Cable Loss Effect on System Return Loss
The insertion loss of the cable needs to be taken into consideration when making system return loss measurements using portable cable and antenna analyzers like the Anritsu Site Master S331E & S332E. The picture below(picture 4) illustrates how the cable loss changes the perceived antenna performance. The antenna itself has a return loss of 15 dB but the 5 dB insertion loss improves the perceived system return loss by 10 dB (5 dB *2). Even though this is something system designers take into consideration when setting up the specifications of the site, it is important to be aware of the effects the insertion loss and also cable return loss can have on the overall system return loss. A very good system return loss may not necessarily be a result of an excellent antenna; it could be a faulty cable with too much insertion loss and an antenna out of specification. This would result in a larger than expected signal drop and once the signal reaches the antenna, a great portion of the signal is now reflected since the match is worse than expected. The end result is that the transmitted signal is lower than needed and the overall coverage area is now affected. In other words, if your system return loss is too good, it is not always a good thing.
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.
Return Loss / VSWR measurement characterizes the performance of the overall system. If either of these is failing, the DTF measurement can be used to troubleshoot the system and locate the exact location of a fault. It is important to understand that the DTF measurement is strictly a troubleshooting tool and best used to compare relative data and monitor changes over time with the main purpose of locating faults and measuring the length of the cable. Using the DTF absolute amplitude values derived from the DTF data as a replacement for return loss or as a pass/fail indicator is not recommended because there are so many variables that affect the DTF readings including propagation velocity variation, insertion loss inaccuracies of the complete system, stray signals, temperature variations, and mathematical limitations; hence it is very challenging for system engineers to come up with numbers that take all of this into consideration. Used correctly, the DTF measurement is by far the best method for troubleshooting cable and antenna problems.
The DTF measurement is based on the same information as a return loss measurement or a cable loss measurement. The DTF measurement sweeps the cable in the frequency domain and then with the help of the Inverse Fast Fourier Transform (IFFT), the data can be converted from the frequency domain to the time domain. In other words, if you forgot to do a DTF measurement but made a return loss measurement and still have access to the magnitude and phase data of the 1-port measurement you don’t have to worry because the magnitude and phase data can be used to create a DTF plot in software.
The dielectric material in the cable affects the propagation velocity which affects the velocity of the signal traveling through the cable. The accuracy of the propagation velocity (vp) value will determine the accuracy of the location of the discontinuity. A ±5% error in the vp value will affect the distance accuracy accordingly and the end of the 80 ft cable could show up anywhere between 76 and 84 ft . Even if the vp value is copied out of the manufacturer’s datasheet, there could still be some discrepancies between the interpreted and actual distance discontinuities. This is a result of adding all the components in the system. Common base station systems can include a main feed line, feed line jumper, adapters, top jumpers and even though the main feed line contributes with the largest amount, the velocity of the signal through the other parts of the system could be different.
The accuracy of the amplitude values is usually of less importance because DTF should be used to troubleshoot a system and find problems so whether a connector is at 30 dB or 35 dB may not be as interesting as if the connector was at 35 dB one year ago and now it is at 30 dB. While the propagation velocity value remains fairly constant over the entire frequency range, the insertion loss of the cable does not and this also affects the amplitude accuracy.
Most handheld instruments available today have built-in tables that include propagation velocity values and cable insertion loss values for different frequencies of the most commonly used cables. This simplifies the task for the field technician as he/she can locate the cable type and obtain the correct vp and cable loss values.
Fault Resolution, Display Resolution, and Max Distance
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 / ΔF (MHz)
Fault Resolution (ft) = 15000*vp / (ΔF*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
Interpreting DTF Measurements
In the ideal world, the DTF measurement would be done with no frequency selective components in the path and just a termination at the end of the cable. Most of the time, this is not the case and the technician needs to understand how to make the measurement with different components in the path and at the end of the cable.
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 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).
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 a portable cable and antenna analyzer 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.
JM Test Systems carries Anritsu Site Master S331E / S332E for rental or purchase
Call us for a quote at 800.353.3411 or email us at [email protected]
JM Test Systems lab calibrates Anritsu Site Masters S331B, S331E, S331D, S331L, S332D, S332E, and S820D/S810D