A contact-less small antenna characterization through impedance modulation B. Monsalve 1 , S. Blanch 2 , J. Romeu 3 and L. Jofre 4 # Antennalab, Signal Theory and Communications department, Universitat Politècnica de Catalunya Jordi Girona, 1-3 Barcelona 1 bmonsalve@tsc.upc.edu Abstract— In smalls antennas S-parameters measurements are highly influenced by the measurement equipment as well as the environment. In order to avoid this inconvenience, a wireless and non-invasive characterization setup is required. In this paper a novel contact-less measurement setup is developed based on the Radar Cross Section (RCS) measurement method using impedance modulation. A small (2x2cm 2 ) autonomous and self- powered communication device is presented as a system able to change the load impedance of the antenna under test (AUT) to produce impedance modulation. The device contains a microprocessor, a switch able to change between three different load impedances and a 5V battery. I. INTRODUCTION Nowadays there is a trend towards the miniaturization of communication systems including the antenna device. Due to the small size of the antenna, the S-parameters measurements are highly influenced by the measurement equipment as well as the environment. It is well known that changing the load impedance in the antenna feeding it is possible to obtain the characteristic parameters of the antenna, such as S-parameters and gain. Different authors developed a measurement setup using the RCS method [1] [2]. If the load changes with a certain modulation we can analyze the received signal of the modulated scattered fields in order to extract not only a near- field map, as in MST theory [3], but also the characteristic parameters of any antenna. In this paper a novel contact-less measurement setup through impedance modulation is presented. The theory formulation of the problem and the experimental setup are presented in sections II and III, respectively. In order to validate the setup, three different antenna characterizations have been done and the results are presented in section IV. II. FORMULATION Taking into account the two one-polarization antennas free space system depicted in Fig. 1(a), the resulting scattering matrix [S] T can be considered as a two port network with a signal flow graph showed in Fig. 1(b). If port 2 of the reciprocal network it is terminated with a reflection coefficient Γ Li , the measured S 11 is related to S ij as follows:                     [1] (a) (b) Fig.1 (a) Scattering parameter representation and (b) equivalent signal flow graph. Where S 11probe , S 22AUT and     are the return losses of the probe antenna, the Antenna Under Test and the system respectively. Due to the fact that the system is a reciprocal two port network, S 21 is equal to S 12 and they are dependent on the gain of each antenna, the polarization coefficient and the distance between antennas. For a complete characterization of the system a double polarization measurement has to be done to obtain the antenna gain but to characterize the reflection coefficient of the AUT only one polarization measurement is needed to extract the S 22AUT parameter. In [1] three unknown have to be solved, S 11probe , S 12 and S 22AUT . So, a 3x3 equation system is needed to characterize the scattering matrix. Changing the reflection coefficient of port 2 between three known states (short circuit, open circuit and matched load) the scattering matrix is characterized solving the result 3x3 equation system. III. EXPERIMENTAL SETUP A. Switching device To produce an impedance modulation at a certain frequency, a change between three different load impedances is needed and also the generation of the required frequency modulation. Ideally, a perfect short circuit, open circuit and matched load are needed to cover the maximum different values in the a 1 b 1 a 2 b 2 [S] T S 12 b 1 a 2 S 21 S 11 S 22 a 1 b 2 Γ Li Γ Li Probe AUT 696 Authorized licensed use limited to: UNIVERSITAT POLITECNICA DE CATALUNYA. Downloaded on July 30,2010 at 11:31:16 UTC from IEEE Xplore. Restrictions apply.