Frequency domain reflectometry aids new antenna installations
New antenna system design trends complicate performance verification. Newly developed analyzers simplify the measurement techniques.
Cellular and PCS expansion continues at a brisk pace in 1997. As new competitors seek subscribers, factors such as coverage, capacity, cost and quality are receiving greater emphasis. Simply activating a radio network no longer guarantees success. Furthermore, attracting thousands of low minute-per-month subscribers produces meager profits. The best customers_those who generate the high daytime minutes_migrate to those providers offering the best service package.
Base station engineers have responded with a new generation of antenna systems equipment including duplexers with built-in, receive-side LNAs; directional antennas, superconducting filters, interference sensing dynamic frequency assignment, interference monitors, repeaters and horizontal and vertical isolation. Although these antenna system designs improve quality and capacity, they also present new performance verification challenges.
It helps to be aware of the trends behind new antenna installation tests and the measurement techniques used to verify performance. Specifications and operating ranges noted herein apply specifically to three new Site Master models from Anritsu Wiltron’s series of portable cable and antenna analyzers for ranges of 600MHz-1200MHz, 1,250MHz-2,350MHz and 1,750MHz-2,500MHz.
Improving RX signal strength Duplexers separate the transmit and receive signals from a single antenna feed. Most base station designers are migrating to a design used for many years in military systems. A receiver preamplifier is installed close to the antenna. A duplexer or filter precedes the low-noise amplifier (LNA) to limit input bandwidth and to minimize unwanted interference. This design boosts signal-to-noise ratio by reducing system noise figure.
For example, a receive antenna feed with 6dB of loss feeding a receiver with a 4dB noise figure has a system noise figure of 10dB. Thus, signals received by the antenna are degraded by 10dB as measured at the receiver output. If the same system is preceded by a preamplifier with 2dB noise figure and 20dB of gain, the system noise figure is a respectable 2.2dB_an improvement of 7.8dB. Today, LNAs and duplexers can be built with high reliability and reasonable cost, so implementation of the design is receiving wide recognition.
Less widely recognized is exactly how to test the LNA gain and duplexer bandwidth after installation. During installation, the duplexer/LNA test is relatively straightforward. Once the weather seals are applied, the test procedure is more complex.
Installation verification is facilitated by connecting a test input cable (typically the installed transmitter feed cable or other installed cable) to the antenna’s connection port on the duplexer/LNA. First, the transmitter cable end is connected to the receiver cable end, and the test analyzer is normalized (path calibration). Next, the duplexer/LNA assembly is inserted between the feeds as shown in Figure 1 on page 10. When the dc voltage supply is applied, the duplexer/LNA’s gain and bandwidth is easily viewed on the analyzer’s display. If duplexer bandwidth verification is not a concern, the procedure may omit the cable-to-cable connection. Instead, path calibration is performed through an LNA bypass switch, which is closed whenever the dc bias voltage is removed.
Once the antenna is attached and the connectors are sealed, injecting a test signal and normalizing the response is more challenging. There are two possible signal injection paths, the transmitter feed through the duplexer or through an alternate, co-located antenna to the duplexed antenna. Depending on the design of the duplexer, the filter skirt roll off between the transmitter and receiver frequencies may be shallow enough to pass a measurable test signal. However, the path calibration vs. frequency has a severe slope_equal to the sum of the receiver and transmitter filter skirts RF attenuation. Further, as systems designs are updated, transmitter-to-receiver isolation is likely to improve. Presuming that test procedures should support long-term maintenance, it is difficult to advocate use of the transmitter feed connection for receiver LNA test signal injection.
Test signal injection using a second antenna has several variations, depending on antenna type. The simplest case is injection through an omnidirectional antenna. Even if the duplexed antenna is directional over a specific cell sector, its isolation to an omnidirectional antenna is typically a reasonable 20dB to 40dB. The test signal is coupled largely through the directional antenna’s back lobes.
If the only available antenna is another duplexed sector antenna, the normalization task must deal with more significant antenna-to-antenna isolation. Isolation can be 70dB at some frequencies, thus the analyzer must sense the low signal level and average appropriately to achieve a flat, normalized baseline. Adding 5dB for each of the cables, duplexers, and jumpers yields a total path loss of 80dB. For a typical laboratory network analyzer or a spectrum analyzer and tracking generator combination, the 80dB dynamic range is nearly the maximum of the standard operating mode. Filter bandwidths, attenuation and averaging must be adjusted to reduce the effects of noise and to obtain a stable calibration. By contrast, an FDR-based antenna system analyzer automatically applies averaging when measuring low signal levels. Because of its design, additional filtering is unnecessary. Sweep speed is maintained.
Figure 2 at the left shows the normalization path through the bypass switch, duplexer and injection antenna into the antenna attached to the unit under test, the duplexer/LNA. For the normalization step, both bypass switches are in the closed position. During measurement, dc power is applied to the duplexer/LNA under test. The analyzer will display the LNA’s gain response.
Improving sectorization As far as the antenna system is concerned, improving sectorization is largely a matter of antenna position and pattern control. The benefit of sectorization is capacity improvement and reduced cell-to-cell RF interference. Other benefits are related to handoff controls, but this is a minor concern for modern handoff control algorithms.
It is common to mount all antennas at the same elevation, and with the relatively modest isolation requirements to date, most antennas are able to pass specifications. However, new antenna systems are demanding higher isolation. One clever approach involves using standard, inexpensive panel antennas that are simply mounted at differing vertical elevations. The fundamental installation issue is mounting the antenna in such a position that the specified isolation performance is achieved. Following installation, antenna isolation data should be stored to a PC database. The historic test data helps verify performance following storms or high winds. Because the antennas are coupled via a combination of front and back lobes, the isolation vs. frequency varies significantly with angular position variations, particularly with the higher isolation designs. Thus, these data can be used to indicate whether antennas have been moved or damaged without climbing the tower.
Seeing these sensitive variations is another convenient aspect of Site Master’s internal design. Since bandwidths and averaging are automatically controlled, the measurements are repeatable. When using laboratory analyzers, each technician tends to set controls differently_reducing measurement repeatability. It is not that laboratory analyzers themselves lack repeatability; it is the way that they are operated that causes measurement-to-measurement differences. The FDR-based antenna system analyzer solves the repeatability problem; thus, mounting and damage conditions can be identified sooner_frequently before failures degrade signal quality.
RF interference Unwanted RF interference affects both cellular signals and test signals. Unless the test instrument is designed to reject interference, the measurement will be corrupted. (See Figure 3 on page 18.) During both duplexer/LNA and antenna isolation tests, an antenna is part of the test signal injection path. Other RF signals are also received by the antenna.
One might assume that the only interference would be relatively low-powered transmissions from handsets, because local transmitters can be deactivated. That assumption is inappropriate for two reasons. First, complete site deactivation is undesirable due to system quality, cost and time-of-day restrictions. Second, other sources of in-band interference are common. For RF system design (such as the near-far handset reception problem), municipal zoning, and economic reasons, RF transmitter antennas are frequently co-located. Further, the explosion in wireless communications implies that the sources of RF interference will increase in the future, not decrease. If the test instrument lacks interference immunity, how can maintenance data be compared to baseline data?
Spectrum analyzers and tracking generator combinations are particularly sensitive to RF interference problems. A spectrum analyzer’s receiver will measure any incidental in-band RF energy. Thus, interference is displayed additively with the test signal input from the tracking generator. As the interference power varies, so do the measured results. Further, since the antenna-to-antenna isolation is commonly high, the spectrum analyzer must be operated with its input attenuated to an operating range where the dynamic range is maximized. One can try to blast through interference using RF test signal amplifiers, but insertion of attenuators during path calibration both compounds frequency response errors and adds to test process complexity.
Laboratory-grade vector network analyzers have better interference rejection, but none available today are optimized for operation in conditions where the RF interference power level exceeds the nominal power level of the measured test signal.
Our analyzer is designed to reject RF interference. It uses second-generation interference rejection capability, which implements phase tracking vector receivers and spread spectrum techniques. Both in reflection tests (return loss, standing wave ratio [SWR], cable loss or distance-to-fault) and in transmission tests (gain, isolation, or insertion loss), the analyzer’s frequency coding is able to track itself and thereby reject interfering signals. Even when the interfering signal power is several orders of magnitude higher than the measured test signal power, only one position of the displayed waveform will be distorted.
RF repeaters RF interference also affects cellular signal performance. For example, the near-far problem has degraded service substantially for some digital services. Namely, a handset initiating a call to a far base station from a position near a competitor’s base station has difficulty with call setup, particularly if the competitor’s base station is analog.
Mini-cells have helped, but repeaters are likely to become another popular solution. In both cases, antenna isolation is important. For direct RF-to-RF repeating, the antenna mountings require high, >90dB, isolation. The antenna transmitting the repeated receiver signal is typically positioned on the opposite side of a building from the receiver antenna receiving traffic from the “far” handsets. Even though many repeater systems automatically back-off their transmit gain, should antenna isolation degrade, most engineers feel it is important to have verification test data during both installation and periodic maintenance intervals.
Many common test instruments lack this dynamic range, and of those achieving >90dB range, most require significant adjustment to reduce noise effects.
Analyzer performance factors Measurement speed is two times faster than previous analyzer models. Transmission gain and loss measurements are displayed at better than 22ms/point. Trace math supports normalization (subtraction) of test setup components. distance-to-fault (DTF) update rate is also increased. * Accuracy: 0.5dB typical, <60dB range. 2.0dB typical, <105dB range. * Resolution: 0.01dB * Dynamic range: >110dB, typical * Trace memory: 50 sweeps * Setup memory: 5 setups with calibration data.
Summary Antenna systems are becoming more complicated and, correspondingly, so are the measurement tasks. Site Master is a family of portable, high performance instruments designed specifically for antenna systems testing. In addition to the analyzer’s accuracy improving vector correction techniques and high RF interference immunity, the straightforward design ensures that measurements are performed more accurately, more conveniently.
The analyzer models incorporate two measurement ports and may optionally be equipped with wattmeter capability.
Single port models for return loss, SWR, cable loss, and DTF measurements are available from 5MHz to 18GHz.
No transmission line component has a perfect impedance match. At any impedance mismatch, some of the energy of an incident signal will be reflected backward toward the source. If these reflections are excessive, the antenna system will not operate properly.
We all know that signal quality needs to be improved, but at what cost? Are airtime prices dropping? Yes, of course, but consumers are still prepared to pay a premium for top quality service. In an expanding market, both can be true simultaneously without adversely affecting profitability. Expanding network capacity creates simpler equation for operations managers. Unless subscribership increases sharply, as more cell sites are installed, cellular service operators must either a) shrink per-base station expenses or b) plan for lower earnings.
Rather than submit to the latter, managers are adopting failure prevention strategies. Historically, repairs are conducted on a fix-after-failure basis: antenna performance problems are allowed to degenerate into failures. Only after the failure does a site technician diagnose the problem.
Similarly, remote standing wave ratio (SWR) monitoring systems are available today that perform the same task. Problems are allowed to degrade until the SWR level is unacceptably high, at which point an alarm is triggered. It is a case of false economy: In this case, the SWR alarm does not translate into preventive maintenance. Rather, it is a failure monitor. The antenna system SWR is monitored continuously, but it cannot predict, and thus, prevent, a failure. Well before the failure occurs, quality has been degraded. And, the failure still requires extensive equipment replacement.
The new maintenance paradigm seeks out the problems before expensive failures occur. Failure prevention simultaneously reduces service costs and improves quality. Because of the relatively high failure rate_due to weather exposure, poor-quality installation or both_RF cable and antenna testing is increasingly important.
For example, a loose connector may have a return loss of 12dB. But if it is at the top of the tower, loss in the antenna cable (say 4dB) will mask that value, making the 12dB failure look like 20dB (12 + 234). A 12dB return loss (SWR, 1.7:1.0) will trip a SWR monitor alarm, but a 20dB return loss (SWR, 1.22:1.0) will not. If frequency domain reflectometry (FDR) techniques are used, the loose connector is easily identified by distance-to-fault (DTF) analysis, and it can be tightened before moisture and corrosion destroy the expensive feed cable. So, what feeds have the most problems? Of course, the tallest, most expensive ones.
The key to preventive maintenance is quick recognition of small SWR changes within the antenna feed. The measurement task is accomplished by FDR. Recognizing changes is best implemented using DTF signature analysis. Because the FDR techniques are both sensitive and repeatable, new measurements can be directly compared to a database of baseline, or “signature” measurements. A graphic overlay comparison, such as found in the Anritsu Wiltron Site Master software tools, quickly identifies the minute SWR changes which precede failure conditions.
At least in antenna system maintenance, high quality doesn’t equate to high maintenance. Good maintenance planning equates to good management. Ken Harvey