Gain and Input Impedance Measurement for UHF Transponder Antennas

It was shown by numerous authors that the characterization of RFID (radio-frequency identification) transponder antennas that are favored to be small, have low gain, and show a strongly reactive input impedance, is far from trivial. We propose a method that utilizes a small, highly-stable, battery-driven signal source to generate the transmit signal directly at the antenna. The signal source is equipped with a tunable matching network that allows to determine the antenna impedance by a source-pull measurement. Antenna gain can be measured by using the battery-powered oscillator as a calibrated transmitter. 1. Motivation It has been published in numerous articles that the characterization of directional pattern and input impedance of small antennas is far from trivial [1], [2]. The main reason is that a measurement cable leading to the antenna-under-test has to be avoided because it carries common-mode currents that may cause radiation by far stronger than the original radiation of the antenna. The fact that in RFID (radio-frequency identification) technology, antennas are not only small but also lossy due to low-cost manufacturing processes, makes the characterization of such antennas particularly difficult. In [3] a method specific for antennas used in mobile equipment is presented. A small, batterypowered VCO (voltage controlled oscillator) is used to generate a CW (continuous wave) signal directly at the antenna. Since antennas in mobile equipment are operated by an RF (radio-frequency) frontend typically presenting a 50Ω impedance, matching issues are not considered in detail. Although maximum gain and efficiency of the antenna can be determined with the method in [3], the directional pattern is left unexplored. A more advanced method that uses fiber optics instead of a measurement cable is presented in [4]. This is a very promising approach because it allows to determine gain, phase patterns, and even the input impedance. On the downside, the assembly needed at the antenna site is by far too large to apply this method to RFID antennas. A very interesting approach to measure the input impedance of RFID tag antennas is given in [5]. The impedance is determined from measurement results obtained for antennas loaded with different calibration standards. But, to our understanding, the results depend on the accuracy of the chip impedance which has to be known or characterized separately. Input impedance measurements are also shown in [6], where an on-wafer-prober is modified to characterize planar antennas. The authors put the antenna on a styrofoam spacer and use absorbing material to reduce the impact of the on-wafer-prober and the probe-head metal. Still, it is argued that the probe head influences the measurement results. We propose to characterize gain and input impedance of RFID tag antennas by means of a small, battery-driven oscillator that is mounted directly on the antenna and generates a CW (continuous wave) transmission signal at 864MHz. The oscillator is equipped with a tunable matching network that allows power matching with the antenna. The input impedance is determined by a source-pull method. For the directional pattern measurement, the autonomously operating oscillator supplies the tag antenna with RF power. The antenna can thus be freely rotated. The accuracy of the measurement mostly depends on the properties of the oscillator that has to provide • sufficient output power to overcome noise at the receiver, • stable frequency and power level, regardless of battery status and load situation, • a matching network that is tunable to the complex conjugate of the antenna impedance, • sufficient battery life to conduct the measurement procedure, and • it has to be small enough to maintain the electrical properties of the antenna-under-test. id103540359 pdfMachine by Broadgun Software a great PDF writer! a great PDF creator! http://www.pdfmachine.com http://www.broadgun.com Figure 1: Oscillator schematic. 0 5 10 15 20 25 30 35 42.8 -20.7 -20.6 -20.5 -20.4 -20.3 -20.2 t (minutes) P ( dB m ) 863.996 863.997 863.998 863.999 864 864.001