Defining and assessing the uncertainty contributions in the line-injection measurements of transfer impedance

The shielding effectiveness of the coaxial transmission lines is currently quantified in terms of transfer impedance, a lumped-element concept. Its measurement is based on two methods, the more recent of which, the line injection method, is intended to extend the technique to higher frequencies. In doing so, wave propagation effects are introduced which produce potential distortion of the quantity we intend to observe. Also, the test arrangement is imperfectly grounded, thus unpredictable common-mode coupling may be present. This paper describes the approximations of the basic equations involved and the uncertainties related to the experimental equipment. A configuration of the injection line intended to minimize common-mode effects, and improve the predictability and repeatability is adopted. An experimental verification based on the comparison with the traditional and more closely defined method of the triaxial coupler is offered.