Basics of cable/antenna test tools for base stations

It is widely accepted that one of the main concerns pertaining to the overall performance of a modern mobile communication network will stem from the numerous base stations on which the network is reliant. If these base stations have not been correctly constructed or there has been a failure for timely maintenance to be carried out, then the level of service that a network operator provides to its subscribers might be put in jeopardy and significant revenue may be lost.

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 is 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 (P_I) that reaches a specified point and the reflected power (P_R) that comes back from it. The equation describing this relationship is:

RL = 10Log₁₀(P_I/P_R)

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.