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RF (Radio Frequency)

RF is loosely defined as cyclically changing (alternating) electromagnetic energy which provides for radiant energy that detaches itself from the source and travels freely through space. The reason this can occur is because the originating source builds a field in free space, and as the originating source intensity collapses and before the free-space field can fully collapse, the reversing originating field pushes the former field out into free space. If the originating source does these alternations continuously, then the field not only travels freely in free space, but is also part of a continuous wavetrain. For a free-space field to form or collapse there is a finite time required, related to the size of the field and the speed of light. If the field is created by a slow alternation of the originating signal, then the field may fully collapse with the generator, unless the field is fed by large amounts of power, creating a dimensionally large field. If the field is created by a fast alternation of the originating signal, then the field will generally not fully collapse before the generator changes field direction. So in general for slower frequencies, there must be a large amount of power for free-space radiation to occur, whereas for faster frequencies, the power requirements become minuscule.

Single Frequency RF is customarily produced by resonant circuits that employ crystalline characteristics of such materials as quartz, or cap-and-coil resonant "tank" circuits. Another type of frequency generation is by electronic sysnthesis. Subsequent circuits are used to condition the resultant electrical alternations for the cartage of information.

After the eventual signal conditioning is accomplished, it is brought to the Antenna, and propagated through space.

The field within about one to two wavelengths of the antenna is known as the induction or near field, because the collapsing and expanding fields interact with each other significantly, returning some energy to the antenna and forcing some out into free space. In this region there are a significant standing waves (or simplistically "echoes") that result from the impedance mismatch between the electrical "resistance" of the antenna (on the order of 50-75 Ohms) and that of free space (377 Ohms). Spatial variance dictates that to properly characterize the fields in this region both the E (electric) and H (magnetic) fields must be quantified. Since for power systems the wavelength is in the order of 3,000 miles or so, you are always within its near field, and must treat the electric and magnetic components as separate entities.

While there is an iterim region between near and far fields, it is of little importance for this discussion.

Beyond about two to three wavelengths from the antenna, the energy that is forced away from the antenna begins to radiate out into free space at the speed of light, this region is known as the radiation or far field. In this region the E and H fields are so intertwined and synchronized that to properly characterize them, it is sufficient to detect only the E field value, and most RF meters do just that. However, using a calibrated antenna, the instrument can determine with accuracy the amount of energy per unit area. A common error in measurement by novices is to hold a meter up to a cell phone to try and determine its output. No regulatory body would think of doing that, as such measurement in the near-field will have large variations of detected field with as little as a few millimeters in any of three directions toward or away from the source. While far-field measurement is straightforward, near-field measurement is best left to SAR calculations by laboratories.

The wavelength is easily determined by the formula: Wavelength = Speed of Light / Frequency, and briefly, for 1 GHZ (close to most 800-900 MHz) the wavelength is about one foot (12"), for 2 GHz it is about 6". So in most cases involving a wireless tower, measurement is made in the far field, in contrast to measurement of wireless emissions from computers and cell phones, which can be in the near field or far field, depending on proximity to the source. Realize that the RF Exposure Limit recommendations (MPEs or Maximum Permissible Exposures) only apply to far field values, so when in the near field, all bets are off, and you take your chances of results being whatever they may, whether you are aware of them or not. But generally speaking, RF exposure (long term or high level) is loosely associated with tumor genesis.

Several related frequencies (sums and differences to either side of the carrier frequency) will be produced by the above circuits using modulation.

Early RF transmitters were of poor quality, because they employed electrical discharge mechanisms to produce RF emissions. They rode the border between radio frequency interference and intelligible data transfer. Nonetheless, their efforts progressed rapidly as they strove to improve their reach and selectivity. Today such contraptions would not be permitted due to their intensive use of wide portions of the EMF spectrum.

A variety of instruments are needed to detect signals over various: 1) frequency ranges, 2) signal level (strength), and 3) modulation characteristics. Their complexity and frequency range of application generally dictates the instrument's price range. These "trivial" factors aside, RF measurement is not always straightforward. Some transmitters, such as for radio and former analog TV, are/were of constant intensity, and one could predictably plunk the intruments in the same spot time and again and expect to acquire the same energy level measurement. Wireless communications, in contrast, are by their very nature not constant, in either time (due to the instantaneous number of connections), or location (due to the irregularities of the antenna emissions).


Shown to the right is a typical wireless antenna tower. Antennas can emit as point sources, where the energy deposited per unit area (away from the antenna) diminishes with the inverse-square law (perhaps like a cell phone or wireless router), as line sources where the energy deposited per unit area diminishes with the inverse of the distance (quite uncommon), or as focused sources (such as that shown) where neither law holds. Focusing energy is a means to extend the physical distance for which the antenna is useful. This is sometimes described as the gain of the antenna, which is simply a measure of how effectively an antenna can direct the input energy. So if 500 Watts is the input energy, the most one can hope for, with ideal focusing, is to have 500 Watts in the main and only lobe of emission. However, as shown, ideal focusing is never achieved, and numerous side lobes exist. Measurement then will vary with where one is relative to the side lobes, how high above the ground one is when measuring, what time of day one makes the measurement, and whether some social or weather phenomenon is occurring, justifying a greatly enhanced presence nearest system transmission capacity, not to mention the frequency response characteristics of the instrument used for measurement.

There are generally three modes of biological interaction with RF.

1) Dipole molecules such as water, having an asymmetric charge distribution, will attempt to align with an impressed electric field. Since the electric field will vary with time, the molecule will continuously try to realign with the field. However, since the molecules do not exist in a void, they meet with resistance from neighboring molecules. This resistance, when above a certain strength, will manifest itself as frictional heating.

2) A second method involves the electric field interference with molecular vibrational and rotational states normally induced by the earth's background magnetic field, and the molecular environment surrounding the affected molecules.

3) A third method involves kinetic energy transfer to free rlectrons and important biologic ions. This is a "windowing" opportunity, where certain discrete energy levels, frequencies, or combination of frequencies, will cause interaction even where the intensity level of the absorbed energy is not sufficient for classical interaction, or even measurement is some cases.

The means to acquire a general sense of the biological interaction is to categorize the energy as to its absorption, as this is the primary means of energy being available for producing an effect within internal biological structures.

One means of providing a standardized measure is with the Specific Absorption Ratio (SAR), depicted in Watts/Kilogram (W/Kg). The SAR has been an accepted standardized measurement protocol only system since about 1980.

From E = I x R or Voltage = Current x Resistance, and
W = E x I or Power (in Watts) = Voltage x Current, and
Conductivity = 1/Resistance = I/E (in Siemens)

SAR = [Conductivity x (Electric Field Strength)2 ] / [Tissue Density]
        = [Siemens/meter x (E/m)2] / [Kg/(m3)]
        = [Siemens x E] / Kg
        = W / Kg

SA or Specific Absorption is the time integral of the SAR for any specific mass. That is, the total absorption over a defined period of time.

The SAR is frequency dependent, tissue composition dependent, and positionally dependent (with the subject's position relative to the impinging energy).

The FCC's limits, and the NCRP and ANSI/IEEE limits on which they are based, are derived from exposure criteria quantified in terms of specific absorption rate (SAR). The basis for these limits is a whole-body averaged SAR threshold level of 4 watts per kilogram (4 W/kg), as averaged over the entire mass of the body, above which "expert" organizations have determined that potentially hazardous exposures may occur. The present MPE limits are derived by incorporating safety factors that lead, in some cases, to limits that are more conservative than the limits originally adopted by the FCC in 1985. Where more conservative limits exist, they do not arise from a fundamental change in the RF safety criteria for whole-body averaged SAR, but from a precautionary desire to protect subgroups of the general population who, potentially, may be more at risk.


FCC articulated exposure limits

A human will also absorb more energy when the frequency approaches the value where the human is about equal to 1/2 or 1/4 wavelength, for an insulated (with shoes) and grounded (barefoot) condition respectively. So the new(er) FCC exposure limits are also based on data showing that the human body absorbs RF energy at some frequencies more efficiently than at others. The most restrictive limits occur in the frequency range of 30-300 MHz where whole-body absorption of RF energy by human beings is most efficient. At other frequencies whole-body absorption is less efficient, and, consequently, the MPE limits are less restrictive.

The fly in the ointment is that these limits and calculations are based on short-term effects, and "marginal reductions in exposure limits are balanced by the impact they have on the economy," not the loss of individual lives, or worsening of the general health or the quality-of-life of those exposed.

In many cases then, long-term exposure to levels well below the generally accepted actionable limits not only can, but will cause biological interaction that is harmful. When someone identifies some tangible perception that can be validated, the individual is left to their own, and more often than not disappears from the landscape because even the best efforts of those who can do something does not provide adequate resolve to the problem. Even when the exposure can, and/or has been reduced by some mechanism, any biological damage may not be fully undone.

Reduction attempts are complicated by the behavior of RF and the different structures in its path. Some materials are transparent, some partially absorb, some fully absorb, and some reflect. Considering that RF behaves like light, one needs to decide whether they want to live in a complete enclosure like a cocoon, or something that produces localized reduction.

Some Indoor Solutions, in order of increasing cost:
1) use a wired phone instead of a wireless one,
2) use a wired-LAN computer connection instead of a wireless one,
3) use shielding to provide a "shadow" with reduced exposure, from single sources,
4) use a RF-reducing canopy over the bed,
5) install RF-abosrbing shielding to produce a Faraday cage,

Some Outdoor Solutions:
1) use partially conductive structures for the outer envelope of the structure, 2) plant a tree buffer, such as Arbor Vitae, in line of sight of major sources.