A fundamental source of error in the scattering method of measuring antenna surface field distributions

An analysis of the measurement of a general standing wave distribution on a transmission line using a modulated scattering probe is presented. This is classified as a measurement in a reactive-field region of a normal-mode of a closed system. Details of theory and validation are discussed and a source of possible error is revealed when it is applied to antennas. In those cases where the forward scattered signal is not reflected back to the measuring system, the law relating the field to the measurable quantity may range from a square law to a fourth-power relationship. Thus, depending on conditions at the detector and whether scattering is occurring on a transmission-line-type structure or not, the measured quantity may be related to the surface field through a power law of index anywhere between 2 and 6. For travelling wave antennas, the distribution that is obtained is not necessarily that which is exclusively associated with the nonradiating surface wave, but it includes components near the ends that contribute to the radiation from the structure.<>