antenna system troubleshooting simplified

8/13/2019 Antenna System Troubleshooting Simplified

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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 diagnosethe 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 fullsize 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, theexact 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.


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

Return Loss in dB by frequency (A much better indication of an antenna systemperformance than SWR).

Distance to Fault (Abnormalities based upon distance). This will allow the technicianor engineer to see every connector, bend in the cable, antenna, lightning protector,grounding kits, and any problem or potential problem with an antenna system.

Cable loss in dB (Without having to climb the tower). All the above in high RF environments (Unlike most other instruments)

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 abovethe 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 thefrequency domain is inherently superior.


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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 toantennasand 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, infeet 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:

Bad connectors Bent (impedance affecting) cable Bad Lightning protectors Moisture in Coax Bad antennas Bad jumpers Subtle changes at connections that are not found otherwise Improperly installed antennas (too close to the tower) Antennas spacing too close to other antennas Improperly installed Ground Kits on transmission lines

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.
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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 lossspecification levels. The following table lists some of the parameters that you will encounter

in actual systems:

Lightning Protectors: 30 dB

Coaxial Cable: 35 dB

Antennas: 14 dB

Jumpers 30 dB

Connectors 25 dB

Ground Kits 35 dB

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


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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.

SWR VS. RETURN LOSS

RETURN LOSS VS. SWR

RETURN LOSS

(DB)

VSWR

xx : 1

100% 100:1

1.7 10:1

4.0 4.2:1

6.0 3.01:18.0 2.32:1

10.0 1.92:1

12.0 1.67:1

14.0 1.5:1

16.0 1.38:1

18.0 1.29:1

20.0 1.22:1

22.0 1.17:1

24.0 1.13:126.0 1.10:1

28.0 1.08:1

30.0 1.06:1

32.0 1.05:1

34.0 1.04:1

36.0 1.03:1

38.0 1.02:1

40.0 1.02:1

42.0 1.01:1


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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 ofWiring for Wireless Sites,

available from Delmar Thompson / Prompt Publishing (www.electronictech.com).

Ira Wiesenfeld has a BSEE from Southern Methodist University in Dallas, Texas; aFCC

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 [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 theresonant 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.
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