What can cause standing waves on a transmission cable?

Standing waves on a transmission cable occur when there is a mismatch between the impedance of the cable and the load or source it is connected to. This mismatch leads to reflections of the signal traveling down the cable. Poor termination refers to inadequate connection at the end of the transmission line, which can create a situation where part of the signal bounces back rather than being absorbed, leading to the formation of standing waves.

In the context of the other options, excessive cable length can contribute to losses and attenuation but does not directly cause standing waves unless it interacts with impedance mismatches. High-frequency signals can exacerbate these issues due to their shorter wavelengths and susceptibility to reflections, but they do not inherently cause standing waves. Lastly, incorrect impedance matching, which refers to a scenario where the impedance of the cable and load does not align, is a more general term that also leads to reflections and standing waves but is not as targeted as the concept of poor termination.

In summary, poor termination specifically and directly creates conditions that lead to standing waves by causing reflections due to inadequate connections at the termination point of the cable.