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A free electronic newsletter covering news and other topics for those interested in RF safety issues.
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Contents
What is spatial averaging? All the major worldwide standards concerned with human exposure to radio frequency radiation have exposure limits based on field levels averaged over the whole body. There are often other limits, such as for extremities and for the sensitive organs such as the eyes and testes. But it is not safe to assume that if one makes a measurement in the general area of interest that this measurement represents the average exposure that would be experienced by a person standing at that location. A spatially averaged measurement is simply an average value derived by making a series of measurements, either in a straight line or over a two-dimensional area, that is representative of the human form. When is spatial averaging important? Spatially-averaged measurements take much longer to make than simple point measurements. Therefore, spatially-averaged measurements should be made when they are relevant. If you move the probe of an instrument around and cover as large a volume of area as possible you most likely will find some locally "hot" areas where the field levels are significant. When field levels are found that approach or are over the standard (the standard that you are referencing in order to determine compliance) then it is time to make a spatially averaged measurement. On the other hand, if you never see field levels that are even close to the exposure limits, then there is no need to go any further. It is only when you start to approach the exposure limits that you must use a more accurate measurement technique. Perhaps nowhere is it more critical to make spatially-averaged measurements than in front of typical wireless antennas. Most of these antennas are collinear dipole arrays. These arrays are designed to form a single beam of energy in the distance. But these arrays all have multiple energy "lobes" in close to the antenna, where the fields are highest. These lobes are normally spaced one wavelength apart. So a typical wireless services antenna that operates at about 900 MHz will have lobes that are 33.3 cm (13 inches) apart. The distance from a peak lobe to a null is half this distance, or about 17 cm (6˝ inches) at 900 MHz. The field level typically varies by 6 dB, or four-to-one, between the peak and the null. So, if two people make a measurement along the exact same vertical axis and one holds the probe a little higher than the other, it is possible to be off by as much as a factor of four! The diagram below depicts the actual field level in red. The field level varies ±3 dB from the average along the vertical axis of the antenna. Spatially variable field from a
typical wireless antenna array.
What are the different techniques used to make spatially-averaged measurements? The traditional method of making spatially-averaged measurements is to use a "storypole". A storypole is a non-conductive pole, often made of wood, equal in height to an average adult with distance marks equally spaced along its length. Measurements are made alongside the storypole in equal increments of height and then mathematically averaged. The height and spacing of each measurement varies from standard-to-standard. For example, the IEEE C95.1 standard specifies measurements from 20 centimeters (8 inches) to 200 centimeters (6 feet-7 inches) in 20 centimeter increments. Canada's Safety Code 6 requires that measurements be averaged across two dimensions—vertically and horizontally. Modern wireless communications sites make these manual techniques not only difficult but very inaccurate because:
The best way to make spatially-averaged measurements is to use a meter that has this feature built-in. If you move the prove at a uniform rate-of-speed over a straight-line distance, then you can use time as a way of converting a time average into a spatial average. The basic technique is to start with the probe very close to or lightly touching the ground. Start the timer and begin moving the probe vertically. If you strive for about ten seconds to move from the ground to a point about six feet above the ground and maintain a fairly constant speed, you should get a very accurate spatial average along that vertical axis. Modern meters will make 30 to 40 measurements per second in this mode. Thus, the vertical average will be based on 300 to 400 discrete measurements. This means that you are making a measurement in approximately 0.5 centimeter increments. A single spatially-averaged measurement along the vertical axis is a vast improvement over random point measurements. However, when the measurements are particularly critical, even this approach may not yield sufficient accuracy. This is because of the influence of your body on the fields being measured. Depending on where you are standing relative to the source or sources of energy and the probe, your body may be either blocking the field or contributing reflected energy to the measurement. Thus, the most accurate spatial averaging technique involves making multiple spatial averages above the same point on the ground or roof surface while shifting your body to different points around the measurement location. For example, you could make four measurements—one while facing East, a second while facing West, a third while facing South, and a fourth facing North. The important point is to move your body, not the location of the probe (which has been suggested by some "experts"). The FCC requested comments on spatial averaging in its Notice of Proposed Rulemaking that was issued in October, 2003. The FCC was specifically looking for comments and where and when to use spatial averaging, when not to use spatial averaging, and recommended techniques. Richard Strickland filed extensive comments on this subject. |
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