WHY MOBILE PHONES CAN DO SO MANY THINGS: THE INVENTION OF THE FRACTAL ANTENNA By Robert Colburn, IEEE History Center [EDITOR’S NOTE: This article originally appeared in IEEE-USA InSight. It is reprinted here with permission from IEEE-USA. Copyright 2021 IEEE.] Mobile phones perform multiple tasks by transmitting and receiving on many different frequencies. For example, when the user first dials, there is a carrier frequency that the phone and the base station communicate with each other to set up the call, establish which cell tower the phone is in range of, and choose which frequency or frequencies to use for the call. Some mobile phone systems use frequency-shift keying, which means the zeros of the digital signal are sent on one frequency and the ones are sent on another. BlueTooth, WiFi, and other applications use yet more frequencies. Prior to the late 1980s, this would have required mobile phones to use an antenna for each frequency. Mobile phones would have needed many different antennas sticking out, each sized according to wavelength. Instead, today’s mobile phones owe their sleek design in part to antennas whose shapes are determined by fractals. In 1982, mathematician Benoit Mandelbrot published his influential book, The Fractal Geometry of Nature. Some of the earliest applications of fractals were in computer graphics. Mandelbrot gave a paper on the landscapes of an imaginary planet at the 99th Colloquium of the International Astronomical Union, held in Balaton, Hungary in June of 1987. Also giving a paper at the conference was IEEE member and radio astronomer Prof. Nathan Cohen of Boston University. Cohen was a ham radio operator, and Mandelbrot’s talk
Satellite sandwich with fractal antenna metamaterial over solar cells. (Courtesy © Fractal Antenna Systems, Inc.) FALL 2021 PROCEEDINGS 34
Scaled up example of an enhanced RCS road reflector for autonomous cars/roads. (Courtesy © Fractal Antenna Systems, Inc.)
intrigued him. He wondered how an antenna shaped according to fractal geometry would work. Cohen found that it worked very well. However, his then landlord had a strict policy about not attaching antennas to the building. Cohen was working with 2-meter FM, meaning that a conventional antenna would need to be about one meter, and thus quite visible. Having found that his fractal antennas could be made much smaller, Cohen made a fractal microstrip antenna out
Wideband fractal elements for a multisector MIMO array. (Courtesy © Fractal Antenna Systems, Inc.)
www.radioclubofamerica.org
