New research on how radio waves interact and exchange energy could lead to the development of more precise indoor location systems.
Like all electromagnetic waves, radio waves propagate at the speed of light, carrying a balance of electric and magnetic energy. The ratio between the magnitudes of the electric and magnetic fields is a physical constant known as the impedance of free space, with a value of about 377 ohms.
When two electromagnetic waves collide, says Q-TRACK Corporation chief technology officer Hans G. Schantz, whose research appears in the July-August issue of the online journal (FERMAT), the impact disturbs the balance of electric and magnetic energy and temporarily changes the local value of the impedance. While the total amount of energy remains the same, some of the electric energy changes into magnetic energy or vice versa. This “surplus” energy temporarily comes to a rest and changes direction, away from the wave it originated from. In effect, a portion of the energy associated with one of the radio waves bounces off the energy in the other radio wave.
Engineers have previously understood that the energy associated with electromagnetic waves reflects from changes in the impedance of the media through which they propagate. Schantz’s work takes this concept a step further by suggesting that the energy associated with electromagnetic waves reflects from the changes in impedance caused by the superposition or interference of the waves themselves.
In the case of two mirror-image waves with identical waveforms colliding, all of the energy associated with the waves stops and changes direction. Here, “if the interaction is a purely destructive interference, the electric field goes to zero, the impedance goes to zero and the energy associated with each wave bounces off the virtual short created by the superposition. If the interaction is a purely constructive interference, the magnetic field goes to zero, the impedance becomes infinite, and the energy associated with each wave bounces off the virtual open created by the superposition,” a Q-Track press release said.
Rather than offering any fundamentally new physics, Schantz sees his ideas as providing an alternate way of looking at how electromagnetics, including radio waves, behave.
“Physicists and RF engineers refer to ’near’ fields because their stationary or ’reactive’ energy will typically be found near to a particular source—typically within about one wavelength. On the contrary,” he argues, “my work illustrates how ’near’ fields are actually all around us. Radio waves interact and combine with sunlight, infrared, and other electromagnetic waves all the time, generating ’near’ fields even arbitrarily far away from the transmitters which create them and the receivers which detect them.”
The research has a few potential applications, including providing a better understanding for why the concept of field diversity offers a mitigation method for multipath interference. Multipath interference is a phenomenon in which a radio wave or other electromagnetic wave travels from a single source to a detector via two or more paths. Under the right conditions, these separate components of the original wave can combine destructively and cancel out the signal. Schantz’s research, however, suggests that while the electric field may suffer destructive interference from multipath at a point, the magnetic field may still have a useable signal. By implementing an antenna diversity scheme employing both electric and magnetic antennas, one can create a compact receiving array that will be more robust in a multipath environment. Schantz and his colleagues at Q-TRACK Corporation also believe the research could lead to better indoor location systems.
Schantz collaborated with researchers from the University of Massachusetts at Amherst in an NSF-funded Small Business Technology Transfer (STTR) project to investigate a similar scheme, devised by UMass Prof. Do-Hoon Kwon, for short-range, low-frequency, near-field wireless links.
A paper on the research, “On the Superposition and Elastic Recoil of Electromagnetic Waves,” is published in the journal Forum for Electromagnetic Research Methods & Application Technologies (FERMAT).
