Wireless Telecommunications Studies

With implementation of the recently revised FCC guidelines on RF exposure, including an emphasis on the wireless telecommunications industry, wireless providers have found an increased need for outside technical support for RF compliance investigations. Typical cell sites, such as the one pictured here, result in only very weak fields at ground level. Nonetheless, even these weak signal levels are often the subject of public hearings.

The real RF safety focus of cellular, PCS, paging, trunked radio and SMR operators, is an occupational one and this is driven by the need for workers to gain immediate access to the radiating elements of active transmitting antennas for maintenance purposes.

Richard Tell Associates has extensive experience in both on-site (and on-tower) RF measurements and theoretical analysis for electromagnetic field exposure assessments. This work has primarily concentrated on near-field issues since virtually all exposures of any consequence relative to compliance with FCC RF rules occur in the near-field.

Another reason for careful examination of the near-field is that the use of conventional far-field modeling methods generally produce gross over-estimates of the field magnitudes near an antenna. This becomes very important when assessing the need for implementing some form of access restriction or re-engineering of the site for compliance. Corrective procedures, based on unrealistic models can be unnecessarily expensive. The lower left figure illustrates the unrealistically high projected field values near the antenna obtained with far-field analysis methods.

  
Projects performed by Richard Tell Associates include technical evaluations of the impact of raising antenna mounting heights to reduce roof-level exposure to RF fields at wireless antenna sites. The upper right figure illustrates the strong influence of antenna mounting height on the fields to which an individual standing beneath the antenna can be exposed. Note the oscillatory nature of the roof reflections near the roof surface and the near-field variation of fields near the antenna elements.

The same considerations hold for personnel working near but beneath active antennas on towers such as this shot related to a project at a rural cellular site in Puerto Rico.

One can envision the near-field as an area within which the gain of the antenna has not yet been fully realized. In fact, what happens is that the beam becomes wider and the gain reduces. This figure illustrates an analysis of a typical 800 MHz band paging antenna and shows the plane wave equivalent power density at different distances from the antenna along a line parallel to the aperture of the antenna and in front of it. One can see that at greater distances the beam becomes apparent as a main lobe of radiation while nearer the antenna, though the field is substantially stronger, it is very non-uniform along the aperture. This variability in field level is related to the multiple elements that make up the antenna. This same concept of near-field gain reduction also applies to conventional aperture antennas like microwave dishes.

Various projects have included characterizing the near-field spatial variation of RF fields via direct measurements. This approach can be a highly cost effective method for deriving empirical models that can be applied generically across large networks using similar antennas without having to conduct expensive field studies for each and every antenna site.

These kinds of studies can be performed by making use of test transmitters and performing careful scans of the RF field levels parallel to the frontal aperture of the antenna. This technique is especially useful for evaluating specific antenna mounting arrangements that may be common to a particular application, such as antennas mounted on electric utility transmission line towers.

Sometimes, these measurements can be conducted under contrived conditions, such as a parking lot or other open area and in other cases making use of installed cell site antennas.

On-site RF measurements are generally performed using frequency shaped, isotropic field probes to determine the peak and spatially averaged field levels in accessible areas at the antenna site. An issue of some importance relative to in-situ field measurements is transmitter up-time.

    
The figure below, obtained from a study performed for a SMR client to help characterize transmitter duty cycles, was obtained by interfacing a scanning receiver to a computer using custom developed software for acquiring data on occupied channels at the rate of 100 channels per second. By acquiring data over a prolonged period, like a week at a time, statistically meaningful results were obtained that provided useful insight to the cyclic nature of RF transmissions by intermittent carriers. By associating frequencies to specific antennas located on the roof-top transmitter site, these data permitted a determination of the actual average antenna duty cycle for any 15 minute period throughout the day. With this kind of information, survey measurements taken at various points in time during the day can be better interpreted.


While many wireless antenna sites occupy building rooftops, there are thousands of communications towers supporting transmitting antennas that may present the possibility of high-level RF exposure of tower climbers. Richard Tell Associates developed the TowerCalc™ software package just for this purpose: to analyze the potential RF field levels that climbers might experience. The software determines the estimated body-averaged power density for every vertical foot of tower height.