# Coaxial Cable Loss & Efficiency Calculator

All coaxial cable transmission lines have loss, called attenuation. As a signal travels down a coaxial cable conductor, a certain percentage of the signal is lost due to the resistive and capacitive effects of the coaxial cable. As a general rule, the smaller the diameter of the coaxial cable, the larger the loss and attenuation values versus larger diameter cables. Also, as frequency of use increases, so too does attenuation.

Cable loss is one of the components that must be taken into consideration at a broadcast tower location because if the cable going up the tower is of smaller diameter and has high loss, anywhere from 20% to 50% of the transmitters' wattage can be lost in the cable itself before reaching the antennas. For example, if a tower is 350 feet tall and 375 feet of cable is needed to interconnect the transmitter on the ground to the antennas at the top of the tower, and the cable is rated at 0.8 dB loss per hundred feet at the frequency of use, 3 dB is lost in the 375 feet of cable alone. A loss of 3 dB represents a power ratio of 50% (wattage x 0.5) which means a 500 watt transmitter is pushing 500 watts up the tower but only 250 watts get to the antenna with the other 250 watts absorbed and lost in the cable. A cost/benefit analysis should always be done to determine how much a less expensive cable is costing in lost wattage given the length and frequency of use.

Use the following to calculate coax cable loss, or transmission line efficiency, for a given frequency.

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