Low Perturbation Antenna Impedance Measurement Techniques

The indirect two-port reflection method for measuring antenna impedances is reviewed using scattering parameter techniques and developed into a more accurate low perturbation optically coupled transmission measurement. This transmission measurement infers the antenna impedance by measuring the voltage induced in a number of loads connected to the antenna port with reference to the antenna used for illumination. Both these techniques have proved to be very versatile: applications to a typically electromagnetic compatibility problem, consisting of a box and wire above a ground plane, and to the design of a challenging multi-band Digital Audio Broadcast antenna are presented. INTRODUCTION In this paper we propose to present our latest results to illustrate the flexibility of the technique. The first application is a classical box-wire-box system, where the radiation impedance is solved by measuring the reflection coefficient from the probe antenna. The second application is an antenna measurement case, where a helical type antenna is measured by the low perturbation two-port measurement technique. In the first case the radiation impedance is solved numerically in the way where the change of the impedance of the known antenna is measured when the wire connection to the box is loaded by three different loads of varying complex impedance. In this method the unknown radiating structure is measured through a two-port network [1]. The network describes the mutual coupling between the probe antenna and the radiation impedance of the measured system. The probe antenna is presented as port one and the transfer impedance as port two. The input impedance of port one is measured by a network analyser. After making reference measurements, the impedance of port two is calculated. In the second case the antenna impedance is solved by measuring the voltage from the antenna port that is loaded by the set of varying loadings. In this case the two-port presentation is also used to present the coupling from the reference antenna through the loaded antenna into the network analyser. An electro optical (EO) system is used to minimise the effect of the the measuring cable that is used in conventional measurement. INDIRECT ONE-PORT MEASUREMENT The unknown antenna can be measured either by indirect [2] one-port measurement or by two-port measurement. The one-port measurement is presented in [3] and [4]. In the indirect method the impedance of an unknown antenna is solved by measuring reflections from the probe antenna for number of loads. The unknown antenna is loaded by three known loadings. Then the impedance is solved from 1-4. Ay = B, (1) where A = [ ZL(2)− ZL(1) Zin(1)− Zin(2) ZL(3)− ZL(1) Zin(1)− Zin(3) ] (2) B = [ ZL(2)Zin(2)− ZL(1)Zin(1) ZL(3)Zin(3)− ZL(1)Zin(1) ] (3)