When it comes to antennas, smaller is better, according to University of Michigan researchers. In an effort that could impact the size of mobile devices and other equipment, researchers have developed tiny, hemisphere-shaped antennas that maintain the same bandwidth as their larger counterparts.
I spoke recently with electrical engineering and computer science professor Anthony Grbic about this work, which was also developed by Stephen Forrest, graduate student Carl Pfeiffer and former Ph.D. student Xin Xu. Below are excerpts from our interview.
You developed small, hemisphere-shaped antennas. How did you go about this?
My colleague Stephen Forrest and I were at a meeting in Ohio. We drove back together and it was a four-hour trip. We started talking about research, as we like to do. He's in solar cells and things like that. I deal with antennas. You can make some really nice, small antennas if you combine these two things. We were able to perfect this process and make these antennas.
The hemispherical shape [of our antennas] takes advantage of volume, essentially. This allows miniaturization. The spiraling effect also contributes to the miniaturization of the antenna.
How much smaller are your antennas than typical?
The way you quantify it is: For a given bandwidth or data rate, what's the smallest antenna you can build? There's this fundamental limit called the Chu Limit. With these antennas, we're riding right on this Chu Limit. This limit that was established in the 1940s by Chu, we're actually building antennas right at this fundamental limit. The antennas out there, they can be very small but their bandwidth is three or four times smaller. I have a bandwidth that I need -- and we're building the smallest antennas you can make for that given data rate.
In terms of size, we built some at cell phone frequencies. This antenna has a radius of about one centimeter. It basically rides this Chu limit. [The maximum cell phone antenna] is about four centimeters. These are light-weight antennas. It's just a thin plastic with a metallic pattern on top.
Why was it necessary to shrink antennas?
The largest component in a wireless gadget is an antenna. Many times, its footprint limits the size of mobile devices. If we can shrink that, we can shrink a mobile device.
I had a friend who was working in antenna design years ago when camera phones came out. When the camera phones came out, you had less space for the antenna. We're adding more and more features to mobile gadgets, so size is becoming an issue. By keeping the footprint the same size, we can pack more in.
You mentioned that these antennas are smaller and more lightweight. Are there any other benefits to them?
This spiral-type antenna was proposed years ago, but there wasn't a nice way to make these. They were demonstrated by manually spiraling a wire around. The question was: How do we mass produce these things? Using our technology, it's a simple stamping process. You have these hemispheres and you stamp the metallic patterns on to them. It's a very efficient, quick, low-cost way of building antennas. Earlier groups have tried ink jet printing, but the issue there is that the metallic ink has lower conductivity than normal metals. That leads to lower efficiency.
In what devices could these antennas be used?
We've had some interest from mobile device manufacturers for Bluetooth and WiFi communication. There's interest in putting these antennas on autonomous micro-vehicles. These can be terrestrial or flying. You want to miniaturize your unmanned aeronautical vehicle to minimize detection.
What’s next for this work?
This antenna operates in one frequency range. The next step is to make an antenna that operates in multiple frequency ranges, so you can use it in multiple applications.
Now that we have a method of printing these metallic patterns onto contoured substrates, we'd like to explore conformal antennas. These are antennas that conform to a surface. For example, conformal to the surface of a car or a plane. This would make them low profile.
Photo, top: Antenna next to quarter, by Carl Pfeiffer
Photo, bottom: Anthony Grbic