Question The CCTV/Surveillance Expert
The Unshielded Twisted Pair Alternative
By Chad Szekeres
Q: Just as with any technology there are possible mis-wiring and set up scenarios which will result in an unfavorable installation when using UTP. I like the fact that it could save money in the installation but I also want to make sure it doesn't cost me. Suggestions?
A: To aid in a smooth, favorable installation, here are some of the obstacles to avoid and the troubleshooting techniques to deploy when faced with these obstacles:
T-Taps or Bridge Taps. One of the advantages to Unshielded Twisted Pair (UTP) technology is the ability to use existing in-house pairs, or unused pairs of vacant UTP cables. If the existing UTP cable was used for phone or voice service in the past, find and remove any T-Taps before beginning your installation. T-Taps, also called Bridge Taps, are ancillary pairs of wires connected in parallel to the main pair being used. They are unused pairs, usually found on telecommunication circuits connected at a punch-down block or pedestal, and in video circuits they cause reflections that appear as ghosts in the display. It is usually possible to tell how many T-Taps there are by the number of reflections in the display.
Crossed Polarity. In order to utilize UTP cabling for your CCTV installation it is necessary to install electronic devices to your CCTV equipment which in turn connect to the ends of the UTP cable. The majority of CCTV equipment today has a 75 Ohm unbalanced BNC connector. In order to utilize UTP cable to send a video signal you must connect a balun type device to the BNC of this device. Most baluns are designed with a BNC to a 2-wire output. The 2-wire output is labeled +/-. If the polarity is crossed the image will appear to be scrambled. To correct this, simply swap pairs on one end of the cable.
Ground Loops. When running video between two buildings or in buildings with long cable runs, the potential for ground loops exists. When looking at installations where a ground loop may be an issue, the use of an active receiver which offers ground loop isolation is recommended. While it increases the cost of the receiver slightly, it is far less than the cost of labor to troubleshoot, diagnose and eliminate a ground loop.
Attenuated Video Signals. Attenuation is the loss of signal. The longer the run, the more attenuation is in the cable. As the signal travels down a cable, part of it is absorbed by the cable and arrives at the end with less strength. With a video signal the high frequencies are affected first. A video image may appear to be washed out or, in severe cases, lacking color altogether. Just as with coax, video amplification may be required to compensate for losses which occur from wire attenuation.
Network Connections. UTP installations being designed around a network wiring scheme use RJ45 jacks, patch panels and telecommunication connection points. Network type installations move away from the traditional 2-wire balun design to a balun with an RJ45 connector. While it is possible to field terminate an RJ45 connector directly to the end of a Category 5/6 cable, it is recommended that you terminate the cable to a jack on one end and a patch panel on the opposite end. The use of patch cables allows for easy connections to the baluns without concerns of crossed polarity, mis-terminated, or poorly terminated RJ45 jacks. In addition each cable run can be certified as a network connection.