History and development  of bench measurement techniques for impedance evaluation

After the introduction of the beam coupling impedance concept by V. Vaccaro it was realized soon that for highly relativistic beams a very close similarity exists between the TEM like field of the charged particles and the field of a wire in a coaxial structure. This is the basis and motivation of the coaxial wire method. Early concepts of this method are shown including their shortcomings which had lead over a certain period to a poor reputation of the coax wire method since not all sources of errors and misinterpretation had been understood properly. Transverse impedance measurements using two wires carrying currents with opposite polarity were already done on the late 70ths and the concept was extended to the evaluation of dipole and quadrupolar impedances by applying a single displaced wire and pair of wires. Advanced and special methods are shown and briefly discussed together with few examples.