These antennas turn anything into a radio station | Science News for Students

These antennas turn anything into a radio station

A low-power approach to hijacks radio signals already moving through the air
May 9, 2017 — 12:00 pm EST
Smart poster

A new device turns posters into radio stations. These smart posters could broadcast sound clips, such as from bands with upcoming concerts.

University of Washington

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.

Look — and listen. That concert poster just might be singing. Engineers have designed antennas that can turn everyday objects, from posters to clothing, into radio stations. Anyone walking or driving by can tune in and hear what’s on. The devices use radio waves, but they don’t generate their own. They hijack the same waves that carry music and news to your smartphone.

Vikram Iyer is a graduate student in electrical engineering. He studies at the University of Washington in Seattle. He co-led the project with Anran Wang, a graduate student in computer science and engineering. Antennas don't send out new waves. So they need little power. A button-sized battery could power an antenna for years. “It's the ideal way to minimize the power consumption for any kind of communication,” notes Iyer. 

The two got the idea for their invention by paying attention to what was already around them. Their research had focused on new types of wireless communications that won't require much energy. They wanted something that would work outdoors in a city. Then they realized the air is already filled with wireless communications in the form of radio stations.

“In most cities, you have all these FM radio stations just pumping out power,” says Iyer. The duo wondered if they could find a way to harness that power.

Radio waves carry energy at the speed of light from tall transmission towers to radios in cars, phones and homes. These waves fill the air — especially in dense cities. The new devices take advantage of that. They take in existing radio waves and change them slightly. Those changes add new sound information. The altered waves are then sent back out into the world where people can listen in. So the device only needs enough power to change the waves, not generate them.

The scientists tested their device with a poster. It advertised a Seattle concert by Simply Three, a string trio. People standing almost 4 meters (12 feet) away from the poster could use FM receivers on smartphones to listen to snippets of the band's music. Those in cars as far as 18 meters (59 feet) away could use car radios to pick up the songs.

FM stands for “frequency modulation.” Frequency is a measure of how many waves pass in one second. The higher the frequency, the closer the waves must pack together. In FM radio, sounds are encoded in the waves by making small changes in the wave frequencies. This is known as “modulating” the frequency.

When a radio wave hits something, it bounces off in a different direction. This is called scattering. Backscattering is what happens when the wave heads back in the opposite direction. The new antenna uses backscattering to send information. It doesn't simply reflect a wave, though. It encodes a different signal on top of the existing wave by changing its frequency.

The enhanced waves don't interfere with other radio stations. The new antenna changes the frequency of the wave so that the new message can be heard at a frequency not being using by an existing radio station. The original sound in the wave remains unchanged.

Electrical engineer Daniel Stancil says the idea of using existing FM signals “is an exciting one.” Stancil is an expert on antennas and communication at North Carolina State University in Raleigh. He did not work on the new study.

Iyer and his colleagues see many uses for their technology. Ads could broadcast sounds. Street signs could send out the name of an intersection or directions. They could also alert people when it's safe to cross the street.

The technology could even extend to clothes. Iyer, Wang and their team stitched conductive thread into a cotton t-shirt. That thread conducts electricity. It turned the shirt into an antenna. It let the shirt talk to the wearer's smartphone. If a sensor in the shirt tracked a person's heart rate during exercise, for instance, the antenna could transmit those data to the wearer’s phone.

The work by Iyer and Wang shows that the devices work. Now they're trying to figure out the best way to use it.

Stancil sees only one possible downside — that people are using FM radios less than they did in the past. “People are normally walking down the street listening to iPods instead,” he says.

Power Words

(for more about Power Words, click here)

antenna     (in physics) Devices for picking up (receiving) electromagnetic energy.

battery     A device that can convert chemical energy into electrical energy.

broadcast     To cast — or send out — something over a relatively large distance. A farmer may broadcast seeds by flinging them by hand over a large area. A loudspeaker may send sounds out over a great distance. An electronic transmitter may emit electromagnetic signals over the air to a distant radio, television or other receiving device. And a newscaster can broadcast details of events to listeners across a large area, even the world.

colleague     Someone who works with another; a co-worker or team member.

conductive     Able to carry an electric current.

electrical engineer     An engineer who designs, builds or analyzes electrical equipment.

electricity     A flow of charge, usually from the movement of negatively charged particles, called electrons.

encode     (adj. encoded) To use some code to mask a message.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

frequency     The number of times a specified periodic phenomenon occurs within a specified time interval. (In physics) The number of wavelengths that occurs over a particular interval of time.

graduate student     Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).

heart rate     Heart beat; the number of times per minute that the heart — a pump — contracts, moving blood throughout the body.

modulation     A variation in some regular aspect of a signal. (In electronics and telecommunications) The process of varying one or more properties of a periodic wave, called the carrier signal. The changes, or modulations, that are introduced typically contain coded information, such as sounds (perhaps as voice or music) or numerical data.

radio     To send and receive radio waves; or the device that receives these transmissions.

radio waves     Waves in a part of the electromagnetic spectrum; they are a type that people now use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals; they also are used in radar.

sensor     A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.

smartphone     A cell (or mobile) phone that can perform a host of functions, including search for information on the internet.

speed of light     A constant often used in physics, corresponding to 1.08 billion kilometers (671 million miles) per hour.

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

transmit     (n. transmission) To send or pass along.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

Citation

Meeting:​​ ​A. Wang et al. FM backscatter: Enabling connected cities and smart fabrics. 14th USENIX Symposium on Networked Systems Design and Implementation, March 27-29, 2017, Boston, Mass. arXiv:1702.07044.