Fundamentals of cable/antenna test tools for base station deployment, upkeep and improvement

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By eeNews Europe

To prevent this from happening in-depth analysis needs to be done regularly. The following article will look at the various parameters that must be considered when sourcing an antenna/cable analyser so that the best suited model is eventually decided upon.

Antenna/cable analysers are an essential piece of kit for today’s field test engineers. By using these items of instrumentation it is possible to obtain detailed information on how well the component parts which make up a base station’s transmission system all integrate together and whether the harsh outdoor environment in which they are situated has had any detrimental influence. By compiling all the information acquired, the operational effectiveness of each base station within a network can be determined. Any problems that have arisen can be identified and dealt with rapidly.

There are a broad choice of analysers now on the market, offered by a number of different highly reputable test equipment manufacturers. Each one of these analysers has attributes that can potentially be of value to a test engineer under the right circumstances, so it is important to recognise which are most relevant for the specific test set you have in mind and base your selection on these.

Key measurement criteria
Regardless of which one is finally specified, for any antenna/cable analyser the ability to take certain measurements will be effectively mandatory. As a result the analyser will have functional modes built in that provision for the following measurements:

Voltage Standing Wave Ratio (VSWR) — This is used to express the power that is reflected from the base station antenna. It relates directly to the reflection coefficient (r) via the following straightforward equation:

VSWR = (1 + r)/(1 – r)

The VSWR can be employed in order to give an accurate assessment of how closely the antenna is impedance matched with the transmission line that it has been connected to. For an antenna that is not adequately matched then a standing wave will form upon the transmission line and the severity of losses will be greater. If the VSWR figure is low then the degree of matching will be better and the power being delivered to the antenna will be higher.

Return Loss (RL) — Measured in dB, this is the loss of signal power that results from a reflection occurring because there is some kind of discontinuity or impedance mismatch on the transmission line. It relates directly to the incident power (PI) that reaches a specified point and the reflected power (PR) that comes back from it. The equation describing this relationship is:

RL = 10Log10(PI/PR)

RL basically gives a measure of how well devices or transmission lines are matched and safeguards against interconnects that have been poorly implemented (so that the connection turns out to be lose, for instance) or kinks within the cabling. When the matching is of a high standard then the RL value will be high. The greater the RL the lower insertion loss will be. The impact of RL was not that large in the past, so it was judged to be of little concern to test engineering professionals. With the emergence of ever higher speed, next generation communications protocols, such as LTE and HSDPA, this is all starting to chance though.

Cable Loss — This signifies the amount of energy that is dissipated across the transmission line, and all of its associated component parts (the cabling, interconnects and protection devices), as the signal passes down it. Caused by the resistance present in the transmission line, this corresponds directly to the total insertion loss (over a given frequency band). The higher the frequency of the signal, the smaller the diameter of the transmission line and the longer the transmission line is, the greater the subsequent loss will be. The size of the losses is of increasing importance to network operators as they now have to meet stringent legislative guidelines in terms of energy efficiency.

Distance to Fault (DTF) — This can be of great assistance in locating the positions of discontinuities and shorts in the base station antenna/cabling that have led to VSWR or return loss issues occurring. Normally complex Fast Fourier Transform (FFT) algorithms are employed by the analyser to translate acquired frequency data into the time domain data, so that signal aberrations can be ascertained in relation to distance.

Other important factors
As well as supporting all the previously mentioned measurement modes there are a wide variety of different features and functionality that are offered by antenna/cable analyser models currently on the market which may prove to be beneficial. Here are just a few major ones.

Obviously, since these instruments are being used in the field and need to be carried up antenna masts, etc., a lightweight, portable format is highly advantageous. Normally an overall weight below 3kg and dimensions of around 200mm x 280mm x 150mm would be expected. Other characteristics like long battery life and robust construction are also elementary.

To make the examining of acquired data as simple as possible to execute, a unit with a relatively large (+7” diagonal), high resolution colour display should be sought. Inclusion of a touch screen has almost become ubiquitous now – this leads to a more intuitive user interface which is easy to operate, however it is worth pointing out that potentially there can be drawbacks associated with touch screen operation. If an engineer is up an antenna mast and is wearing gloves, then a touch-enabled user interface can become difficult to manipulate.

Another feature, which is available from some manufacturers, is the ability for engineers to write their own test procedures for controlling the instrument. This allows a company to guarantee that every one of its field engineers is following exactly the same procedure when testing a specific base station. This reduces the possibility of errors occurring – such as selecting the wrong the frequency when testing.

It is likely that two port transmission measurement will prove itself to be useful, as it will lead to results that are more accurate than those from single port measurements. 3G/4G base stations today use diplexers and duplexers to increase cell coverage. Via two port transmission measurement it is possible to carry out gain, isolation and insertion loss measurements to deal with this. Furthermore if the analyser has a split screen facility, the user can examine two different measurements (such as DTF and VSWR) at the same time.

Access to superior accessories to accompany these analysers is also important. For example, low loss cables and probes with mean that more accurate test data can be acquired. Furthermore by utilising precision calibration kits, that need to be maintained in good order and regularly recalibrated themselves, it can be ensured that the instrumentation fully complies with all the relevant standards throughout its operational life. There is thus a direct correlation between the quality of the measurements taken and the quality of the calibration kits.

So that more can be done with just a single item of equipment, additional functions may be integrated into unit. Spectrum measurement, for example, can be of use as it allows sources of interference to be determined. As a result more advanced models will often incorporate a spectrum analyser. Increasingly network operators will expect field engineers to accurately log the position where testing was undertaken, so GPS is becoming an important supplementary feature. Furthermore, passive inter-modulation (PIM) is now regarded as one of the critical phenomena that needs addressing when base station infrastructure is being deployed. Some analysers now also have the capacity to provide a basic indication of the presence of PIM, though utilisation of a separate dedicated PIM tester will be needed to scrutinise it properly. The detailed mapping out an interference profile, using a directional antenna, can be beneficial to the field engineer, so units that provide this facility are likely to be very attractive.

In many cases analysers are only required for short periods of time. This means that it may not be economically viable to purchase such items of equipment. Instead, in many cases, it will be more attractive, both technically and financially speaking, to partner with an equipment rental firm. Among the products in test sourcing specialist Livingston’s portfolio that are widely used in antenna/cable analysis are Anritsu’ Site Master, JDSU CellAdvisor and the ZVH from Rohde & Schwarz. These are offered with a full range of accessories, plus a comprehensive calibration management service. The flexibility that this sort of engagement permits means that as well as having complete control of how long the analyser is kept in use (so that the monetary investment matches well with the revenue generated), different analysers options can be tried out. If one analyser model proves to not be as suitable as first expected, then an alternative can quickly be sought without any cost penalties being accrued.


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