Anritsu S362E Site Master Understanding Cable & Antenna Analysis White Paper
Anritsu S362E Site Master Understanding Cable & Antenna Analysis White Paper
By Stefan Pongratz
TABLE OF CONTENTS
1.0 Introduction 2.0 Frequency Domain Reflectrometry 3.0 Return Loss 4.0 Cable Loss 5.0 Cable Loss Effect on System Return Loss 6.0 Distance-To-Fault (DTF) 7.0 Fault Resolution, Display Resolution, and Max Distance 8.0 DTF Example 9.0 Interpreting DTF Measuements 10.0 Summary
2 2 3 5 5 7 8 9 9 11
1.0 Introduction
The cable and antenna system plays a crucial role of the overall performance of a Base Station system. Degradations and failures in the antenna system may cause poor voice quality or dropped calls. From a carrier standpoint, this could eventually result in loss of revenue. While a problematic base station can be replaced, a cable and antenna system is not so easy to replace. It is the role of the field technician to troubleshoot the cable and antenna system and ensure that the overall health of the communication system is performing as expected. Field technicians today rely on portable cable and antenna analyzers to analyze, troubleshoot, characterize, and maintain the system. The purpose of this white paper is to cover the fundamentals of the key measurements of cable and antenna analysis; Return Loss, Cable Loss, and Distance-To-Fault (DTF).
FDR
Sour ce s Spe ct ral Densi ty
f1
f2
The return loss and VSWR measurements are key measurements for anyone making cable and antenna measurements in the field. These measurements show the user the 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 transmission and will instead end up in the transmitter. This extra energy returned to the transmitter will not only distort the signal but it will also affect the efficiency of the transmitted power and the corresponding coverage area. For instance, a 20 dB system return loss measurement is considered very efficient as only 1% of the power is returned and 99% of the power is transmitted. If the return loss is 10 dB, 10% of the power is returned. While different systems have different acceptable return loss limits, 15 dB or better is a common system limit for a cable and antenna system.
RL : 10 d B 10 % pow er retu rn ed
While an antenna system could be faulty for any number of reasons, poorly installed connectors, dented/damaged coax cables, and defective antennas tend to dominate the failure trends.
Return Loss and VSWR both display the match of the system but they show it in different ways. The return loss displays the ratio of reflected power to reference power in dB. The return loss view is usually preferred because of the benefits with logarithmic displays; one of them being that it is easier to compare a small and large number on a logarithmic scale. The return loss scale is normally set up from 0 to 60 dB with 0 being an open or a short and 60 dB would be close to a perfect match.
In contrast to Return Loss, VSWR displays the match of the system linearly. VSWR measures the ratio of voltage peaks and valleys. If the match is not perfect, the peaks and valleys of the returned signal will not align perfectly with the transmitted signal and the greater this number is, the worse the match is. A perfect or ideal match in VSWR terms would be 1:1. A more realistic match for a cable & antenna system is in the order of 1.43 (15 dB). Antenna manufacturers typically specify the match in VSWR. The scale of a VSWR is usually defaulted to setup between 1 and 65. To convert from VSWR to Return Loss: VSWR = 1+10-RL/20/ 1-10-RL/20 Return Loss = 20 log |VSWR+1/VSWR-1| The trace in picture 1 shows a Return Loss measurement of a cellular antenna matched between 806-869 MHz. The Return Loss amplitude scale is setup to go from 0.5 dB to 28 dB. The VSWR display in the right graph measures the same antenna and the amplitude scale has been setup to match the scale of the Return Loss measurement. The two graphs illustrate the relationship between VSWR and Return Loss. 8.84 dB RL 2.15 VSWR
Increasing the RF frequency and the length of the cable will increase the insertion loss. Cables with larger diameter have less insertion loss and better power handling capabilities than cables with smaller diameter.
Antenna RL 15 dB
Cable Loss 5 dB
Picture 6 show the cable loss measurement of two 40 ft cables connected together. The combined cable loss averages about 4.5 dB. The graph in picture five illustrates the differences between measuring the return loss at the antenna and measuring the return loss of the entire system including the 4.5 dB insertion loss of the cable. The cable loss graph shows how the insertion loss of the cable increases with frequency. The delta in picture 5 is proportional to 2*CL and the careful observer can also notice the the difference between the two traces in Picture 5 is greater at 1100 MHz than it is at 600 MHz. The majority of this delta is a result of the cable loss increasing as the frequency increases. If both the return loss of the antenna and system return loss is known, the cable loss can be estimated from this information.
Prop Velocity
0.88 0.92
1000 MHz
0.073 dB/m 0.054 dB/m
2500 MHz
0.120 dB/m 0.089 dB/m
The term resolution can be confusing and the definitions can vary. For DTF, it is important to understand the difference between fault resolution and display resolution because the meanings are different. The Fault resolution is the systems ability to separate two closely spaces signals. Two discontinuities located 0.5 ft apart from each other will not be identified in a DTF measurement if the fault resolution is 2 ft. Because DTF is swept in the frequency domain, the frequency range affects the fault resolution. A wider frequency ranges means better fault resolution and shorter max distance. Similarly, a narrower frequency range leads to wider fault resolution and greater maximum horizontal distance. The only way to improve the fault resolution is to increase the frequency range. The MATLAB simulations below based on the DTF algorithm show how two -20 dBm faults simulated to take place 2 ft apart at 9 ft and 11 ft, only show up when the frequency range has been widened from 1850-1990 MHz to 1500-1990 MHz. The 1850-1990 MHz sweep gives a fault resolution of 3.16 ft (vp=0.91) and the 1500-1990 MHz sweep gives a faults resolution of 0.9 ft. More data points in the example in picture 7 would have given us finer display resolution, but it would only be a nicer display of the same graph. It would not matter if we had 20000 data points, the two faults would still not show up unless the frequency range is widened. The curious observer will also note that the amplitude of the two discontinuities show up at -20 dBm in Picture 8. In the first example, the two amplitudes add up to create one fault with greater amplitude than the two individual faults.
Fault Resolution (m) = 150*vp / F (MHz) Fault Resolution (ft) = 15000*vp / (F*30.48) Using the example in Picture 9, Fault Resolution (ft) = 15000*0.88 / ((1100-600)*30.48) = 0.866 ft Dmax is the maximum horizontal distance that the instrument can measure. It is dependent on the number of data points and the fault resolution. Dmax = (datapoints-1)*Fault Resolution Using the example in Picture 9, (ft) = (551-1)*0.866 ft = 476.3 ft
Picture 10 and 11 below show graphs of the DTF measurements of the same instrument setup. The two 40 ft LDF4-50A cables are connected together with an open at the end of the cable in Picture 10 and a PCS antenna connected to the end of the cable in picture 11. The only difference between the two graphs is the amplitude level of the peak showing the end of the cable. Picture 12 shows a kink in the cable just 7 ft before the antenna.
10
Picture 14 shows how the electrical length of the TMA in picture 13 affects the distance measurement of the system. The graph in Picture 13 shows a Transmission Measurement of a 2-port dual duplex LNA. Picture 14 shows the DTF measurement of this system swept with the TMA in the path and the end connection shows up at 106ft because the TMA was swept over both the uplink and downlink bands of the TMA. The end of the same system without the TMA in the path shows up at 83 ft (Picture 11).
10.0 Summary
The cable and antenna system plays an important role in the overall performance of the cell site. Small changes in the antenna system can affect the signal, coverage area and eventually cause dropped calls. Using portable cable & antenna analyzers to characterize communication systems can simplify maintenance and overall performance significantly. The return loss/VSWR measurements are used to characterize the system. If the match is outside the system specification, the DTF measurement can be used to troubleshoot problems, locate faults, and monitor changes over time.
11
Anritsu Corporation
5-1-1 Onna, Atsugi-shi, Kanagawa, 243-8555 Japan Phone: +81-46-223-1111 Fax: +81-46-296-1264
Sweden Anritsu AB
Borgafjordsgatan 13, 164 40 Kista, Sweden Phone: +46-8-534-707-00 Fax: +46-8-534-707-30
Finland Anritsu AB
Teknobulevardi 3-5, FI-01530 Vantaa, Finland Phone: +358-20-741-8100 Fax: +358-20-741-8111
Anritsu All trademarks are registered trademarks of theirrespective companies. Data subject to change withoutnotice. For the most recent specifications visit: www.us.anritsu.com
White Paper No. 11410-00427, Rev. B Printed in United States 2009-07 2009 Anritsu Company. All Rights Reserved.