The interaction of radio waves in free space has never been much of a concern to this wireless engineer. Propagation models do a pretty good job of estimating received signal strength. Receivers are designed to select desired signals and reject undesired ones, within limits. “Interference takes place in the receiver,” I’ve heard more than once. Yes, there are a lot of signals in the air at once, but I’m more interested in the end result.
So, it’s with some unease I read that radio waves are interacting with each other — bouncing off one another — all the time, resulting in reversal of power flows in free-space, and in near-fields being found in far-fields. Those are among the conclusions of Dr. Hans G. Schantz, CTO of Q-Track Corporation, a maker of indoor location systems. A press release was issued last week and the underlying paper has been published by the journal FERMAT.
Schantz starts his analysis with a simple example of one-dimensional electromagnetic waves on an ideal free-space transmission line. Using principles of superposition and energy conservation, Schantz analyzes equal-strength signals moving in opposite directions, combining both constructively and destructively. He shows that at one point in this process, power is zero. No power is available to propagate in the same direction, so the power must be “recoiling,” or reversing direction. He refers to this as an “electromagnetic Newton’s cradle.”
What about in the real world, away from an ideal model and with widely varying signals and frequencies? The same concept is said to apply, but in the case of unequal signals the reflection involves a portion of the propagating energy. The smaller signal gives energy to the leading edge of the larger, and the larger provides energy for the smaller to continue.
Schantz finds this rebounding behavior results in near-field effects that are typically assumed by engineers to occur only near the source of a signal:
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, 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.
He says there is no new physics involved in his findings. He examines alternate explanations, preferring his own. This subject has been looked at before, but the last comprehensive discussion of the topic Schantz finds dates to 1909.
Schantz sees possible applications of his findings to indoor location equipment, such as that made by his company, and to more-robust multipath mitigation techniques that exploit both electric and magnetic fields.