radio waves

Device transmits radio waves with almost no power – without violating the laws of physics

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A new ultra-low-power method of communication at first glance seems to violate the laws of physics. It is possible to wirelessly transmit information simply by opening and closing a switch that connects a resistor to an antenna. No need to send power to the antenna.

Our system, combined with techniques for harvesting energy from the environment, could lead to all manner of devices that transmit data, including tiny sensors and implanted medical devices, without needing batteries or other power sources. These include sensors for smart agriculture, electronics implanted in the body that never need battery changes, better contactless credit cards and maybe even new ways for satellites to communicate.

Apart from the energy needed to flip the switch, no other energy is needed to transmit the information. In our case, the switch is a transistor, an electrically controlled switch with no moving parts that consumes a minuscule amount of power.

In the simplest form of ordinary radio, a switch connects and disconnects a strong electrical signal source – perhaps an oscillator that produces a sine wave fluctuating 2 billion times per second – to the transmit antenna. When the signal source is connected, the antenna produces a radio wave, denoting a 1. When the switch is disconnected, there is no radio wave, indicating a 0.

What we showed is that a powered signal source is not needed. Instead, random thermal noise, present in all electrically conductive materials because of the heat-driven motion of electrons, can take the place of the signal driving the antenna.

No free lunch

We are electrical engineers who research wireless systems. During the peer review of our paper about this research, published recently in Proceedings of the National Academy of Sciences, reviewers asked us to explain why the method did not violate the second law of thermodynamics, the main law of physics that explains why perpetual motion machines are not possible.

Perpetual motion machines are theoretical machines that can work indefinitely without requiring energy from any external source. The reviewers worried that if it were possible to send and receive information with no powered components, and with both the transmitter and receiver at the same temperature, that would mean that you could create a perpetual motion machine. Because this is impossible, it would imply that there was something wrong with our work or our understanding of it.

A graphic in the top half showing a horizontal cylinder on the left with a pipe extending to the right with a 90-degree bend upward connecting to an upside-down triangle with pairs of curved lines on either side, and in the bottom half the same but disconnected
Electrons that naturally move around inside a room-temperature resistor affect electrons in a connected antenna, which causes the antenna to generate radio waves. Connecting and disconnecting the antenna produces the ones and zeros of a binary signal. Zerina Kapetanovic, CC BY-ND

One way the second law can be stated is that heat will flow spontaneously only from hotter objects to colder objects. The wireless signals from our transmitter transport heat. If there were a spontaneous flow of signal from the transmitter to the receiver in the absence of a temperature difference between the two, you could harvest that flow to get free energy, in violation of the second law.

The resolution of this seeming paradox is that the receiver in our system is powered and acts like a refrigerator. The signal-carrying electrons on the receive side are effectively kept cold by the powered amplifier, similar to how a refrigerator keeps its interior cold by continuously pumping heat out. The transmitter consumes almost no power, but the receiver consumes substantial power, up to 2 watts. This is similar to receivers in other ultra-low-power communications systems. Nearly all of the power consumption happens at a base station that does not have constraints on energy use.

A simpler approach

Many researchers worldwide have been exploring related passive communication methods, known as backscatter. A backscatter data transmitter looks very similar to our data transmitter device. The difference is that in a backscatter communication system, in addition to the data transmitter and the data receiver, there is a third component that generates a radio wave. The switching performed by the data transmitter has the effect of reflecting that radio wave, which is then picked up at the receiver. An example of backscatter unpowered wireless communications.

A backscatter device has the same energy efficiency as our system, but the backscatter setup is much more complex, since a signal-generating component is needed. However, our system has lower data rate and range than either backscatter radios or conventional radios.

What’s next

One area for future work is to improve our system’s data rate and range, and to test it in applications such as implanted devices. For implanted devices, an advantage of our new method is that there is no need to expose the patient to a strong external radio signal, which can cause tissue heating. Even more exciting, we believe that related ideas could enable other new forms of communication in which other natural signal sources, such as thermal noise from biological tissue or other electronic components, can be modulated.

Finally, this work may lead to new connections between the study of heat (thermodynamics) and the study of communication (information theory). These fields are often viewed as analogous, but this work suggests some more literal connections between them.

Snapshot of cosmic burst of radio waves

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A strange phenomenon has been observed by astronomers right as it was happening – a ‘fast radio burst’. The eruption is described as an extremely short, sharp flash of radio waves from an unknown source in the universe. The results have been published in the Monthly Notices of the Royal Astronomical Society.

Snapshot of cosmic burst of radio waves

Over the past few years, astronomers have observed a new phenomenon, a brief burst of , lasting only a few milliseconds. It was first seen by chance in 2007, when astronomers went through archival data from the Parkes Radio Telescope in Eastern Australia. Since then we have seen six more such bursts in the Parkes telescope’s data and a seventh burst was found in the data from the Arecibo telescope in Puerto Rico. They were almost all discovered long after they had occurred, but then astronomers began to look specifically for them right as they happen.

Radio-, X-ray- and visible light

A team of astronomers in Australia developed a technique to search for these ‘Fast Radio Bursts’, so they could look for the bursts in real time. The technique worked and now a group of , led by Emily Petroff (Swinburne University of Technology), have succeeded in observing the first ‘live’ burst with the Parkes telescope. The characteristics of the event indicated that the source of the burst was up to 5.5 billion years from Earth.

Snapshot of cosmic burst of radio waves
The intensity profile of the fast radio burst, showing how quickly it evolved in time, last only a few milliseconds. Before and after the burst, only noise from the sky was detected. Credit: Malesani/Petroff

Now that they had the burst location and as soon as it was observed, a number of other telescopes around the world were alerted – on both ground and in space, in order to make follow-up observations on other wavelengths.

“Using the Swift space telescope we can observe light in the X-ray region and we saw two X-ray sources at that position,” explains Daniele Malesani, astrophysicist at the Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen.

Then the two X-ray sources were observed using the Nordic Optical Telescope on La Palma. “We observed in and we could see that there were two quasars, that is to say, active black holes. They had nothing to do with the radio wave bursts, but just happen to be located in the same direction,” explains astrophysicist Giorgos Leloudas, Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen and Weizmann Institute, Israel.

 

Further investigation

So now what? Even though they captured the radio wave burst while it was happening and could immediately make follow-up observations at other wavelengths ranging from infrared light, visible light, ultraviolet light and X-ray waves, they found nothing. But did they discover anything?

“We found out what it wasn’t. The burst could have hurled out as much energy in a few milliseconds as the Sun does in an entire day. But the fact that we did not see light in other wavelengths eliminates a number of astronomical phenomena that are associated with violent events such as gamma-ray bursts from exploding stars and supernovae, which were otherwise candidates for the burst,” explains Daniele Malesani.

But the burst left another clue. The Parkes detection system captured the polarisation of the light. Polarisation is the direction in which electromagnetic waves oscillate and they can be linearly or circularly polarised. The signal from the radio wave burst was more than 20 percent circularly polarised and it suggests that there is a magnetic field in the vicinity.

“The theories are now that the radio wave burst might be linked to a very compact type of object – such as neutron stars or black holes and the bursts could be connected to collisions or ‘star quakes’. Now we know more about what we should be looking for,” says Daniele Malesani.