Which Wire Should I Use?
So what characteristics should you look for?
Interference Immunity. Coax is an unbalanced transmission line and is therefore susceptible to interference. That’s why technicians are taught to avoid placing it near noise sources such as power cables. Unshielded twisted pair (UTP) – and its cousins, foiled twisted-pair (FTP) and shielded twisted-pair (STP) – are balanced transmission lines. As such, they are inherently immune to interference (subject to the performance of the transceivers).
Ground-Loop Immunity. Coax has no ground-loop immunity and can suffer from hum-bars in the image. UTP with passive-to-passive transceivers is slightly better; however, UTP with passive-to-active transceivers produces hum-free transmission every time.
Low-Frequency Attenuation. At low frequencies (<100KHz), the only thing that matters is wire resistance. Thicker conductors yield lower losses, visible as picture brightness. Coax has slightly better low-frequency performance than UTP wire. Active receivers compensate for this loss, delivering a signal equivalent to zero wire distance.
High-Frequency Attenuation. At high frequencies (>1MHz), the copper losses are dwarfed by inductive and capacitive parasitics. These losses can be substantial and are the dominant limit to distance, showing up in the image as soft detail or loss of color. A good receiver can boost these frequencies, delivering a flat frequency response. The result is “lossless” transmission that delivers every detail coming from the camera.
Plenum or Non-Plenum Cable? Plenum-rated wire is typically Teflon-based and is used within buildings where toxic gasses released during a fire could enter ventilation systems. It is more expensive than PVC-jacketed non-plenum cable.
Direct-Burial Rated Cable. This cable must be used in any environment where it could be exposed to moisture. This includes underground conduit, as condensation is inevitable. This wire is polyethelene-jacketed (looks like drip-irrigation pipe) and gel-filled. Never use PVC jacketed wire in these environments, as water will permeate through the jacket and the increased capacitance will ruin the high-frequency performance. Such a fault cannot be detected with a multimeter.
Installers have many cable choices. While some choices are clearly wrong (STP, untwisted wire, failure to meet environment-specific needs), others are more about performance and cost. The world is rapidly migrating to standards-based UTP solutions because they offer competitive improvements in both.
Wire Performance Characteristics
Siamese cable is a coax with attached power conductors. The result is fewer installation pulls. This wire is more bulky than coax. All other coax considerations apply.
UTP cable is the most immune to interference, smallest diameter, and easiest to terminate. It also meets EIA/TIA568B structured building wiring standards, allowing it to be used for video, telemetry, camera power, voice, and IP/Ethernet. Performance varies from voice-grade to gigabit (Cat6), and any category-rated UTP may be used for video.
Un-Twisted Conductor cable is susceptible to interference and should not be used for video applications.
FTP and STP cable, a foil or braid shielded cable, is designed for datacom environments. Before using it, check its data sheet for parasitic capacitance, also called “mutual capacitance.” With special exceptions, this value should be below 20pF per foot. Because of its high capacitance, STP wire should never be used for passive-to-passive video, as the picture will have little or no color and the resolution will be poor. Category-rated FTP has low capacitance and is fine for video applications.
Multi-pair cable with an overall shield is suitable for video transmission provided there are six or more pairs in the bundle. Fewer pairs means that the shield is close to each wire pair, with resulting elevated capacitance. Check the data sheet beforehand.