Do Radio Waves Bounce Off Each Other?
The answer is “yes,” according to Q-Track Corporation CTO, Hans G. Schantz, who presents his results in a paper appearing in the July-August issue of the online journal FERMAT (e-fermat.org). Schantz’s surprising result follows from an examination of how radio waves interact and exchange energy. His discovery could enable more precise indoor location systems.
Superposition and Electromagnetic Energy
Radio waves, like all electromagnetic waves, propagate at the speed of light, carrying an equal balance of electric and magnetic energy. The ratio between the electric and magnetic intensity of waves propagating in free space is a fundamental physical constant called the free space impedance. The value of the free space impedance is about 377 ohms. The collision of two electromagnetic waves upsets the equal balance of electric and magnetic energy and momentarily changes the local value of the impedance. Although the total amount of energy remains the same, some of the electric energy changes into magnetic energy or vice versa. The unbalanced surplus energy momentarily comes to a rest and changes direction. In effect, a portion of the energy associated with one of the radio waves bounces off the energy in the other radio wave. RF engineers have long understood that the energy associated with electromagnetic waves reflects from changes in the impedance of the media through which they propagate. Schantz’s work suggests 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 mirror-image waves with identical waveforms, all of the energy associated with the two waves comes to a rest and then changes direction. 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.
Schantz sees his ideas not as offering any fundamentally new physics, but rather as providing a different way of looking at how electromagnetics in general and radio waves in particular 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.”
This perspective has a couple of potentially useful applications. Radio links often suffer from multiple radio waves taking different paths. When they combine destructively, they can cancel out the signal, making reception difficult. One may mitigate this “multipath” interference in a variety of ways. Schantz’s work provides a clear picture for why “field diversity” offers a mitigation to multipath interference. Although one field, say the electric field, may suffer destructive interference from multipath at a point, the other field may yet have 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 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.
Schantz and his colleagues at Huntsville, AL, based Q-Track Corporation also believe these insights may lead to better indoor location systems. Schantz is CTO and a co-founder of Q‑Track Corporation, a company that commercializes near-field wireless systems for indoor-location applications. Q-Track’s Near-Field Electromagnetic Ranging or NFER® Real-Time Location Systems apply near-field physics to solve the difficult problem of precise indoor location. The company uses low-power RF signals operating at around 1MHz with wavelengths of about 300m to localize tag transmitters to about 40cm rms accuracy. Because the wavelengths are so long, the signals bend around or penetrate through most obstructions, making it an excellent choice for difficult industrial location applications.
About Q-Track Corporation
Q-Track (http://q-track.com) provides Real-Time Location Systems – similar to GPS, but more accurate and capable of working indoors. Q-Track’s NFER® products are deployed at about one third of U.S. nuclear plants, tracking workers in training exercises aimed at reducing their radiation exposure. NFER® proximity detection systems keep workers safe from overhead cranes and moving equipment in a variety of manufacturing facilities. Recently, Lockheed-Martin selected Q-Track for a $1.7 million effort to locate soldiers training for urban operations. This release describes work performed by Q-Track and UMass Amherst under NSF STTR Grant 1217524 “Robust Emergency Data (RED) Link.”
About FERMAT (Forum for Electromagnetic Research Methods & Application Technologies)
FERMAT (http://e-fermat.org) is a journal dedicated to publishing high-quality papers written by leaders in electromagnetics, promoting freedom of scientific expression, providing a forum where burning issues are aired and diverse viewpoints are debated, and facilitating social networking with colleagues like never before in any other electromagnetics-related publication.
- Hans G. Schantz, “On the Superposition and Elastic Recoil of Electromagnetic Waves,” FERMAT, Vol. 4, No. 2, July-August 2014 [ART-2014-Vol4-Jul_Aug-002]. See also http://arxiv.org/abs/1407.1800.
- Hans G. Schantz, “Demystifying Electromagnetic Superposition,” Invited Talk, 2014 Texas Symposium on Wireless & Microwave Circuits & Systems, Waco, Texas, April 3, 2014 (http://prezi.com/5-tidsglo58x/demystifying-electromagnetic-superposition/).
- Hans G. Schantz, “Theory and Practice of Near-Field Electromagnetic Ranging,” Proceedings of the 2012 International Technical Meeting of The Institute of Navigation, January 30 - 1, 2012, Newport Beach, CA, pp. 978-1013 (http://www.ion.org/publications/abstract.cfm?articleID=10005).
- M.A. Nikravan, H.G. Schantz, A.H. Unden, D.H. Kwon, “Channel Multiplexing Technique Utilizing Electric and Magnetic Components of a Radio Wave,” IEEE Communications Letters, Vol. 18, No. 2, February 2014, pp. 317-320.
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