Optimal transmission frequency for ultralow-power short-range radio links

• Published in: IEEE transactions on circuits and systems. I, Regular papers Vol. 51; no. 7; pp. 1405 - 1413
• Main Authors: Yates, D.C., Holmes, A.S., Burdett, A.J.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01-07-2004; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Antennas; Circuit topology; Circuits; Energy consumption; Frequency; Links; Loop antennas; Optimization; Oscillators; Personal digital assistants; Power transfer; Radio transmitters; Transceivers; Transmitters; Transmitting antennas; Ubiquitous computing; Wireless communication
• ISSN: 1549-8328; 1558-0806
• DOI: 10.1109/TCSI.2004.830696

Analysis determining the optimal transmission frequency for maximum power transfer across a short-range wireless link is introduced, including a comparison of near-field transmission with far-field transmission. A new near-field power transfer formula has been derived, which allows direct comparison with the well-known far-field Friis transmission formula. Operating charts are presented, which provide the designer with the preferred transmission frequency as a function of distance and antenna dimensions, together with surface plots which show the power transfer for this frequency. The analysis, performed for loop antennas, has been used to evaluate the oscillator transmitter as a low-power topology. It is shown that the requirement of a high-Q factor to realize a low-power oscillator need not be contradictory to achieving optimal far-field radiation characteristics. Based on this fact an approach to sizing loop antennas for low-power oscillator transmitters is suggested.