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Cable and Antenna Analysis Troubleshooting Guide – Utilizing Anritsu’S Handheld Site Master™ S331L

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Cable and Antenna Analysis Troubleshooting Guide – Utilizing Anritsu’S Handheld Site Master™ S331L Cable and Antenna Analysis Troubleshooting Guide – utilizing Anritsu’s Handheld Site Master™ S331L Antenna Sweeping Antenna Sweeping (continued) Transmission Line Concepts Return Loss and VSWR Distance to Fault (DTF) Why Sweep Antennas? Maintenance Sweeps Antenna cables are a type of transmission line, a Measuring Reflections Comparing Traces Commissioning Sweeps Troubleshooting Sweeps cable that has constant impedance throughout its Setting Cable Types Propagation Velocity and Cable Loss Common Faults length. Any change in impedance causes a partial Limit Lines Frequency Range Why sweep antennas? Poor VSWR/Return Loss radio signal reflection. can damage transmitters, reduce the coverage area, Maintenance sweeps can catch faults early, which and lower data rates. For instance, a return loss of will increase network uptime, reduce dropped calls, 10 dB means that 10% of the total power is not and allow consistently higher data rates. radiated and (if the transmitter is still running) that the coverage area is 10% smaller than the Since every cable has a different sweep signature, it transmitter power settings might imply. can be difficult to judge changes. But a reference trace, from the commissioning sweeps, makes it easy to see changes. Once a problem is spotted, whether from a VSWR alarm or a maintenance sweep, it’s time to troubleshoot the problem. Troubleshooting sweeps are inherently different from maintenance or commissioning sweeps, since Return Loss, or VSWR if you prefer, can be used as they require a flexible use of the instrument, DTF is a way to locate faults identified by Return a one-number screening tool. calibrations, and settings to accurately locate the Loss or VSWR measurements. fault. FlexCal™ is a Site Master capability that As seen above, the markers for this sweep are set Trace comparisons are often used for diagnostics allows users to change frequency range without a at the edges of the antenna’s pass band. The trace because small changes in cables will have large new calibration, which is helpful when between the markers is better than 15.5 dB, (or a effects on the DTF trace. Because of this, it is Reduced coverage increases dropped and blocked troubleshooting. VSWR of 1.40) a common limit for sweeps with an accepted practice to take reference sweeps of each calls due to weak signal areas and network loading antenna at the far end. cable at commissioning time for later comparison. imbalances. Common faults include connector, cable and antenna faults. When looking for faults, it’s This trace would typically be accepted as good. Changes are often more significant than actual On the data link side, the increased signal distortion important to know that most faults are connector Reflections are measured using either VSWR or values. Even so, typical values with a good setup caused by a low return loss means that data rates related. This includes Changes in impedance, in turn, are caused by Return Loss. These are two different ways to are: are reduced and capacity suffers. loose connectors, mismatches, or physical changes in the cable, such measure the same thing. Return Loss is a Finally, poor Return Loss or VSWR can cause corroded connectors, as: logarithmic scale, and Voltage Standing Wave Ratio Open or Short 0 to 5 dB transmitter shutdown and even damage, taking out and poorly installed Narrow spots, perhaps caused by clamps, (VSWR) is a linear scale. Your choice can be made Antenna 15 to 25 dB a sector until repaired. connectors. Most sharp bends, cable stretch, or other by personal preference, the unit’s limit numbers are Connectors 30 to 40 dB remaining faults are external pressure given in, or by company requirements. Here’s the Keeping antenna systems in shape means the cable related. This conversion formula: system as a whole runs better. Uptime is increased, Change in the internal insulating material, Propagation velocity (PV or Vp) Is sometimes includes water in the Return Loss = 20 log |VSWR +1/VSWR-1| call drops go down, data rates go up, and both the dielectric, for instance, when water referred to only as Velocity, or Velocity of cable, loose weather —RL/20 -RL/20 managers and customers are happier. gets into the cable VSWR = 1+10 /1-10 propagation. It is commonly specified as either a wrap, pinched cables, decimal value Eg: 0.87 or as a percentage Eg: 87%, However the quickest conversion is to just change Commissioning sweeps provide the basis for poorly installed ground Connectors, particularly when improperly and it is a measure of how quickly the RF signals the Site Master units to the preferred settings. acceptance of the antenna systems. They also kits, bullet holes, and installed, loose or corroded. can propagate through the cable. The value is even nails in the cable! A small portion of the faults provide a reference trace for use when looking for Physical damage such as bullet holes or Limit lines can be created on the S331L referenced to the speed of light, so 87% Vp means are antenna related. changes later. For best accuracy, they use a short nails Site Master and limit pass/fail or limit alarms can be the signals will travel at 87% of the speed of light. and a load at the top of the antenna run. They may It is possible to damage activated as desired. This makes judging results fast Vp directly affects distance accuracy in DTF The term Voltage Standing Wave Ratio (VSWR) also be done with an antenna attached. GPS antennas by and easy. measurements. Vp must be set either manually or comes from how radio waves are distributed along a sweeping. Their active by selecting a cable type from the built-in cable list. The same data that is used for Return Loss or VSWR transmission line. When reflections are present, a components are not is also used for the Distance to Fault (DTF) combination of the forward (incident) and reflected Cable Loss also needs to be set accurately, either intended to take higher calculations and display. Users can change between wave produce a standing wave that forces the RF manually, or by selecting a cable type. Incorrect power levels so they the two displays at will. This reduces the number of voltage to vary with distance. The ratio between the cable loss values can mask Return Loss or VSWR should be replaced by a traces that must be archived and tracked by half. high and low voltage in the transmission line is the problems. load (for Return Loss) or Cable loss is also an important commissioning Voltage Standing Wave Ratio. The log version of a short (for DTF) before The Frequency range for DTF sweeps should be check. Excessive cable loss reduces the radiated VSWR is called Return Loss. measuring the line. set to stay within the load’s bandwidth. If an power, but also, can mask return loss issues, antenna is used for the load, any portion of the DTF creating false good readings later. Measurement accuracy is critical. That’s why there are a variety of calibration routines as discussed in sweep that goes outside of the pass band is mostly other parts of this guide. Improper use of the reflected, reducing the accuracy of the vertical axis calibration standards, or use of an antenna sweeper Return Loss or VSWR measurements. with lower specifications, can lead to inaccurate A wider frequency range improves distance measurements and unnecessary equipment resolution, but lowers the maximum measureable Return loss, with an antenna on the far end, should replacement due to false fails. What is the cost of a distance. However, if an antenna is in place at the be between 15 to 25 dB. For a line with a load on false fail? other end of the cable, the DTF frequency range the far end, the value should be between 30 and 40 should be restricted to the antenna’s pass band. dB. Further information on Return Loss and VSWR Further information on DTF testing can be found in testing can be found in the application note the application note “Distance To Fault ” at “Understanding Cable & Antenna Analysis” at www.Anritsu.com www.Anritsu.com. Visit us at www.anritsu.com Cable and Antenna Analysis Troubleshooting Guide – utilizing Anritsu’s Handheld Site Master™ S331L Calibration and Accuracy Which Calibration to Use? Line Sweep Tools (LST) SweepMasters DIRECT Handy VSWR/RL Conversion Chart Antenna and cable sweepers need to be calibrated Open, Short, and Load (OSL) Collect traces Online Trace Delivery Service to correct for the very small reflections that will InstaCal, FlexCal Verify traces VSWR RL(dB) VSWR RL(dB) otherwise lower the accuracy of the measurement. Open, Short, Load (OSL) Report results Capture and upload multiple traces Capture 1.00 ∞ 2.50 7.4 The accuracy of the from the Anritsu S331L. LST is a PC based tool that makes life easier for instrument depends OSL (standard) is the most accurate calibration 1.01 46.1 2.60 7.0 people testing antennas and antenna cables. It on the accuracy of for one port tests such as Return Loss, VSWR, helps with collecting traces from the instrument, Deliver multiple traces in a timely 1.02 40.1 2.70 6.8 the Open, Short, and DTF. An OSL calibration requires the use of verifying that the traces are correct, and reporting manner and Load (OSL) three precision standards, and is as accurate as Deliver 1.03 36.6 2.80 6.5 the standards. results. LST has several features that make life Prevent un-authorized users from used for calibration. 1.04 34.2 2.90 6.2 simpler for the users of Anritsu’s cable and antenna accessing the traces.
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