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Distance To Fault – Effective Method for Isolating Problems

Olivier Lespinasse was recently appointed as Service Support Manager for AllForSite; a job where he will have to develop the AllForSite Solutions range of products, in addition of selecting more suppliers, and answering all technical questions that the site construction community may have.

He has already answered to some operator’s queries including questions about the antenna line measurement. As it is highly related the KPIs that most of our consultants manage on daily basis, we found relevant to provide you with some of the answers he prepared with the support of Teracom Sweden.

When a new site is installed, return loss is measured to check compliance with specifications and to check that components and cables are functioning correctly. This also provides data about the site that will be useful for follow-up and future modifications of the system.

Two primary measurements are taken at a site. A frequency sweep within the relevant frequency range measures the total return loss at the site, and provides an absolute value that shows what TX and RX will be working against. Distance To Fault (DTF) measurements show the engineer the location of deviations in the antenna line. Today’s instruments can identify deviations caused by e.g. improperly installed connectors or damaged cables. These measurements also provide technical data about the site that will be of use later when comparing the line’s current performance with that achieved at the initial commissioning.
In addition, many operators request measurements of total loss (attenuation) to indicate how much power actually reaches the antenna from the transmitter. This gives a clear indication of the line’s ability to do what it is intended to do – transmit RF power or receive an RF signal.

Diagnostic test… but no more
It is important to remember that the DTF test does not provide absolute values that accurately describe the performance of the site.
According to the instrument specification, an actual value of 1.06 can range between 1.08 and 1.04. This assumes that precision measurement cables and a precision calibration kit are used, and that they are in optimal condition. In reality, however, variations in the cable and the calibration kit mean that a field measurement is more likely to be between 1.09 and 1.03.
Instrument manufacturers recommend that DTF measurements only be used for fault localization and comparisons, and not for measuring absolute values. This is because instruments have insufficient resolution and, moreover, the method lacks sufficient precision at low VSWR values.

DTF readings
The actual length of a fault could be less than a millimeter. At the same time, a frequency sweep from 1900 MHz to 2200 MHz has a resolution of about 44 cm, depending of the velocity of propagation in the cable. For this reason, a 1 5/8” connector fitted with a 1/2” connector will not be seen as two connectors on a DTF reading. Instead, the reading will be a combined value for the two connectors that will not accurately reflect the component values for each individual connector.
Even a damaged cable is likely to be interpreted as a connector fault if the damage is located close enough to a connector. In fact, a fault induced close to any connector will not show up as an individual fault, but, depending on the distance between it and the connector, will either add or subtract to the magnitude of the fault.

Inappropriate use of DTF testing
Because of this low resolution, actual readings can be easily manipulated. Therefore, results can be “improved” by e.g. bending or pinching the cable, loosening connectors, or switching to connectors with another electrical length. In other words, inducing a fault can trick the instrument into giving better results while in reality the actual value for the antenna line, and hence its performance, is worse.
If correctly used, a DTF diagnostic test is an ideal method for locating faults when measurements from a frequency sweep fall outside the specification range.




Ideally, real reflection should correspond to measured reflection as depicted by the black line in the above graph. Inaccuracy of the instrument, however, makes this impossible. The diagram shows the range in which the reflection lies. With a reference load calibrated to -35 dB, the red line is used to identify the range of values for real reflection. If for example the instrument shows a measured reflection of -20 dB, then real reflection will be somewhere between -18 and -22 dB.


Relevant links:
For information & support, Olivier is reachable at www.allforsite.com     

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