Printed Multi-Turn Loop Antennas for RF Biotelemetry

Compact antennas afford hemispherical coverage at any linear polarization.

Printed multi-turn loop antennas have been designed for contactless powering of, and reception of radio signals transmitted by, surgically implantable biotelemetric sensor units operating at frequencies in the vicinity of 300 MHz. In the original intended application of these antennas, the sensor units would be microelectromechanical systems (MEMS)-based devices now being developed for monitoring physiological parameters of humans during space flights. However, these antennas and the sensor units could just as well be used for physiological monitoring on Earth.

Figure 1 depicts one such antenna, consisting of a thin metal strip laid out in a multi-loop pattern on a dielectric substrate. Other components are also mounted on the dielectric substrate. For maximum sensitivity in reception, a Pi network (which comprises lumped-element inductors and capacitors) is used to match impedances between the antenna terminals and the input terminals of a monolithic microwave integrated circuit (MMIC) low-noise amplifier. The output of the amplifier is sent to other circuits through a coaxial cable. The antenna and the other components and circuits mentioned above are all parts of a handheld telemetry-reception unit.

In comparison with prior spiral, diskcoil, and solenoid-coil antennas that have been considered for use in receiving telemetry from implantable sensors, the antennas of the present type are smaller. In addition, the present antennas offer potential for further miniaturization in that, in principle, signal-processing circuits could be mounted in the central regions of the loops.

In a typical medical diagnostic situation, the signal radiated by an implanted sensor could have vertical, horizontal, or slanted polarization. Hence, to emulate all possibilities, the radiation pattern of a loop antenna of the present type was measured in reception of a signal transmitted by a dipole antenna in vertical, horizontal, and slanted (45°) orientations. The results of these measurements (see Figure 2) show that the antenna provides hemispheric coverage and is capable of receiving signals under the aforementioned linearpolarization conditions.

This work was done by Rainee N. Simons, David G. Hall, and Félix A. Miranda of Glenn Research Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Bio-Medical category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17879-1.