The procedure to measure the Radiating Cable Coupling Losses is defined by the IEC 61196 – 4.
According to this standard, two measurement configurations are permitted, i.e.: the “ground-level method” and the “free-space method”; these two configurations give results that may be quite different.
In addition, the standard allows to specify either a Coupling Loss for a “single orientation” (i.e. radial, orthogonal or parallel) or a mean value calculated with a specific formula.
The fact that the standard is not very restricting may be very confusing, especially when the performances published in the manufacturer data sheets have to be compared. The aim of this note is to provide some clarifications and to assist the radio engineer in making the most accurate link budget calculations.
Ground-Level versus Free-Space Method
In the ground-level method, the Radiating Cable is laid at 10 to 12cm above a concrete ground. The centre of the antenna is positioned vertically at 2m above the Radiating Cable. The field strength is recorded when moving the antenna along a path parallel to the Radiating Cable.
In the free-space method, the Radiating Cable is hung to non-metallic posts at a height of 1.5 to 2m. The antenna centre is at 2m from the Radiating Cable, these two elements being at the same height. The field strength is recorded when moving the antenna along a path parallel to the Radiating Cable.
Ground-level and free-space method rarely give the same Coupling Loss results.
For a Radiating Cable working in coupled mode, the difference is important and may even exceed 10dB.
For modern Radiating Cables, operating in radiated mode, differences of 2 or 3dB are common, but rarely exceed 6 or 7dB. There is no general rule regarding the difference being positive or negative. In some cases, the ground-level method gives higher Coupling Losses than the free space method and, conversely, lower Coupling Losses in other cases.
Radiating Cables are in most cases installed at a short distance from a surface, ideally 10 to 12cm from a ceiling or wall. This is producing reflections that may either improve or impair the Coupling Loss.
In Tunnel versus Free Space
Measurements taken under operational tunnel conditions with cables from different Manufacturers are frequently between 15 and 20dB worse than the values given in their Data Sheets that are usually based on Free Space measurements. This possibly leads system designers to use high safety margins in their link budget calculation.
Since many years, Eupen Cable carries out measurements on Radiating Cables in accordance with the Ground-level method inside a real tunnel and these are the values published in its Data Sheets.
Thanks to this, the measurements taken under operational conditions are usually very close to the values given in the Eupen Radiating Cables Data Sheets. This has clearly been demonstrated by several independent measurements.
Although, according to the standard, the antenna orientation has to be specified, most manufacturer data sheets do not provide this information. Consequently, Radiating Cable performance comparisons are very difficult, as the given Coupling Loss could correspond to any of the three orientations or could even be mean value.
Except in some very special cases, it is strongly recommended to use the IEC Average in the link budget calculation, as in practice, the antenna orientation is nearly never perfectly radial, parallel or orthogonal with respect to the Radiating Cable, but rather a combination of those three possibilities. Indeed, the mobile antenna is often down tilted and is rarely in a Radiating Cable plane at all. This makes it obvious, that the Coupling Losses should ideally be expressed as a mean value, calculated with a specific formula, as defined by the IEC 61196 – 4.
Eupen Radiating Mode Cable (RMC Range)
For the reasons mentioned earlier, all the EUCARAY® Radiating Cable Data Sheets state the Coupling Losses (50 and 95% probability) measured in tunnel according to ground-level method and are expressed as a mean value, as defined by the IEC 61196 – 4.
Furthermore, the EUCARAY® RMC range of Radiating Cables is using a patented slot design, which, amongst other, benefits from the reflection phenomenon. We achieve this by minimising the phase difference between direct and reflected waves. This also explains why the Eupen Data Sheet Values and actual Values measured after installation are so closely matched.