Power / Grounding for Audio and Video Systems – International Version Page 30 of 43 ground, and should generally be isolated from ground.] 2) Good RF performance The antenna must be coupled to audio/video receiving equipment in a manner that does not create excessive loss. 3) Prevent Earth Loops The cable shield should be broken by some form of RF transformer before the input to the receiving equipment (but not between the antenna and the connection of the lightning protection device to earth). Excellent isolation transformers are available from Jensen Transformers.
Fig 18 – RF Surge Suppressors for use on receiving antennas A form of shunt mode protector called a Gas Discharge Tube, or GDT is the preferred protection device for most signal wiring, like antennas, telephone lines, audio circuits, and video circuits. Like any other shunt mode device, suppressors using GDTs that protect balanced circuits must installed very close to the common point for all earth connections within the building, and must have a very low impedance bond to that point at the frequency of lightning. GDTs can also protect coaxial inputs and outputs of equipment by wiring them in parallel with the signal path. Fig 19 shows three shunt-mode protectors designed to protect receiving antennas. Although they are shunt-mode devices (GDTs), RF protectors are packaged with input and output connectors, built so that the transmission line impedance is maintained through the protector, and so that the protector enclosure can be bonded to a ground bus. The Nextek unit (left) is the most flexible – it passes RF signals from 0 to 2.5 GHz at up to 25 watts, and up to 3A DC at up to 48V to power an outdoor preamp – and it is also the least expensive. It is designed to mount as a "feed-through" connector on a metal plate. The shield (screen) connection of the GDT must be bonded to the system ground common point. The GDT protects the electronics from the differential voltage on the coax, and the bond to the ground system provides the discharge path for lightning on the shield (screen). POWER CONDITIONING Power conditioning is a rather broad term, describing processes to correct one of more of the problems noted above. In it’s broadest meaning, it connotes voltage regulation to correct for the line voltage being higher or lower than normal, surge suppression to eliminate short term faults that can damage equipment, and bandpass filtering to reduce noise. Some may even attempt to reduce harmonic currents. Unfortunately, much of the equipment sold in the name of power conditioning does more to relieve purchasers of their money than to improve power quality. ISOLATION TRANSFORMERS
Figure 19a – An Ordinary Transformer
Common mode noise on the power line can be coupled into audio equipment via the power supply. In an ordinary transformer, shown schematically in Fig 19a, stray capacitance between the primary and secondary windings will couple high frequency energy across the transformer – the higher the