Research on time-varying, electrically-small antennas featured on IEEE TAP journal homepage

A team of researchers in Electrical and Computer Engineering has published a new paper, “Space-Time Modulation of a Multimode Electrically Small Antenna for Increased Matching and Efficiency Bandwidths,” that was selected to be featured on the homepage of the IEEE Transactions on Antennas and Propagation website.

The paper describes a method to increase the bandwidth and efficiency of small antennas using time-modulation. The paper was co-authored by PhD students Zachary Fritts and Amirhossein Babaee, Professor Anthony Grbic, and Assistant Research Scientist Steve Young.

We talked to first author Zachary Fritts about the research, how he became interested in the topic, and more.

Tell us about the research featured by IEEE Transactions on Antennas and Propagation.

The miniaturization of wireless devices is ultimately limited by their largest wireless component, the antenna. It is especially challenging to design antennas that are much smaller than the waves that they radiate, known as electrically-small antennas. One of the main difficulties in designing these electrically-small antennas is that they can either be made efficient or broadband, but not both. Practically, this limits the rate of data transmission through the antenna. In our paper, we proposed and theoretically showed how to leverage the time variation of an antenna’s electrical properties to improve the bandwidth to unprecedented levels without corresponding losses to efficiency.

Why is this research important?

The new method of analyzing time-modulated antennas that we described allowed us to break the glass ceiling on the bandwidth-efficiency product of electrically-small antennas. Electrically-small antennas are important whenever the wavelength of operation is large or when the antenna simply must be physically small, so this development of highly efficient, miniaturized antennas will enable ultra-small wireless environmental sensors and communication devices for next generation IOT systems. The potential applications are quite broad, including remote sensing of the earth or atmosphere, long-range communications, small sensors of interest in biology, and more.

Can you describe the research and your results in a bit more detail?

Most antennas are time-invariant systems, meaning that their shape and their electrical properties do not change in time. We designed the electrical properties of our antenna to be time varying in a very special way, using tunable elements. We rapidly vary the capacitance of these elements, known as varactors, by applying a voltage to them.

If you imagine the antenna to be divided into sections, like the seating in the Big House, and the varactors are like football fans in different sections of the stadium, our antenna design is analogous to causing them to do “the wave,” where fans sequentially raise and lower their arms to make it look like a wave is going around the stadium. Varying the varactor capacitance in this way enables the antenna to operate over a much wider bandwidth than would otherwise be possible. 

Our latest simulations show 10X improvement compared to the state-of-the-art linear time invariant antennas.

What were some challenges you faced on this project and how did you overcome them?

My own personal inclination at the beginning of the project was to try to mathematically model the system. But, the model quickly became so complicated that it was hard to interpret. So, I turned to simulating the system to get a “feel” for how it behaved. After having performed extensive simulations, and gaining a lot of intuition for how the antenna worked, I was able to understand the way that the math should work out and model the antenna mathematically. 

This is a really new kind of antenna. The existing IEEE standards that define the efficiency of an antenna assume that nothing about the antenna is changing in time, so we had to figure out how to meaningfully talk about the efficiency of our new antenna and its bandwidth. 

Did any of your findings surprise you?

Yes. This project was definitely my first big experience of finding, “Oh, wow! This works unexpectedly,” and some of these discoveries happened the day before a big program review, which was exciting, to say the least.

How did you get interested in this idea and project?

I was studying parametric systems at the very beginning of my PhD. These are electromagnetic systems where a parameter is varied in time. They have a long history dating back to the 1950s. I was working on a project that was essentially trying to model electromagnetic scattering (the process that causes a radar signal to be reflected). In particular, I was studying spherical scatters whose properties vary in time. I had demonstrated that the scattering cross-section of time-varying spheres could be broadened by coupling to non-radiating modes that store energy at frequencies other than the operating frequency. 

Our team read a call for proposals on broadening the bandwidth of electrically-small antennas (those much smaller than a wavelength) and started brainstorming what it would look like to use time variation in such an antenna. It was natural to consider using non-radiating modes, just like in the case of time-varying scatterers. After much trial and error, we showed that this concept could be used to broaden the bandwidth of electrically-small antennas, and then set out to theoretically show how this was possible.

What has it been like collaborating with others on this project?

There are a number of other students working on different aspects of the project, and I’m also being advised by Tony Grbic and Steve Young. They’re great mentors. Tony has a knack for thinking that something might be a good idea—and every time I’ve tried one of his ideas, it’s worked out in the end. He’s not always certain exactly how his idea is going to work out, but it does. Steve has often encouraged me along the way to try something different, and his advice works really well. So it’s been great to have both of them as mentors on the project.

What are your next steps in this line of research?

We are currently working with collaborators at SRI International to test a prototype of the antenna. I am also working on mathematically showing that the antenna is stable. If it weren’t stable it would behave like a microphone that screeches after being placed too close to a loudspeaker, which is just as undesirable for an antenna as it is for human ears! Initial measurements of the prototype seem to indicate that we can avoid instabilities, which is promising.

How does it feel to have your paper featured on the IEEE TAP homepage?

This year, IEEE TAP transitioned  from having both a print and online presence to just an online presence.  So, to have it featured is gratifying. It’s as if the IEEE Antennas and Propagation Society or journal is saying, “Okay, this research is worth putting closer to the eyes of other researchers that are looking at this kind of thing.”