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Keysight Technologies
Techniques for Precise Cable and
Antenna Measurements in the Field
Using FieldFox Handheld Analyzers
Application Note
This application note introduces the practical aspects of cable and
antenna testing, interpreting measurement results and instrument
operation including calibration options such as CalReady and QuickCal
using a Keysight Technologies, Inc. FieldFox handheld analyzer conigured
as a cable and antenna analyzer. Measurement examples are provided
showing techniques for measuring insertion loss, return loss, and
locating faults in a transmission system. Carry precision with you.
Introduction
Cable and antenna measurements are often required to verify and troubleshoot the electrical
performance of RF and microwave transmission systems and antennas. In RF and microwave
communications industries, measurements are often made along the coaxial cable connecting a
transmitter to its antenna and/or between an antenna and its receiver. This process is sometimes
referred to as Line Sweeping and results in the measurement of the signal attenuation and return loss
as a function of frequency. Line sweeping may also be used to estimate the physical location of a fault
or damage along the transmission line using the Distance-to-Fault (DTF) measurement available on
many RF and microwave signal analyzers such as FieldFox. Testing the performance of transmission
lines is not limited to only coaxial cable types but systems using waveguide and twisted pair cables
may also be characterized once the appropriate adapter is installed between the transmission line
and the coaxial interface on the analyzer. In addition, antenna measurements, in the form of signal
relection, or namely return loss and VSWR, may be used to verify the performance of an antenna at
the installation site. When multiple antennas are required at an installation site, whether the antennas
are associated with the same system or different systems, the antenna-to-antenna isolation may also
be veriied using FieldFox.
Figure 1 shows a typical return loss measurement (upper trace) and DTF measurement (lower trace)
for an X-band antenna connected to a 3-meter length of coaxial cable. The antenna was mounted
on a vertical mast at a roof-top installation and measured using FieldFox conigured with a preset
calibration called CalReady. The return loss measurement includes two frequency markers placed
across the speciied operating band at 8 GHz and 12.4 GHz. The DTF measurement includes a
distance marker placed at the location of the antenna at 3.23 meters.
This application note will introduce the practical aspects of cable and antenna testing, interpreting
measurement results and instrument operation including calibration options such as CalReady and
QuickCal using FieldFox.
Figure 1. Measured return loss (upper trace) and distance-to-fault (lower trace) for an X-band antenna
and coaxial feed cable
03 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Why is cable and antenna testing important?
During the installation, operation and cable manufacturer. For example,
maintenance of a telecommunication low-loss coaxial cable may have a specified
system, the connecting cables, adapters minimum bend radius of 1 inch to over 10
and antennas may become damaged or inches and bending the cable below the
show reduced performance over time. It is specification will cause damage to the
known that fifty to sixty percent of cellular cable and substantially degrade its
base station problems are caused by faulty electrical performance.
cables, connectors and antennas. Failure
in these components causes poor coverage Verifying and maintaining the operation of
and unnecessary handovers in the cellular the RF and microwave transmission system,
system. In any communication system, it including coaxial cable, waveguide and
becomes important to have a process and twisted pair line, and antennas, requires fast
the appropriate instrumentation to verify and accurate measurements of the insertion
the performance of the cables and loss and the return loss as a function of
antennas when issues are discovered frequency. Also important to field operations
at the system level. is the capability to quickly troubleshoot and
locate faults and damage in cables and
Cabling and antennas may be installed in a connectors. A modern handheld analyzer,
variety of environments including outdoor such as FieldFox, configured for cable and
and indoor installations. Outdoor installa- antenna testing (CAT), is an extremely
tions may include roof-top, tower mounted useful tool for characterizing the entire
and underground cabling. Indoor installa- transmission system as well as the
tions may include equipment shelters, office performance of individual components in
buildings, shipboard, aircraft and certain the system. In the next few sections of this
automotive applications. At an outdoor application note, measurement examples
installation of a wireless cellular system, the will be provided showing techniques for
antenna and portions of the coaxial cabling measuring insertion loss, return loss and
are often exposed to extreme weather con- locating faults in a transmission system.
ditions including rain, snow, ice, wind and The measurements will be made using a
lightning. Severe environmental conditions FieldFox N991XA analyzer with frequency
can potentially create physical damage to range up to 26.5 GHz.
the system components including failed
waterproof sealing at interfaces between
cables and connectors, failed sealing at
cable splices, and weather-induced cracks
in the insulation resulting in an ingress of
moisture into the cables and interfaces.
Sheltered installations can expose compo-
nents to mishandling, stress, heat, vibration,
and oils and other containments that could
leak into the system. Cable fault can also
occur at any interface between the cables
and connectors where solder joints and
cable crimps become fatigued and break. It
is also possible that cables can be dented,
crushed or improperly routed during or
after installation. Conditions may also be
created where the minimum bend radius is
exceeded beyond the value specified by the
04 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Cable insertion loss measurements
The insertion loss of transmission line or cable using a two-port technique. Figure 2b The ratio between the input and output
coaxial cable is often measured as a function shows a novel technique, available on the signals represents the total insertion loss
of the intended operating frequency band. FieldFox, for measuring the insertion loss of of the cable usually expressed in decibels
The insertion loss, also referred to as the a cable using measurements obtained from (dB). An ideal lossless cable would have 0
cable attenuation, is the amount of energy only one end of the cable. Both of these dB insertion loss. The cable manufacturer
dissipated in the cable. The insertion loss techniques will be discussed. usually provides a table of the insertion loss
also includes energy lost due to mismatch as a function of frequency. For example,
reflection between the source and load. It is In the traditional two-port setup, the cable an LMR900 coaxial cable would have 2.2
expected that the source (transmitter), the under test is connected to two separate ports dB insertion loss for a 100 meter length of
transmission line (coaxial cable), and the on FieldFox. FieldFox injects a test signal into cable measured at 150 MHz. The same cable
load (antenna) are all designed for the same the cable from the RF OUT port conveniently would have 7.4 dB loss at 1.5 GHz. Those
characteristic impedance, Zo, usually 50 positioned along the top of the analyzer. As familiar with vector network analyzers (VNA)
ohms or 75 ohms when using coaxial cable. the test signal passes through the cable, a will recognize this insertion loss measure-
It has been shown 1 that coaxial cable having small portion of the energy is absorbed by ment as the absolute value of the S21 trans-
an air dielectric has the lowest insertion loss resistive and dielectric losses in the cable. mission scattering parameter (S-parameter).
at 75 ohms and the highest power handling Discontinuities from cable connectors, Additional information regarding VNA options
capability at 30 ohms. The Cable TV (CATV) cable splices, damage and other factors will on FieldFox and the basics of network anal-
industry has standardized on 75-ohm cable reflect a portion of the energy back to the ysis can be found at the following references
in order to maximize signal transmission over source resulting in an additional increase in [1] and [2]. It should be noted that accurate
very long cable distances required by the the measured insertion loss. The remaining insertion loss measurements require an initial
system. On the other hand, the majority of signal exiting the cable is then measured by calibration of FieldFox.
the RF and microwave industry has standard- FieldFox at the RF IN port.
ized on 50-ohm impedance as a compromise
between the lowest loss and the highest
power handling. Most RF and microwave
analyzers, such as FieldFox, are configured
with 50-ohm test port impedances. When
measuring 75-ohm cables and components,
50-ohm to 75-ohm adapters, such as the
Keysight Technologies, Inc. N9910X-846,
should be connected to the analyzer.
Once the transmission line cables are
installed into a system, it is often difficult and
costly to remove them in order to verify their
operation and troubleshoot cable failures.
Also with very long cable runs, access to
both ends of the cable at the same time
is typically impossible, especially when
attempting to connect the cable to the test
instrumentation. Under these conditions,
techniques that allow cable insertion loss
measurements to be made from only one
end of the cable are preferred. Figure 2
shows two configurations for measuring the
insertion loss of a cable using FieldFox with
CAT mode. Figure 2a shows the traditional
method for measuring the insertion loss of a
Figure 2. Configurations for measuring the insertion loss of a cable using FieldFox with CAT mode
05 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Cable insertion loss measurements (continued)
The calibration removes the insertion loss Once the reflected measurement is The load is removed and the cable is termi-
effects of the equipment, adapters and complete, FieldFox, using a built-in model nated in an open (or short), same as before.
jumper cables as a function of frequency. for coaxial cable dispersion and knowing The measurement of the open (short) is then
The calibration of FieldFox for making that the measured signal contains twice the subtracted from the memory using the Data
one and two-port measurements will be cable insertion loss (due to the round trip Math, Data-Mem. This additional mea-
discussed later in this application note. path), can now report the cable insertion surement step may improve the observed
Figure 3 shows the insertion loss measure- loss as a function of frequency. Figure ripple in the 1-port cable measurement. In
ment of a coaxial cable using the 2-port 3 shows a comparison of the measured general, the traditional 2-port insertion loss
technique over the frequency range of insertion loss using the 2-port and 1-port measurement will be more accurate than
30 kHz to 18 GHz. This figure also shows a techniques. In this figure, the blue line a 1-port cable measurement but having a
measurement of the insertion loss using a represents the 2-port insertion loss mea- measurement process that does not require
1-port cable loss technique to be surement and the yellow line represents an instrument connection to both ends of
discussed next. the 1-port cable loss measurement. As the cable is a great benefit when character-
observed in figure 3, the 1-port technique izing installed cabled systems.
As previously mentioned, when a cable is contains additional amplitude ripple across
installed into a system, it is often difficult the measured frequency range due to
to physically connect FieldFox to both ends mismatch effects between the open (short)
of a very long cable without introducing and the input connector. Using FieldFox, the
an equally long jumper cable into the test amplitude ripple may be reduced with an
setup. Fortunately, FieldFox has a conve- additional measurement of a 50-ohm load
nient technique to measure cable insertion placed at the end of the cable under test. In
loss from only one end of the cable under this case, the measurement of the 50-ohm
test. This eliminates the need to carry an load is saved to memory using the Trace,
extra-long, high-quality test cable as part Data-> Mem.
of the equipment requirements. Figure 2b
shows the simple 1-port configuration for
measuring cable insertion loss by attaching
one end of the cable to FieldFox and
leaving the other end open or terminated
in a short. It is preferred at microwave
frequencies to use the short circuit to elimi-
nate fringing fields found in an open-ended
cable which could alter the measured
results. In this configuration, FieldFox
injects a test signal into the cable from the
RF OUT port, same as before. The test sig-
nal passes through the cable, is completely
reflected from the open end (or shorted
end), passes through the cable a second
time and lastly measured by FieldFox using
the same port. FieldFox is configured with
an internal factory-calibrated "reflectom- Figure 3. Measurement of coaxial cable insertion loss using a novel one-port technique (yellow) and a
eter" 1 for measuring reflected signals at traditional two-port technique (blue), both available on FieldFox
the RF OUT port. This internal calibration,
called CalReady, is available at instrument
turn-on and instrument preset and will be
discussed later in this application note.
06 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Cable insertion loss measurements (continued)
There is another configuration for measuring Other techniques to reduce the displayed
cable insertion loss that does not require a noise level include increasing the number of
long coaxial cable in the return path back trace averages or decreasing the IF band-
to FieldFox. In this configuration, FieldFox width setting. Trace averaging will reduce
is configured as a power meter connected the effects of random noise on the measure-
to a Keysight U2000 Series USB power ment by averaging the results from multiple
sensor. As shown in figure 4, one end of sweeps. FieldFox trace averaging can be set
the cable under test is directly connected to over a range from 1 (no averaging) to 1000.
the RF OUT port of FieldFox. The other end Knowing that trace averaging requires the
of the installed cable is connected to the measurement of multiple traces, it should
USB power sensor. FieldFox is configured to be expected that there will be an increase
generate a CW signal at the RF OUT port. in the total measurement time as multiple
This CW signal is the test signal that will be sweeps are required. Reducing the IF
transmitted along the cable and measured bandwidth setting on FieldFox will also
by the USB power sensor. If the cable ends reduce displayed noise. The IF bandwidth
are physically separated by a large distance, can be adjusted over the range of 100 Hz
the USB sensor can be connected back to to 300 kHz. It should be noted a lower IF
FieldFox through a USB cable extender. bandwidth will also increase the overall
USB cable extenders are commercially measurement time as lower bandwidths
available with ranges of 500 meters or increase the instrument sweep time.
more. This configuration overcomes the lim-
itation of running an expensive coaxial cable
between one end of the installed cable
back to the instrument. The main constraint
using the power meter solution is that swept
frequency measurements are not available
and therefore changing test frequencies will
require manual tuning of FieldFox's settings.
When measuring cables with high insertion
loss, the displayed measurement trace may
exhibit a relatively high level of noise as
the measured signal amplitude approaches
the noise floor of the analyzer. One way
to improve the relative signal to noise and
improving the associated measurement
accuracy is to set the output power on
FieldFox to High Power mode. High Power
mode is the default setting for FieldFox and
is typically used for most passive device
measurements. The Low Power mode is
typically used when measuring high-gain or
low-noise amplifiers to prevent saturation
and potential damage to the amplifier. The
High Power setting results in a port power
of approximately +5 dBm, the Low Power
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