Exploit the power of VSWR measurements VSWR measurement, one of the most basic measures of RF system performance, lets system operators stay on top of base station antenna performance without leaving the office and can predict impending failures.

The current explosion of personal communications services (PCS) brings with it a raft of new measurements geared toward evaluating various elements of system performance. The testing of communications protocols gets the most recognition, although the accurate measurement of forward and reflected power and voltage standing wave ratio (VSWR), continue to be basic determinants of RF system performance.

VSWR is perhaps the simplest qualifier of system health, and it can yield important information about whether or not that health is deteriorating. Surprisingly, system operators sometimes assume that because there have been no complaints about a particular cell in a cellular system or a particular sector of an SMR system, no periodic VSWR measurements need to be made. However, ignoring this simple performance diagnostic can have costly consequences.

Fortunately, it has become quite simple to perform VSWR measurements on a regular basis and to send the resulting data to a remote location on either a dial-up line or via a wireless telemetry system.

The test environment Nearly every land mobile, cellular or PCS system uses multiple transmit and receive antennas. Each one is responsible for blanketing a sector of the desired coverage area. Many systems have six transmit and six receive antennas, and some even more.

Monitoring the VSWR of the transmission chain from the transmitter and receiver through the transmission line and connectors to these antennas can be extremely helpful. VSWR is the ratio of power that is sent forward to the antenna to the amount of that power that is reflected back toward the transmitter because of standing waves on the transmission line and the terminating load, the antenna.

As VSWR increases, transmitted power decreases to the point at which no power is radiated. Beyond the obvious degradation of system performance caused by a high VSWR, the condition places an increased burden on transmitter components because the mismatch presented is well beyond the optimum.

This scenario applies equally, yet less obviously, to the receive side of the system, where performance degrades rapidly as VSWR increases. Most often, the primary concern in the receive link is receiver sensitivity and a low LNA (low-noise amplifier) noise figure. Of course, a poor antenna feed circuit with a high VSWR completely negates the best LNA noise figure.

The consequences of a high VSWR condition are most obvious to the users of the system. In cellular service, calls are dropped, reception is poor and overall performance is unacceptable in the cell (or section of cell) covered by the base station antenna with the high VSWR. Performance degradation is most noticeable as a lack of coverage in some areas, as poor reception and, in the worst cases, as nearly complete disruption of communication or as a dropped cell.

Perhaps more onerous is the possibility of gradual degradation from an extremely low (favorable) VSWR at the time of installation to an extremely high VSWR over a long time. There are many potential causes for this condition, including damage at the time of installation, weathering, corrosion, vandalism and storm damage.

As competition for consumer and business communications dollars increases, a poor-performing RF transmission system causes a crescendo of complaints and finally a search for alternative communications providers. All for the lack of measuring a basic RF performance parameter.

If it ain’t broke…. In an ideal world, every system operator would have the time and resources to take RF system performance into account and to conduct periodic checks of VSWR to ensure optimum system behavior. However, this has been difficult to do, because sweeps of antenna performance over the full system bandwidth require skilled technicians and expensive analytic equipment. In addition, traffic must be disrupted while measurements are made, and the collected data must be compiled manually and presented to RF engineers for review. As a result, these measurements have been made infrequently, and sometimes not at all.

Because many base stations are in remote, even severe environments, it is a rare occasion when someone volunteers to journey to a base station simply to make a measurement on a system that “appears to be working OK.”

On arriving, the technician is faced with making measurements on each transmission line, one by one, and recording the data manually. This can take a long time on a large, multi-sectored system. It can also require taking the system down to install a calibrated, dual-directional coupler with high directivity as the power sampling port.

In addition, making VSWR measurements on the receive section requires injecting a signal onto the line, which, if done manually, shuts down that sector until the measurement is made, which in turn halts communication in that sector while the test set-up is being installed and the receive antenna feed measurement is being made.

Finally, a single measurement such as this is of questionable value, because transmission line conditions can change the moment the technician “goes back down the mountain.” Multiple measurements over time are required to produce enough data to generate a history of VSWR performance. It is this trend information that is the most powerful exploitation of VSWR as a system evaluation tool.

Continuous monitoring The only cost-effective, useful way to make VSWR measurements pay off is to perform them with a system in place at each base station or cell site. Results can be transmitted to a remote point (via a modem and telephone line or a microwave link), or to a local point (either a notebook computer for immediate analysis, or to a workstation for later retrieval and analysis.

One product, a continuous antenna monitoring system VSWR/power monitor, provides this type of functionality while providing complete characterization of the transmission path on both transmit and receive antennas without operator intervention.The in-line system simultaneously monitors the forward and reflected power of the transmit antennas and determines the VSWR of the antenna feed circuit. The information gathered by the system can be communicated from the cell site or base station to the operations center for analysis, and preset alarms can be set to alert system operators that an out-of-limit condition has occurred. In addition, receive antenna status is determined without degradation of system performance.One model of the system also has on-board storage of up to six months of daily receive antenna sweeps and transmit power measurements. It includes Windows-based CELLPRO Plus software that allows a trend analysis to be conducted, and it has many other functions such as alarm functions, accurate transmitter power measurement and passive receive antenna frequency plots. The monitoring system operates at standard cellular and land mobile frequencies. On the transmitter side, the system can monitor power levels as high as 600W average and 7kW peak, with +/-0.5dB measurement accuracy from 0[degrees]C to +50[degrees]C. (See Table 1 at the left). Even more significant, at room temperature, the worst-case accuracy is +/-0.3dB. Each model provides more than 30dB of dynamic range for transmit power measurement in three ranges of maximum power (50W, 200W and 600W). Models are available to meet AMPS, NAMPS, TDMA, and GSM requirements.*

The system includes an RS-232 interface for direct connection to a computer or another remote monitoring device. The transmission monitoring section employs individual antenna sensors, which are high-directivity, in-line couplers that work with an embedded microprocessor to monitor the forward and reflected power levels. The system uses the microprocessors for control, calibration and communications, and GaAs (galenium-arsenide) circuitry is used for making precise multisignal power measurements with high dynamic range. The transmit monitors also have an extremely accurate true RMS (root-mean-square) power meter, which determines transmit power at any time, to confirm that all power amplifiers are performing acceptably.

The system’s receive antenna sensor portion consists of a multiplexed ultra-low-power synthesized transmitter and a receiver. The transmitter is actually a phase-locked-loop, precision frequency synthesizer stabilized by a crystal reference oscillator. It injects test signals into a cell site’s receive circuitry to check forward and reflected power levels. Only -20dBm of test signal power is used in the monitoring system’s receive antenna monitor, so that the signal reflected back from a defective antenna will not disturb the system or radiate spurious signals.A low-loss, high-directivity coupler with less than 0.10dB typical insertion loss is used in the receiver monitoring architecture, which provides a virtually transparent receive signal monitoring port. The coupler’s insertion loss is kept low through the use of design techniques proven in our manufacture of similar products for aerospace applications for more than 45 years.

Individual sensors within the system are connected via an RS-485 bus that supports remote control and data transfer. The RS-485 bus is converted to an RS-232 output bus within the monitoring system. The sensors are addressed by means of individual identification numbers.

The continuous antenna monitoring system VSWR/power monitor can be used for single- or multiple-frequency measurements. (See Photo 1 on page 10.) In the single-frequency mode of operation, the controller requests a measurement from the addressed sensor. The sensor sets its synthesized source to the commanded frequency, performs a calibration, transmits the test signal, measures the reflected power level and terminates the transmission. The monitor calculates the VSWR and reports it back to the controller. A new frequency, antenna or both are then selected by the controller.

In full-band operation, frequency switching is automated to provide VSWR measurements at 25 frequencies across the receive band. Band A, band B or both can be selected by the user, as well as any 30kHz band from 800MHz to 960MHz. The calculated VSWR values are stored. Upon request, the monitor sends the antenna identification number, measurement frequencies, and measured VSWR values to the controller. This process can be repeated for any other receive antennas in the system and can be used to provide a running log of VSWR as a function of frequency and time for all of a site’s receive antennas.

The system uses CELLPRO Plus Windows-based software to facilitate setup and measurement. (See Photo 1.) Combined with the popular ProComm Plus communications software, CELLPRO Plus forms a comprehensive remote site RF monitoring package. RF-specific menu choices include power/VSWR meter, status, set up, alarm, alarm reset and logging functions. The software can be used to initiate a number of responses to an alarm condition, including automatic calling or beeping of service personnel and sending of fax messages incorporating the latest data. The program can also automatically initiate a logging function at a predetermined time and at any desired sampling rate.

The system with the data storage option allows as much as six months worth of data to be stored in order to develop trend studies and statistical analyses. The data can be displayed in tabular or graphical formats, and can be used to profile changes in performance over frequency or time. It is this ability to view performance over time that offers the system operator the ability to spot gradual degradation, which can lead to failure. Even a change from one day to the next can signal that an event took place that requires attention.

Simple test, big results VSWR is such a basic measurement that its potential use for service providers is often overlooked. Simply stated, when VSWR goes up, performance, customer satisfaction, and, ultimately, profits go down. VSWR measurements, whether conducted manually or with sophisticated automated equipment, can go far toward keeping communications systems performing well without catastrophic failures. *AMPS is Advanced Mobile Phone Service. NAMPS is Narrowband Advanced Mobile Phone Service. TDMA is time-division multiple-access. GMS is global system for mobile communications, formerly Groupe Speciale Mobile.