Antenna system troubleshooting simplified

What is the quickest and most efficient way to determine the root cause of a failed transmission system? In most failure conditions, after determining that the transmitter or receiver itself is operating correctly, the technician or engineer must then attempt to diagnose the antenna and associated cabling. Is the failure due to an RF jumper, a connector, a defect in the cable, a resonant filter cavity, a lightning protector, or the antenna itself? Making these determinations is one of the most difficult and potentially expensive parts of troubleshooting and repairing any failure.

In this article we will discuss the technology of Frequency Domain Reflectometry.

Frequency Domain Reflectometry technology has been around for years, but until recently, its use was limited primarily to lab applications and microwave systems where the large bulky test equipment needed for precise measurements could be made. Even under ideal conditions, making the necessary measurements was a cumbersome process and usually a full size plotter was needed to provide any meaningful output.

What has changed to bring the technology of Frequency Domain Reflectometry into the practical realm?

The new site analyzers on the market have brought Frequency Domain Reflectometry technology out of the lab and into the field. Many of these analyzers are the size of a large textbook and their operation has been dramatically simplified. The output and analysis processes also have been refined. Today, an output file can be easily sent via e-mail to a client or consulting engineer in a matter of seconds.

In many related magazine advertisements and catalogs you will always see the terms “distance to fault” and “return loss.” This article will explain these and all of the other technology associated with Frequency Domain Reflectometry.

If every system was perfect, all installations were done correctly, all connectors were truly weatherproof, lightning never damaged a system, all components worked as labeled, coax cables never got bent or crimped, and nothing ever deteriorated over time, there would not be a need for site analyzers. But in the real world, all of the above does happen, and the analyzers allow the installer, maintenance technician, and engineer to all know that the antenna system is either functioning correctly, or there is a problem. If there is a problem, the exact location of that problem will be shown to the operator. On a long cable run, of 400 feet for example, the problem can be localized to within a few feet; no other technology can do this. Even the slightest abnormality is reported back to the operator.

Taking advantage of the new technology

A Frequency Domain Reflectometer (FDR) is a test set that allows the operator to test and diagnose problems with a cable or antenna system from the ground. The FDR “looks” into the cable in the exact same way as the radio does and “sees” all of the elements between the insertion point and the antenna. Any problems with the coaxial cable, connectors, jumpers, or the antenna will show up as an abnormality on the display.

Not only does the FDR give an SWR reading of the antenna system, but it also shows:

The new FDRs do all of this in a self-contained, battery operated, portable unit the size of a textbook (about 10″ × 6″ × 2.5″). Once you have used one of these instruments to check an antenna system, it is hard to imagine life without it. The time savings and potential reduction in repair expense can, in many cases, easily justify the capital expense of acquiring an FDR.

What is time domain reflectometry?

Time Domain Reflectometry (TDR) is an alternate analysis technique in which a direct current (DC) pulse is injected into the cable and antenna system. The DC pulse traverses the cable at nearly the speed of light. The actual speed varies based on the medium, but is always a known constant programmed into the testing device.

The “reflection” of this pulse is measured and calculations are made based on the elapsed time between when the pulse was sent and when the reflection was received. In addition, since the voltage of the pulse being sent is known, the voltage of the reflection is also measured and used in a TDR analysis.

Based on these measurements, several assumptions can be made as to the location of a potential fault.

Frequency domain reflectometry

The focus of this article is on Frequency Domain Reflectometry and here the basic differences will be highlighted. As previously discussed, the TDR process utilizes a DC pulse as a tool for analysis. An obvious shortcoming of this technique is that the cable, the couplings, and antenna are not intended to be a medium for transmitting direct current.

An FDR-based analysis uses frequency specific pulses first below, then on, and finally above the actual band and frequency used by the radio system. By using pulses of discrete frequencies, a more realistic analysis of the cable and antenna system is possible.

In a theoretical model, all of the elements of the cable and antenna system are either “invisible” or resonant. This is to say that cable, connectors, grounding kits, lightning arrestors, and other non-radiating components would not affect signal transmission at all.

Also, and again in the theoretical model, resonant filter cavities and the antenna are perfectly resonant at the operating frequency. In the real world, however, every element of the cable system and the antenna have performance characteristics that affect signal transmission. Since many of these characteristics are frequency specific, an analysis that allows for the frequency domain is inherently superior.

An FDR analysis is initiated as described above and measurements are made similar to those in a TDR analysis. Reflections are measured, elapsed time is measured, however in the FDR analysis much more information is available. By testing with several frequencies, an extremely accurate representation of the cable and antenna system can be presented to the operator in a very short time. The operator immediately knows the impedance of every element of the system and in many cases faults can be identified instantaneously.

What is a match?

In electrical engineering school, one of the problems that professor’s pose to students is to calculate the maximum power transfer from box A to box B when the output impedance of box A equals the input impedance of box B.

In the world of radio, everything relating to antennas and transmission lines is based on both having an impedance of 50W; therefore 50W is the standard that all related equipment is designed to measure.

Since a “match” is when the antenna equals 50 ohms, this also corresponds to the antenna working correctly at the frequency assigned, we say there is a “match” when the SWR is less than 1.5 :1.

This corresponds to a return loss of -14 dB or better.

What is distance to fault?

Distance to Fault (DTF) is a measurement that shows how far from the end of the cable faults occur. The instrument sees impedance disturbances, determines where they are located, in feet or meters, and displays the fault location information to the operator. Today’s analyzers can and do identify bad connectors, pinched cables, or even a faulty grounding kit. Faults, in many cases, are isolated to within 1 percent of the total length of the cable.

Some of the problems that can be located with the FDR technology:

What is cable loss?

All cables have a certain amount of insertion loss as the RF signal travels through the cable. As frequency increases, losses increase as well.

The FDR Analyzers can easily verify the cable loss at the operating frequency; a normal requirement imposed by RF Engineer’s for the system records.

What are typical system specifications?

If a system is properly constructed, different components will meet different return loss specification levels. The following table lists some of the parameters that you will encounter in actual systems:

You can only see these parameters on a DTF sweep.

How does one run a sweep?

In most systems, the radio transmission line is removed from the radio and temporally replaced with a site analyzer for these measurements. A Return Loss Match Measurement and a Distance To Fault Measurement will reveal how the antenna system is working.

What will a sweep show?

An FDR sweep will show the frequencies where the antenna is resonant as a function of frequency, and what the return loss match, of each component in the system, reflects as a function of distance.

Archiving data

Most instruments have the capability to store 200 to 300 sweeps and allow the user to assign symbolic names, specific to a particular site, for later review. In addition, the date and time are automatically added.

Comparing archived data and new data

Most instruments have provisions to recall previously saved or stored sweeps enabling the user to compare present readings to a previously stored readings to determine if anything has changed.

Any changes, no matter how small, indicate a potential problem with the cable and antenna system.

Conclusion

There are many different methods that technicians and engineers use to isolate problems in cable and antenna systems. In many cases, the design and physical layout of the system will dictate the first approach.

There are tried and true methods of troubleshooting that have been used by radio engineers and technicians for many years. While these are as valid today as they were twenty years ago, their shortcomings are now more apparent. Replacing an antenna or committing to a 500-foot cable replacement is common practice when using outdated troubleshooting techniques.

In today’s environment, time spent in isolating and replacing a faulty component can be quickly translated to a company’s bottom line. Less time spent in “maintenance mode” means more time spent in “production mode.” Repair and maintenance budgets are being stretched; therefore knowing that a component is actually faulty, before replacing, makes good business sense.

The new site analyzers, that perform FDR measurements, are one excellent example of a new technology that easily justifies the investment.

Ira Wiesenfeld, P.E., is a consulting engineer who has been involved with commercial radio systems since 1966. He has spent time working in the broadcast, two-way, mobile telephone, paging, microwave, military, and public safety radio systems, and has consulted with most of the major manufacturers in the radio industry. Ira is the author of Wiring for Wireless Sites, available from Delmar Thompson / Prompt Publishing (www.electronictech.com).

Ira Wiesenfeld has a BSEE from Southern Methodist University in Dallas, Texas; a FCC General Radiotelephone Operator License; BellCore Certified Radio Technician; and is a licensed Professional Engineer in the State of Texas. Ira can be reached at [email protected].

Robert Smith is a technical consultant based in Dallas, Texas. Most recently, Smith was employed by Arch Wireless Inc. as Vice President-Technical Operations after serving for thirteen years in various capacities with MobileComm prior to its acquisition. Robert attended the University of Tennessee and graduated from the Tennessee Institute of Electronics. Smith began his career in telecommunications in 1984 with BBL Industries in Atlanta. Robert can be reached at [email protected].

What is Velocity Factor?

Radio signals travel at the speed of light in free space. If the signal is sent down a coaxial cable, the speed is reduced by a set amount in the cable. The quantity of this reduction is known by the manufacturer of the cable, and is contained in the published specifications for each type of cable. The actual speed relative to the speed of light is called the Velocity Factor, also known as the Propagation Velocity. Since the time is how the Distance to Fault measurement knows where a problem is, then the Velocity Factor has a major impact as to how this measurement is computed.

Why do we use 50 Ohms?

A quarter wave Marconi (vertical) antenna has an impedance of 50 Ohms at the resonant frequency. A half wave Hertz (dipole) antenna likewise has an impedance of 50 Ohms at the resonant frequency. Since the antenna has a 50 Ohm input, then the maximum power will transfer if the transmission line is also at 50 Ohms. In addition, the radio manufacturers set the output impedance of their transmitters to be at 50 Ohms, and the input impedance of the receivers to also be at 50 Ohms. Thus, all of the power transfers between components, are set as the impedance of the antenna itself: 50 Ohms.