What Is the Maximum Length of BNC Coaxial Cables?

Attenuation

• The most common concern in coaxial cable length is the amount of signal strength it will lose, often called attenuation. Cable specifications usually show loss statistics in dB per 100 feet or 100 meters. Each reduction of 3 dB cuts the power approximately in half. A signal of 10 dBm has a power of 10 milliwatts. If the cable reduces it by 3 dB, the signal at the end will roughly be 5 milliwatts. If it reduces it 10 dB, the signal will 0 dBm or 1 milliwatt. Larger diameter cables usually have lower loss.

Frequency Response

• A coaxial cable does not have the same loss at all frequencies. As frequency increases, so does loss per 100 feet. Most coaxial cables can carry signals from zero Hz or direct current to 1,000 MHz or 1 GHz, but the loss at the higher frequencies can be significantly more than at lower. Manufacturers show specific cable loss per foot per frequency on statistic sheets often called frequency response curves. System designers should always consult these specifications to select an appropriate type of cable. Specialized cables can carry frequencies higher than 1 GHz, but should not use BNC connectors.

Cable Design

• The loss and frequency response of a coaxial cable depends on specific design features. The size and shape of the center conductor determine the resistance (R) to direct current. and the inductance (I). The distance between the center conductor and the shield and the material used to insulate them determine the capacitance (C). Complex combinations of R, I and C determine the electrical characteristics. Different metals in solid, stranded or hollow configurations for the center conductor vary resistance and inductance. Foil, braided or corrugated shields combined with different insulating materials and thicknesses vary capacitance.

Applications

• The main considerations for the type of cable used with a BNC connectors are the power of the originating signal and the sensitivity of the distant component. If a signal starts at 10 dB and the receiver can read any signal higher than -30 dB, the system will work with an overall cable loss of 40 dB. Before the age of digital technology, AT&T Long Lines carried signals for several miles using coaxial cables capable of using BNC connectors. However, a ham radio operator may upgrade an 50-foot antenna cable to gain 1 dB for maximum transmitted power.