How to Calculate Loss in a Coax Cableby Dwight Chestnut
Coax cable is used to connect many electronics devices, including televisions, DVD players, cable television boxes, radio antennas and computers. The standard coax cable consists of an inner conductor, outer conductor and an outer layer of insulation material. Although, both the inner and outer conductors are made of metal, they both have some resistivity that causes power loss as the signal travels down the conductors. This loss is referred to as signal attenuation. Pure attenuation calculations are complicated, requiring the use of higher order mathematics. However, you can shortcut the approximations with the aid of manufacturer or industry cable data.
Refer to the MIL-C-17 Attenuation and Power Handling tables (see References). Find your cable part number in column one and locate the corresponding value for K1 and K2. K1 and K2 are the resistive loss constant for the cable.
Determine the frequency or range of frequencies for which you are looking to determine the cable loss.
The unit "dB" is a unit of attenuation. Calculate the attenuation or loss for each frequency using the following equation: Attenuation = K1 multiplied by the square root of F + K2 x F, where F is the frequency in MHZ. The results will be in dB per 100 feet of cable, or "dB/100 feet." As an example, assume that K1 is 0.444, K2 is 0.00126 and 100 MHZ. The attenuation will equal K1 times the square root of F + K2 x F, i.e., (0.444 x 10) +(0.00126 x 100) = 4.44 + 0.126 = 4.56, or 4.6 dB per 100 feet of cable.