The basic component of any CCTV
system is the cable. The strange chalk markings seen on pavements around our big cities in recent times point to places where illegal access to wire-less networks can be obtained, allowing the suit-ably-equipped ‘hacker’ free time on the Internet.
This development indicates that wireless networks are not secure enough and that cable appears the better bet when privacy and freedom from unwanted intrusions are important. This raises the old question; “Who guards the guards?”
In this case it means, “How secure is co-axial cable?” Of course, not all interference getting into a cable is caused by villains. There are many innocent sources of interference such as radio transmitters, cordless telephones, baby alarms and electrical equipment with low spark suppression characteristics.
We need to defend our systems against both kinds of intrusion. How does cable measure up to its alleged superiority?
It all depends on the quality of the cable, of course, and to quote another old adage, “You get what you pay-for!”
Triple screened co-axials
Even so, modern cables are coming onto the market that combine excellent performance in respect of radiation and interference pick up with a reasonable price. These are the new generation of triple-screened co-axials that combine light weight, excellent bending and crushing resistance and superior ageing characteristics with very high screening factors.
Screening factor is the most important characteristic of co-axial cable used in security-conscious applications.
Other parameters such as attenuation, VSWR and resistance to UV exposure rank highly too, for they determine the performance of the cable network that carries video and audio signals to the control room. With a poorly-screened cable it is possible to inject interfering signals into it without making a direct connection, and it is equally possible to pick up radiation from the cable by the same means for illicit monitoring purposes. Ageing also produces increased cable losses making the likelihood of early replacement high.
Triple-shield cable has a performance approaching that of copper tube co-ax, and rivals that of 150 micron copper tape screening. This quality is available at a lower price with triple-shield cable, and it is more easily installed, is slightly smaller and has the same signal loss per 100 metres. It has a higher velocity factor and a much lower increase of attenuation with age (See Fig 1).
Triple-shielding aside, the reason for the high performance and improved longevity is the use of gas-injected polyethylene foam for the dielectric – the white insulator that surrounds the centre conductor – instead of the older chemical foam process.
Contamination zone
Problems with the chemical foam are several, firstly the chemical residues of the process attack the inner conductor and braid or tape and braid screen over a period of time and reduce the conductivity of them. This is because high-frequency currents tend to flow on the surface of a conductor, the “skin effect”, and this is precisely where the contamination takes place.
Secondly, the gas-injection process allows the use of high-density polyethylene and the result is a tougher but lighter dielectric that has lower capacitance, allowing the cable to be made smaller for a given through-loss per distance, and, as a bonus, a higher velocity factor – the time a signal takes to travel through the cable compared to the same distance in free space.
Earlier this year on a visit to Italy I was invited to tour the factory of Italiana Conduttori sri, makers of Cavel brand cables. I was permitted to inspect all production departments and the laboratory at the factory near Milan with my camera and tape recorder. The construction of this A Class triple shield co-ax cable is interesting.
The triple shield consists of three layers (See Fig 2). The first, next to the dielectric is a polyester film tape with aluminium foil on both sides placed longitudinally on the cable. There is sufficient edge-to-edge overlap to allow good electrical contact, making a double-walled aluminium tube over the dielectric. Over this is woven a 75 per cent density tinned copper braid. Finally, a second tape is placed over the first two layers. This tape is a polyester film with aluminium foil on one side, and is again placed longitudinally over the braid. The inner edge of the tape is folded outwards to form a “shorting fold” that ensures the aluminium foil again takes the form of a tube over the braid.
Good workmanship vital
The technical sophistication of the modern criminal demands higher levels of security consciousness and this type of cable goes a long way to providing a high degree of immunity to interference ingress and, equally, a very low level of radiation. The long life-expectancy it offers allows it to be installed in less accessible places and to be built into new construction.
No matter how good the cable is, the quality of the workmanship must follow good RF engineering practice, and the quality of the connectors, glands and hardware enclosures must also be of high quality to achieve a good and lasting result.
This is a vital requirement in a vitally important CCTV industry.
