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 like the Anritsu Site Master S331E & S332E 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).
Frequency Domain Reflectrometry
Most modern analyzers used today to characterize the antenna system use the Frequency Domain Reflectrometry (FDR) technology. This technology uses RF frequencies to analyze the data, providing the ability to locate changes and degradations at the frequency of operation. Analyzing the data in the frequency domain enable users to find small degradations or changes in the system and thus can prevent severe system failures. Another major benefit of analyzing the system using RF sweeps is that antennas are tested at their correct operating frequency and the signal will go through frequency selective devices such as filters, quarter-wave lightning arrestors, or duplexers which are common to cellular antenna systems.
Return Loss / VSWR
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.
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
As the signal travels through the transmission path, some of the energy will be dissipated in the cable and the components. A Cable Loss measurement is usually made at the installation phase to ensure that the cable loss is within manufacturer’s specification.
The measurement can be made with a portable vector/scalar network analyzer or a power meter. Cable Loss can be measured using the Return Loss measurement available in the cable and antenna analyzer. By placing a short at the end of the cable, the signal is reflected back and the energy lost in the cable can be computed. Equipment manufacturers suggest to get the average cable loss of the swept frequency range by adding the peak of the trace to the valley of the trace and divide by two in cable loss mode or divide by four in return loss mode (to account for signal travel back and forth).
Most portable cable & antenna analyzers today are equipped with a cable loss mode that displays the average cable loss of the swept frequency range. This is usually the preferred method since it eliminates the need for any math. The graph in picture 3 below shows a cable loss measurement of a cable between 1850 and 1990 MHz. The markers at the peak and valley can be used to compute the average. This particular handheld instrument computes the average cable loss for the user as can be seen in the left part of the display.
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.
Next: Part 2 of Understanding Cable & Antenna Analysis
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