower.
Because of this expense and associated difficulty, absolute
testing is normally reserved for initial commissioning or
critical troubleshooting.
Feed line manufacturers publish product specifications in
various formats which must be properly applied. Some present
impedance characteristics such as “50 ohms +/-1 ohm”.
Others may supply VSWR instead of Return Loss, or
impedance may be given in the frequency domain only rather
than as a Distance To Fault (DTF) specification. Regardless
of format, the specification provided by the manufacturer is
what should be applied in determining feed line health.
2) Relative Testing
Relative testing relates to performing tests outside of a
controlled environment and without controlled test
terminations. This type of testing occurs when an installed
system is tested without the benefit of calibration standards. In
a relative testing environment the matching network or “load”
of the RF system is the antenna itself. Antennas have
significantly varying impedances and matching characteristics,
depending on the frequency, design, quality, antenna type and
installation. They can also be affected by movement,
proximity to other objects including people, and RF signals
from other systems. Because of the uncertainty of the antenna
as a precision load, the results cannot be referenced back to
manufacturer specifications other than performance
specifications of the antenna itself. However this type of
testing is beneficial when compared against a benchmark
portfolio of tests and sweeps that were performed during the
initial installation. As the name implies relative testing must
be compared with something. Absolute testing is compared
with the manufacturer’s testing and specifications; but relative
testing must be compared with the initial installation test
results. When initial test results and sweeps are available, a
comparison with the current sweeps will show changes that
may affect operation. Since the cable matching in the same
network is used in the initial benchmark tests, the comparison
establishes a benchmark to which later tests can be reliably
Release V1.1 January 14, 2013 www.anritsu.com Anritsu Company Document # 11410-00700-A
4
compared with. As long as the comparison is equal, or close,
it can be assumed that the RF network has not changed
significantly.
IV. STANDARDIZED TESTS
Proper evaluation of an RF network involves performing a set
of standardized tests consistently. If performed in the same
manner the results will be consistent and reflect real world
performance. If not performed properly they will be of no use
to anyone trying to evaluate the system.
1) Return Loss (RL)
The Return Loss (RL) and VSWR measurements are key
measurements for anyone making cable and antenna
measurements in the field. These measurements show the user
the impedance match of the system and if it conforms to
system engineering specifications. If problems show up during
this test, there is a very good likelihood that the system has
problems that will affect the end user. A poorly matched
antenna will reflect costly RF energy which will not be
available for use at the load. This extra energy returned to the
source will affect the efficiency of the transferred power and
the corresponding coverage area.
An Absolute Return Loss (or VSWR) test is taken with a
known calibrated load at the end of the RF network to ensure a
perfect match. This allows the network to be the limiting
factor in most reflections. With the calibrated load in place of
the antenna, most reflections that occur will be the result of
impedance mismatches in the network itself. This test allows
the network to be compared to manufacturer specifications that
were taken in like manner. The return loss measurement
should also be taken with the final antenna connected and
installed in the final location as this shows the delivered return
loss of the system and takes installation distortions into
consideration. This relative test will uncover irregularities not
caused by the hardware such as mounting too close to other
metal objects.
Figure 1 shows a typical RL sweep. This sweep over
frequency can be used to validate manufacturer specifications
only when a calibrated termination is used.
Typical Return Loss Sweep
Figure 1
2) Insertion Loss (IL)
As the RF signal travels through the RF network, some of the
energy will be dissipated in the cable and the components. A
Cable Loss measurement is usually made during the
installation phase to ensure that the cable loss is within
manufacturer’s specification. Cable Loss measurement is not
isolated to the transmission line but all components in the
network. When performing Cable Loss measurements be
aware of components that may have frequency characteristics
which could affect the results.
There are two types of Cable Loss measurement. Two-Port
Insertion Loss (2PIL) uses a test instrument in which the test
signal is generated on one RF port and received by a second
RF port on the same instrument. This method directly
measures the loss of a system with high accuracy, however it is
not always possible to connect physically to both ends of a
cable, so a second method is needed. One-Port Cable Loss
(1PCL, or just CL) uses a measurement method in which the
RF energy is generated and received by a single port. In
effect, 1PCL is Insertion Loss divided by two, and as such it
must be understood that it suffers from the same uncertainties
as Return Loss. One-Port Cable Loss is only an absolute test
when done with a calibrated open or short connected to the
end of the RF network, because only a calibrated open or short
provides a consistent and total reflection to the test signal.
The test instrument compares the generated signal against the
reflected signal and divides the difference by two. The One-
Port Cable Loss data is normally the average of the
maximum/minimum value. The 1PCL measurement can only
be accurate if the reflection is total; i.e. a calibrated short or
open must be used. Relative CL tests cannot be performed as
the energy used to measure system loss will be radiated by the
antenna and not reflected. Figure 2 shows a typical CL sweep
taken with a calibrated open or short at the end of the cable.
The results of this sweep can be compared with the insertion
loss of the cable, connectors, and any other devices present to
the manufactured standard.
Typical Insertion Loss Sweep
Figure 2
3) Distance to Fault (DTF)
The most controversial test is the Distance to Fault (DTF).
The DTF maps the RL (or VSWR) over the length of the
complete network; this is referred to as DTF-RL. While the
DTF sweep is a great troubleshooting tool, it is also a great
quality analysis tool. There are times when the CL and RL
sweeps meet the manufacturer specifications, but irregularities
along the cable cause failure in DTF expectations. Two very
specific situations will be shown in the Case Studies that show
the DTF can fail and real problems exist, even when the RL
and CL sweeps pass. The DTF sweeps can only be performed
Release V1.1 January 14, 2013 www.anritsu.com Anritsu Company Document # 11410-00700-A
5
reliably and effectively in the Absolute Testing mode. DTF is
not reliable in the Relative mode for determining failure,
unless another previous Relative Sweep is available for
comparison. Figure 3 shows a typical DTF-RL sweep.
The most questionable item is the DTF Return Loss
threshold to be used for pass/fail. Cable manufacturers are just
now beginning to perform DTF on their products, and have not
published specifications for DTF Return Loss. While there is
not a true standard for the acceptable level of DTF Return
Loss, it is a reasonable expectation that it should be between -
40 dB and -50 dB when sweeping primary feed lines. A DTF
Return Loss of -50 dB will have fewer imperfections and
irregularities than a network measuring -40 dB. Risk and
system requirements are the two determining components in
selecting a tolerance threshold. A mission critical Public
Safety system may require a DTF Return Loss better than -45
dB where a commercial cellular system may require only -40
dB. The expectation must be identified before testing begins.
Failure of DTF Return Loss is associated with impedance
changes along the cable or at the transition of a connector.
These changes can be caused by cable bends that kink,
improperly installed connectors, stretched cable, dents which
change the dielectric spacing, or water intrusion. Because one
or two irregularities can have minimal effect on the overall RL
characteristics of the cable, the absolute RL Sweep may not be
affected. Nevertheless, if dents, kinks, or other impairments
exist the cable could be considered bad. DTF not only helps
identify where irregularities occur on the cable but also their
severity.
Typical Distance to Fault (DTF-RL) Sweep
Figure 3
V. METHOD OF PROCEDURE
A Method of Procedure (MOP) is needed to ensure a
standardized approach to testing. The MOP is a written
document of procedures and testing methods that outlines
exactly what and how the data is collected. The MOP is
similar to the checklist used by airline pilots to ensure
everything is completed in a specific order and completely.
When an engineer or technician is performing a deployment
verification, hardware validation, or relative testing of an
existing system; an understanding of what and how the tests
were performed is critical in obtaining acceptance of the
results by reviewers that were not on site. The MOP
establishes a consistent foundation for the testing
methodology, and removes uncertainties that could cause
invalidation of the data.
Antenna system commissioning is necessary to verify the
integrity and performance of an antenna system. Antenna
system commissioning involves both physical inspection and
electrical testing. Physical inspection of the antenna system
should include an installation audit which includes an audit of
the cable while still on the reel, transmission line ground kits,
transmission line mounting hangers and lightning suppressors.
A physical audit is important to ensure any damage that may
have occurred during transit is found before an expensive
installation occurs. The electrical testing includes a series of
tests using a Frequency Domain Reflectometer (FDR).
The MOP is not intended to replace proper training in the
field of antenna system concepts, nor is it intended to replace
training in the use and operation of the test equipment. The
MOP is designed as a guideline for trained and experienced
technologists and engineers.
The MOP must contain several important components to
consider as line sweeping is performed. If these components
are not included in the process the results may be questionable.
Below are components that should be considered in any
MOP. While the expected results may vary depending on the
system type and requirements of the system deployment team,
the components and the processes should vary little.
1) Data Collection and documentation
Before antenna and line commissioning or testing is started,
it is necessary to have the electrical specifications for all the
RF network components.
The system designer should supply this information or make
it available before the MOP is begun. This site specific
information establishes the expectation and allows rapid
comparison of the collected data. This data is also needed to
program the test equipment to ensure the equipment knows the
cable type and characteristics. Drawings also allow the person
performing the test to fully understand the components
included in the network which will assist in understanding and
interpreting the results.
The electrical specifications needed are:
Antenna frequency range and return loss specifications
Jumper cable type, velocity factor, insertion loss, and
return loss
Transmission line type, velocity factor, insertion loss, and
return loss
Lightning suppressor frequency range, insertion loss, and
return loss
RF connector type, insertion loss, and return loss
Expected transmission line system insertion loss
Release V1.1 January 14, 2013 www.anritsu.com Anritsu Company Document # 11410-00700-A
6
2) Analyzer and Test Requirements
The Analyzer used in the field is considered laboratory grade
equipment and must be treated and used accordingly. Proper
setup and configuration of the analyzer is critical for
meaningful and accurate measurements. Below is a list of the
analyzer and test configuration requirements:
Analyzer should be loaded with the most current firmware.
Analyzer must be in known good working order and
serviced at the factory as recommended by the manufacturer.
Precision load must be in known good working order.
The precision load is a very delicate piece of equipment and must
be treated with care. If the precision load is dropped from any
height it should not be used again on projects until its proper
operation is verified by the manufacturer.
Analyzer must be calibrated at the ambient temperature in
which it will be operated.
Analyzer must be re-calibrated whenever its temperature
changes significantly, or when the analyzer display indicates
that the calibration is no longer valid due to temperature
change.
Analyzer must be re-calibrated whenever the setup
frequency changes.
Analyzer must be re-calibrated whenever the test port
extension cable is added, removed, or replaced.
Analyzer must be re-calibrated if it has been turned off for
any significant length of time.
Analyzer must be re-calibrated anytime “noise” or “picket
fencing” appears at the bottom of the display during a
distance-to-fault measurement (down around -50dB).
The calibration results and precision load should be tested
by performing a return loss test on the load after calibration.
A return loss of -42dB or better should be obtained from the
precision load.
Analyzer resolution should be set to maximum for the
highest quality and most accurate printouts.
When using a load, only a precision load shall be used.
Adapters should be avoided whenever possible. If adapters
are needed, only precision adapters shall be used.
If an extension cable is needed, only a phase stable cable
shall be used.
Never use your primary calibration standard on the tower or
at the remote end for absolute testing. Keep your calibration
standard in a controlled environment to ensure integrity.
3) Testing Documentation
When antenna system commissioning is performed, it is
necessary that all tests are properly documented for use in the
system manual and for future antenna system testing and/or
troubleshooting. All relative testing accuracy will depend on
the quality and the attention to detail used in the
commissioning documentation.
The results of the MOP tests should be readily available in
the system manual as a viewable file, such as *.pdf or *.wmf
formats. To allow side by side comparison of relative data the
raw data files should also be available. To coordinate use, all
traces should contain common information and be taken in
similar formats.
As a minimum, all software traces should identify the
following:
Site name
Clear identification of Antenna system tested (e.g., TX 1,
TX 2, TX North leg, RX, Blue, Red, etc.)
Test type (e.g., Return Loss, Insertion Loss, or Distance-
to-Fault)
Test details (e.g., with jumper, terminated with precision
load, terminated with open/short or terminated with
antenna)
4) Standardized tests to perform
A list of all tests that should be performed is critical in
ensuring complete and thorough testing. These tests will
involve both Relative Testing and Absolute Testing. While
the procedure for performing these tests is very important, it is
beyond the scope of this paper to present. Each MOP
developed should outline exactly how and any special
considerations to be used in performing the tests.
Standardized testing must be preceded with visual inspection
of the cable and reel to identify any shipping or other damage
that may have occurred.
Tests that should be considered a part of the MOP are:
Jumper Insertion Loss and Return – All jumpers should
be tested and verified before installation. (Absolute Test)
Antenna Testing – Each antenna should be tested on the
ground before installing. When testing an antenna the
antenna test location should not be near metallic materials
and needs to be above the ground as much as possible. If
using a directional antenna, the antenna shall be pointed
vertically and/or away from possible sources of RF
energy. (Relative Test)
Verify End of the Antenna System – This test involves
performing a DTF using precision terminations (Open or
Short) and is the absolute verification of the cable that can
be compared with the manufacturer specifications. This
test verifies that cable supplied is the length specified, and
that the end of the cable is actually visible on the sweep.
This test may involve installing test connectors on a reel
of cable. (Absolute Test)
Antenna System Insertion Loss (Cable Loss) – This test
is used to measure and validate the insertion loss of the
entire antenna system (i.e., main transmission line,
jumpers, and lightning suppressor) with a calibrated open
or short. The results of this test can be compared with the
design engineer’s theoretical expectations. (Absolute
Test)
Transmission Line Distance-to-Fault While
Terminated with Antenna Jumper and Precision Load
– This test is similar to the “Verify End of the Antenna
System” test with the exception that this test is performed
after installation and will show installation errors.
(Absolute Test)
Transmission Line Return Loss While Terminated
with Antenna Jumper and Precision Load – Final
Release V1.1 January 14, 2013 www.anritsu.com Anritsu Company Document # 11410-00700-A
7
Return Loss test after installation. This validates the RF
network match. This test will be used as the foundation
for operational validation. (Absolute Test)
Complete Antenna System Return Loss – Similar to the
Absolute Test of Return Loss, except this test is
performed with the actual antenna connected. This test
will be used to validate changes in the antenna system
over time without the need for a tower climb. (Relative
Test)
Complete Antenna System Distance-to-Fault - Similar
to the Verify End of Antenna System, except the final
antenna is used for termination. This test will be used to
validate changes in the antenna system over time without
the need for a tower climb. (Relative Test)
VI. EXAMPLE CASE STUDIES
To fully understand the mistakes and uncertainties that can
exist we will present three case studies which demonstrate how
lack of discipline in testing and evaluation produced
significant miscommunication and mistakes. These case
studies will also demonstrate how organized testing can
eliminate problems before installation. These case studies
point to the logic of a standardized MOP, in order to control
data collection and evaluation.
1) Case Study Number 1
The first case study is an actual city-wide public safety
project which experienced significant delays and unnecessary
complexity. The customer wanted to reuse their existing
cables and connectors on a new communication system. While
it was strongly recommended that new cable and connectors be
used because of the age of the existing cables, the customer
was adamant concerning the reuse of the old cable. The
customer hired an independent “line sweeping consultant(s)”
and used a contractor’s staff members to conduct line sweeps.
Because of the nonstandard tests performed and lack of
understanding of the testing conditions the results were not
readily accepted. Debate over what the data showed occurred
over the next three month between the project team, the feed
line manufacturer, two test equipment manufacturers, multiple
line sweeping experts and others with line sweeping
experience and expertise. This debate and review by experts
could not produce agreeable results.
Four major points resulted in the data not being acceptable or
useable:
Documentation was poor or non-existent for most sites
Procedures used were sloppy and were not standardized.
An MOP was not followed
The test equipment used was older and not in verifiable
condition
Calibration certification of terminations was not available.
The results of the sweeps produced evidence of cables that
did not meet manufacturer specifications.
Questionable DTF Sweep
Figure 4
For one specific DTF sweep, the sweep showed several
spikes that were questionable. (Figure 4) Depending on the
threshold used for acceptability, the line could be considered
bad. Nevertheless, there are issues indicated on the cable.
Following absolute testing procedures would have provided
conclusive non-controversial data to evaluate the quality of the
cable.
After considerable debate over the quality and condition of
the line, an inspection was conducted. One of the spikes of
questionable quality occurred at 74 feet (-40 dB). The tower
crew was asked to return to the site to verify what was at the
physical location. The physical inspection revealed incorrect
cable clamps (Figure 5) that did not provide uniform pressure
to the cable.
Improper Cable Clamps
Figure 5
Case Study Number 1 conclusion: Proper documentation
and following an acceptable MOP would have made a
significant difference in the outcome of this discussion. As
recommended elsewhere in this document, a systematic
approach to sweeping the lines and determining the health of
the system requires all the tests to be executed properly. In
this instance, it was found that the testing was not complete;
the system contained nearly 30% of feed lines which should
Release V1.1 January 14, 2013 www.anritsu.com Anritsu Company Document # 11410-00700-A
8
have been condemned and replaced, but were instead left in
place potentially compromising coverage and reliability.
Many of the feed lines showed signs of water intrusion and
corrosion at the connectors, problems not seen or noted by the
crew conducting the testing.
2) Case Study Number 2
After the receipt of a new reel of cable, a project team
performed absolute line sweeping tests, following the certified
MOP tests, on the cable. Performing these tests while still on
the ground and not installed is a very good procedure. This
testing identifies “As Received” before installation and can
prevent installing bad cable. The test identified an issue with
the cable. In this case the team jumped to the conclusion that
the cable had manufactured defects. Testing of the reel
showed a good RL sweep (Figure 6) but a questionable DTF
sweep (Figure 7).
Good RL Sweep
Figure 6
Questionable DTF Sweep
Figure 7
The cable was returned to the manufacturer for testing and
analysis. The manufacturer compared the quality data on the
bulk reel before it left the manufacturing facility with the
returned cable. The bulk reel is a large 10,000 foot reel that is
used to supply cable of different lengths from a cutting
process. This cutting process may be done by distributors or
by the manufacturer. The cable did exhibit a DTF spike
greater than -40 db as found by the field team. Visual
examination of the cable showed a large dent in the cable.
(Figure 8)
Dented Cable
Figure 8
The dented area was removed and the cable retested. The
DTF sweep was performed again and the cable tested to better
than -50 dBm. The jacket was removed from damaged area to
expose a dent in the cable. (Figure 9).
Cable damage
Figure 9
Case Study Number 2 conclusions: The dent in the cable was
the cause of the DTF spike greater then -40dB. The dent was
not caused during manufacturing as the cable would not fit
through the jacket extrusion die with the deformity. In
addition, the bulk cable passed DTF testing. While the exact
cause of the dent and defect is unknown, it is anticipated to be
transportation damage. The dent was on the outside layer of
the cable reel and thus could have been found with visual
inspection. This case study demonstrates the importance of
inspecting cable and performing Line Sweeping tests prior to
installation.
3) Case Study Number 3
After the receipt of a new reel of cable a project team
performed absolute line sweeping tests following the certified
MOP test procedure. While the tested roll of cable exhibited a
good RL sweep (Figure 10) it had a bad DTF spike of -28 dB
(Figure 11) at the 148 foot point on the cable.
Release V1.1 January 14, 2013 www.anritsu.com Anritsu Company Document # 11410-00700-A
9
Good RL Sweep
Figure 10
Bad DTF Sweep
Figure 11
The reel of cable was returned to the manufacturer for
evaluation. Manufacturer evaluation validated the DTF spike
seen in the field but showed the bulk reel had no deformities
when it was shipped. The cable was unwound from the cable
reel and the point of failure identified. The area was cut out
and the reel was re-tested. After the area was extracted, the
cable performance was better than -50 dB which compared
with the bulk reel performance. When the damaged area was
examined it was found to be stretched. (Figure 12) Since the
damage was found before it was installed, the damage was
believed to be caused between the manufacturer facility and
delivery to the field. Stretching of the cable is a possible
defect in cutting cable. Cable is distributed in very large
quantities and cut to length. Cutting involves unrolling the
cable from the bulk reel and re-spooling it for shipment. This
re-spooling process can cause stretching if the tension is
incorrect. This is the probable cause of this cable distortion.
Bad DTF Sweep
Figure 12
Case Study Number 3 conclusion: The stretched cable
(corrugation deformation) caused a DTF spike greater than -
50dB. The stretch point in the cable was at 148 ft. While this
can occur during cable manufacturing (start up), the bulk
length spool did not show this DTF spike, and the defect was
determined not to be manufacturing related. The cut off length
was supplied by a distributor who purchases bulk cable and
then cuts it down to custom lengths to fulfill orders from
customers. Although the distributor’s facility was not
inspected, they do have cable processing capability, and it
could be possible that the cable was stretched during cable
processing (cutting for customer order).
Release V1.1 January 14, 2013 www.anritsu.com Anritsu Company Document # 11410-00700-A
10
FINAL CONCLUSIONS: All of the preceding case studies
support the premise as set out in this position paper; that Line
Sweeping executed using current Frequency Domain
Reflectometers is a valid test of feed line and component
viability; that when properly executed and the results are
properly and thoroughly documented, line sweeping using a
combination of all three tests (RL, IL, and DTF) are important
in determining the reliability of feed line systems; and that
these tests will determine if coverage predictions can and will
be met. Distance To Fault (DTF), Frequency Match Return
Loss sweeps and Insertion or Cable Loss sweeps all form a
valuable set of field tests usable at site commissioning to show
that the system, as installed, will meet design specification,
and further are reliable techniques for determining system
health and finding faults later in the life time of the system
under test.
VII. CONSENSUS RECOMMENDATIONS
Consistency relies on data collected by different parties
being usable with confidence by others. To collect consistent
data, discipline must be used and standardized procedures
must be followed. Below are several important considerations
the industry must consider and followed to ensure useable and
correlatable data.
• Before beginning any tests have a good MOP and
follow it carefully. The MOP ensures the tests are
performed correctly and completely. Don’t cut
corners.
• Formal training in the proper use of test equipment
is critical to ensure measurements are performed
reliably and consistently.
• Maintain all test equipment and calibration
standards with the respect they deserve.
• Calibration of the test equipment and all calibration
standards is critical for reliable and consistent
results.
• Perform physical inspections and absolute testing on
new cables as they arrive. Don’t assume the cable is
intact.
• Always keep in mind that measurement data from
any test is only as good as the precision and diligence
used to perform the test.
• Measurement results can only be objectively
evaluated when the results are documented in a
professional manner. Evaluation relies on having
confidence in the data presented. Accurate and
consistent record keeping provides a foundation
upon which your team can have confidence in the
data.
Release V1.1 January 14, 2013 www.anritsu.com Anritsu Company Document # 11410-00700-A
11
Appendix 1 – List of manufacturers participating in the Line Sweeping workshop
Below is a list of manufacturers that participated in the two day workshop to discuss all aspects of
sweeping RF networks and what is required for reliable and consistent results. These manufacturers
worked diligently to produce this consensus document of understanding.