Researchers Propose Energy Harvesting - Radio World

Researchers Propose Energy Harvesting

Transmitted Power From Decades of Broadcasting Are Building to Highest Levels, Reaching Steady State
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APRIL 1, 2009 According to sources within federal research laboratories, considerable attention is now being given to the residual levels of radio frequency (RF) energy present in the environment.

The basic problem being studied is that all of the RF signals from decades of high-power transmission leave residual levels of energy that remain in the environment and continue to propagate nearly endlessly. The result is that the cumulative levels have been increasing.

As stated by one source, "While the instantaneous transient levels of energy being radiated are not a major concern, what has been discovered is that the steady state levels of energy within various frequency bands, especially in VHF and UHF, have been slowly increasing from year to year."

It is a complex problem not easily solved by a closed form equation due to numerous nonlinear effects and multipath, but the phenomenon can be illustrated using the classic Friis transmission equation, given by:

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where the power received at a given point is related to the power transmitted, the gains of the transmit and receiver antennas, and the distance R between the transmit and receive points.

Clearly the received power decreases by the square of distance of separation, but the power never goes to zero. As the power available to be received continues to propagate, an accumulation of power over time results in the phenomenon of steady state effective power increase as the energy continues propagating.

A periodic function can be written to show an infinite series that predicts power levels at a given location, where each successive term in the summation represents a power term due to a transmission that occurred further back in time. The expanded Friss equation taking into account temporal effects is given by:

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where Rn is the distance term, k is the collection of fixed terms from the Friis equation, and Rn is expanded by the number of repetitive cycles of transmission over the additional distance D to an infinite number of terms. Note that the power level decreases in each term but is additive to all prior terms.

The Friis equation obviously doesn't take atmospheric loss into account. The atmospheric loss varies greatly across frequency but is less at lower frequencies. Therefore the typical broadcast frequencies at VHF and UHF are the greatest contributors to this energy buildup.

The effect is non-linear in that it cannot be predicted by standard equations for transmission, reflection, absorption and energy transfer. The best way that it can be described is that a propagation resonance has built up that now is very easily influenced and maintained by even small transmissions at the same frequency.

Consider how a group of soldiers marching across a bridge are able to make the bridge resonate to the point of destruction by maintaining a steady cadence in their march (the "Angers Bridge" effect). Once the resonance condition begins, it is easily maintained and magnified.

Worsening matters is another phenomenon referred to as sympathetic resonance where transmissions at one frequency can contribute to the amplification of oscillations at another nearby frequency. The net impact of greatest interest to broadcasters is that the VHF and UHF bands tend to invite the greatest level of impact from sympathetic resonance — for instance unlicensed devices in the UHF band, though typically at a lower power level than TV transmissions, will tend to have a collective impact on the higher power bands used by TV stations.

Why not harvest it

One researcher in the field, Dr. Mead Citron, has presented a number of possible mitigation plans which are now being pursued.

One of these mitigation plans coordinates with other efforts to promote "green" business practices by requiring producers of transmitted energy to recycle unused transmitted signals through energy harvesting.

Methods of energy harvesting are well documented for many military applications. For transmitted broadcast communication signals, it is clear that there are many producers of signals at a given frequency, i.e. at a specific channel. If a law were to be put in place requiring recycling of unused signal, it is likely that an energy harvesting device with a modulation scrubber could be used to return a signal to a clean tone at carrier frequency that can then be reused and remodulated by any station.

In addition, since the recycled unmodulated signal already exists at the carrier frequency, less energy will be needed from the oscillator. While requiring individual stations to procure and maintain their own energy harvesting equipment may be cost prohibitive, support from the government stimulus package is expected.

Reharvested energy credits may also be used as a means of allowing an energy harvesting cooperative to capture unused energy from a large number of broadcasters while paying them credits for the portion of energy attributable to individual stations. This may ultimately provide a new source of revenue to broadcasters to help defray costs, especially in the current economic times.

Dr. Citron also stated that his lab is experimenting with capturing stray ambient light for energy harvesting applications. The impetus for light energy harvesting is due in part to the amount of light pollution that occurs, especially near metropolitan areas.

Experiments have been conducted with various surface collector designs for use in a large Carnot engine using the earth as a heat sink. Experimental black surface collectors have been investigated, but the radiation efficiency of these limits the performance. A proprietary green design, based in part on the photosynthesis process found in nature, is currently being used.

The process seems to be providing promising initial results, and simulations predict the process will help the environment both by reducing background light and by reducing global warming caused by excessive carbon dioxide buildup from artificially induced photosynthesis.

Another proposal being considered, according to our sources, is that a one-week moratorium on all RF transmissions is being considered in order to let the current steady state RF levels die down to negligible levels.

Scientists suggest that there exists a threshold above which the increase in signal level due to dynamic loading resonant transfer occurs. The moratorium would reduce levels below the threshold so that the transfer of momentum from newly transmitted signals will be diminished.

Though not made public, the National Oceanic and Atmospheric Administration (NOAA) has developed a record of historic RF steady state levels at a number of locations and has correlated the data with other meteorological data. They have shown that the RF levels are reduced following certain weather events such as rain and snow, which indicates that precipitation serves to absorb some of the energy and reduces the absolute level of atmospheric energy. Global warming and drought conditions may then also accelerate the problem of RF energy build-up.

While not confirmed, one computer model shows that if the rate of increase in steady state conditions continues at the current pace, a tipping point will be reached in the year 2012, at which point an extreme acceleration in resonant activity will begin. The impact of this acceleration was not disclosed.

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