A) An impedance mismatch
Amateur radios require an impedence match of 50-ohms in the coaxial cable, antenna, or any other device connected. A mismatch between the radio and any of the devices in the chain will cause a greater than 1:1 (perfect match) SWR.
SWR is an abreviation of standing-wave ratio. SWR on a transmission line in an antenna system represents the the ratio of foward to reflected voltage on the line. A perfect 1:1 ration occurs when all parts of the antenna system are matched correctly to one another.
Written by: NJ2X, Michael Maher