When reading Specifications for Radiating Cables, prepared for the purpose of a public tender, one often finds that values are given which may restrict a System Integrator interested in bidding.
It is quite common that Tender Documents not only specify the important System parameters, but go a few steps beyond, specifying a cable size, a longitudinal loss in dB per Kilometre and sometimes a frequency range that is unnecessarily broad. This, unfortunately, often restricts the System Integrator, usually a company chosen for their expert knowledge in the field of Radio Re-broadcasting, leading in many cases to higher than necessary costs.
The following is a brief listing of what should and, importantly, what should best not be specified in a public tender:
It is advisable to restrict the information set in a public tender to sow the important parameters, like frequencies to be used, lengths of the tunnel and type of the tunnel, allowing the System Integrator to choose the most suitable cable for the given project, free of restrictions, to allow the experienced System Integrator to design his solution in the most cost effective manner.
- Specify the Areas to be covered by RF Services
- Give a general description of environment, tunnel (or other area) dimensions & length…
- Specify the required Frequency Bands
- Specify the minimum Signal levels
- Specify the permissible System Loss for the different frequencies.
For a Radiating cable, the System Loss, expressed in dB, is usually the sum of longitudinal loss over 1000 meter (or any other relevant distance for the particular application) plus the coupling loss C95 or C50.
- You may specify the Coupling Loss for either 50% or 95% Probability, depending on the primary usage of the RF System. If the System is to be used for mission critical communication services, then you have no option, it has to be 95% Probability.
- Do NOT specify Longitudinal Losses, they are purely a function of the Cable Size.
- Do NOT specify the Cable Size, as that is a function of the Permissible System Loss.
Specifying a cable size should be avoided, as it can exclude innovative cable designs and the cost savings that can be achieved by installing smaller sized cables.
- Encourage the use of Radiating Cables that are optimised for particular Frequency Bands.
As most Operators of Mission Critical RF Services do not encourage the use of Commercial Services, like Public Mobile Services, on the same Radiating Cable, it would be beneficial to the propagation of all RF Services, if two separate Radiating Cables, optimised for their respective Frequency Bands, are used.
As an example, let’s look at a typical Road Tunnel of about 1.2 to 1.4 Kilometres in lengths:
The services required are usually:
88-108 MHz – FM Radio, with or without Break-in facility
150-172 MHz – VHF Radio for Maintenance and some legacy Emergency Services frequencies
380 – 512 MHz – UHF, including TETRA and TERTRAPOL for Emergency Services frequencies
And, sometimes, a single GSM 900MHz channel for use by the Motorist using the Tunnel.
Past experience shows that often a 1”1/4 or even a 1”5/8 size radiating cable, with a frequency range from 50 to 2500MHz is being called for, whilst a good System Integrator would be able to fulfil all technical requirements to provide the above listed services with, for example, a 7/8” Radiating Cable that is optimised for the required frequencies.
That the potential for cost savings can be quite substantial is easily understood.
The required Flame and Fire features of a Radiating Cable depend on the Geographic Area where the Cable is to be used. In the United States of America and in Canada, Underwriter Laboratories (UL) is the leading standard, where in most other parts of the world, standards in accordance with IEC or EN are adopted. Local regulation may also apply in some countries.
We refer here under to the relevant IEC Standards usually required for indoor application. The designer should always refer in priority to the local laws & regulation.
Low Smoke emission:
One important feature a Radiating Cable should meet is the Low Smoke rating.
It is a well-known fact, learned from incidents in tunnels, that it is seldom the actual fire that leads to casualties; it usually is smoke that is responsible for loss of life.
A cable is considered as “Low Smoke” if it passes the IEC61034-1 & -2 tests.
To avoid the emission of corrosive gases in case of fire, the cable should be Halogen Free according to IEC 60754-2.
Flame or Fire retardant cables do not propagate the flame, limiting by this the propagation of the fire.
A radiating cable is considered as flame retardant if it passes the IEC60332-1 test for single cable.
Further, even though radiating cables are never installed in bundle, it has become of common praxis to specify the more severe IEC60332-3 cat C test for Cable bundle.
– Circuit integrity test (sometime called Fire Resistant Test) according IEC60331
Do NOT specify this test, unless you intend to supply DC Power through the radiating cable.
IEC60331 is not a Radio Frequency Test, but a purely DC test, it gives no indication concerning the performance of a radiating cable during a fire.