Origins of RFID concepts in early wireless-based scale model airborne antenna pattern measurements

It is interesting that the RFID concepts were utilized as early as 1941, in making wireless-based scale model airborne antenna pattern measurements by means of scattering modulation. Prior to 1941, such scale model airborne antenna pattern measurements were made as in Figure 1, where the airborne antenna referred to henceforth as “antenna under test” (AUT) was operated in the receiving mode, while typically a measuring or horn antenna was used as the transmitter. In this measurement procedure, a 1/20 or 1/40 scale model was employed at a correspondingly higher (20 or 40 times higher) antenna frequency. This approach of Figure 1 required a long cable connecting the AUT receiver to the recording equipment back near the transmitting horn antenna. The use of the cable led to measurement inaccuracies resulting from radiation leakage and other cable distortion effects. The use of such a cable could, of course, be avoided if the AUT was now used as a transmitter instead, while the measuring horn antenna was operated in the receiving mode. However, during the 1940s a transmitter with self-contained batteries was not available, at sufficiently high frequencies, for installation into the AUT in such scale model. The above limitation due to the non-availability of high frequency transmitters for scale model airborne AUTs was overcome, in an ingenious fashion, by G. Sinclair in 1941 [1,2], based on a suggestion by W. L. Everitt, both of the Ohio State University, upon utilizing the fields backscattered by the AUT scale model aircraft configuration after it was illuminated by the transmitting measuring horn antenna. This new wireless method thus did not require the cable in Figure 1. The electromagnetic (EM) field scattered by the AUT model can be shown to consist essentially of two parts; one being the scattering by the external structural features of the air-craft-antenna combination while the other part is the field re-radiated by the illuminated AUT model in the presence of the rest of the aircraft model. The latter field re-radiated by the AUT, which is of interest here, results directly from a load impedance mismatch at the terminals of the AUT model after the latter is energized by the field incident from the measuring horn. The horn also acts as a receiver in this backscattering configuration as depicted in Figure 2. The load impedance mismatch was introduced by means of a battery operated switch that opened and closed in a periodic fashion. Such a switch provides a short circuit when closed, and an open circuit when it is opened, at the terminals of the AUT model. As a result, the desired signal re-radiated by the AUT model after it is energized by the illumination from the measuring horn is modulated and hence can be separated. This novel wireless measurement of the AUT scale model aircraft radiation patterns has all the characteristics of a modern RFID system; namely, the measuring horn antenna (acting here as both a transmitter and receiver) operates like a RFID reader antenna, while the modulated scale model aircraft AUT operates like an RFID tag antenna placed on the object being identified. An analytical description of the AUT model pattern measurement of G. Sinclair, and hence of the general RFID principle, which is essentially based on EM antenna scattering, is summarized below.