Coax cable is the most common type of cable used in CCTV installation. When video signals or sometimes PTZ data are transmitted via coax cable they are transmitted as “unbalanced transmission”.
Below diagram show a cross section of a coax cable. Center conductor transmit video signals and shield is used as ground (GND). Copper shield is also used to protect the video signal from any unwanted foreign interference or known as electro magnetic interference (EMI).
All unwanted electro magnetic interference will be induced by the copper shield and when it is ground at camera’s end
How to run your cable free from EMI (Electro magnetic interference)?
As a CCTV installer, you always want to keep your coax cable free from EMI. It does not matter how well the coax cable is protected by shield, it can still be a challenge to keep your signals away from electro magnetic interference. It all depends on how well the copper is screened and suspected EMI’s frequency.
Coax cable quality is mostly determined by the percentage of the copper screening in the cable. It is quite normal to have copper screening in between 90% to 99%,you can check this in cable manufacturer’s specification.
Technically saying, only frequencies above 50 kHz are prevented by coax cable. Any frequencies below this level can induce current and it can cause EMI. The frequency of a main circuit is normally around 50 Hz and this can sometimes cause issues, that is why we do not run coax cable in parallel with the mains. It is recommended that you have at least 12.5 inch gap between the mains and coax cable to avoid EMI.
Most common used coax cable in CCTV installation is RG59 which can easily transfer black and white video signals up to 300m and colour video signals up to 200m. The performance of this cable is affected by several other factors such as cable run, bends in cable run and what actual the cable is made of for example whether the cable is copper or copper plated.
Coax or RG59 is not a bad option for CCTV cable run, but you should consider other factors such whether the cable is manufactured in good quality and strict guidelines are followed in cable run.