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A direct current (DC) block is a frequency filtering device used in coaxial antenna systems. The DC block allows the passage of radio frequency (RF) signals while blocking audio and DC frequency interference that can damage the system. DC blocks are used in three different forms, known as inner, outer, or inner/outer. These components are further classified by connector type, voltage, and maximum frequency.
The DC block functions by using a blocking capacitor to capture the flow of low-level DC currents through a coaxial cable. It does this while allowing the higher frequency AC currents used in radio signal transmission to pass through in a process called coupling. The DC block can produce limitations on the transmission of all lower level frequencies, including AC frequencies, so modification of the system is sometimes required. This process of modification is called tuning the circuit.
These devices make use of a capacitor to filter out unwanted frequencies that could damage sensitive electrical components of laboratory testing equipment or radio transmitters and receivers. The larger capacitors offer a lower cutoff frequency while smaller capacitors cut off the transmission in a higher range. Where the various DC blocks differ is their placement in the coaxial system.
An inner DC block is connected inline as an integral part of the coaxial system's main conductor. Connected along the conductor, also called the hot wire of the system, this configuration is sometimes built into specialized equipment to avoid the need for aftermarket parts and modifications of the coaxial system. When an outer DC block is used, it is attached to the ground plane of the circuitry, in series with the outer conductor of the coaxial cable. Combinations of the two systems are known as inner/outer DC blocks. These blocking capacitors are incorporated along the ground plane and the hot wire of the coaxial system.
The outside and inside/outside varieties of DC block exhibit one special characteristic that limits their use. Due to the fact that these devices are placed in series with the electrical wiring of the ground plane, they have the potential to carry current. This means the configuration makes it possible for the equipment operator to receive a shock if he or she were to touch a charged piece of the system. These systems are sometimes used when it is necessary to inject current into a system for modulation of the transmission. While the set-up does present an electrical shock hazard, the risk is considered negligible when weighed against the benefits to the transmitted signal.