ANTENNAS AND TOWERS

HUMAN EXPOSURE  TO  RF RADIATION:

STANDARDS

In 1.966  the American National Standards Institute (ANSI) issued its first standard that recomended máximum exposure values for electromagnetic field intensities.  This early guide on exposures was relatively simplistic in that it set a power density limit of 10.000 µW/cm²     across the frecuency range of 10 MHz to 100 GHz..  Since 1966,  the ANSI Standard has been twice revised and is presently in the stages of being revised again to reflect new research findings that are relevant to the setting of realistic and accurate RF exposure standards.   In addition to the ANSI Standard, there have been developed numerous standards, guidelines, ordinances, exposure policies, and recomendations by federal, state, and local governments.  This section is intended as an overview of some of these documents to provide a perspective on what presently exists in the regulatory world of RF fields and some of the trends that are occurring in regulation development.

Virtually all modern RF exposure guides are now frequency dependent; i.e.,  recomended maximun exposure levels vary according to the frecuency of the exposure fields.  This fundamental commonality is base on the finding that the body absorbs RF energy from electromagnetic fields differently at different frecuencies.  While research continues to provide a better understanding of how RF fields interact with the standard .

The ANSI guide for exposure is the most stringent; i.e. it limits the fields to the lowest value anywhere in the electromagnetic spectrum. This is because the adult human body exhibits a whole-body  resonance peak at around 70 – 90 MHz, while the body of a small child resonates at a considerably higher frecuency, nearer 300 MHz.  Thus the flat limit line between 30 MHz and 300 MHz protects individuals of all sizes from absorbing RF energy at excessive rates.

There are three additional qualifications of the limiting values of RF exposure given in fig. 2.  First, RF fields measured as being equal to the ANSI limit will normally incorpórate a significant degree of additinal safely since  it is generally unlikely that the body will be exactly aligned with the polarization of the incident fields.  A second qualification is that, while the RFPG power density limits were base on the concept of a human body being uniformly exposed to a given power density (that value that would result in a whole-body averaged SAR of 0.4 W/kg), the Standard does not explicity state that the field must be uniform over the body to be of significance, nor does it explicity state  that exposure of only part of the body to a field greater tan the specified power density limits is acceptable.

Hence,  a conservative view is that exposure of any part of the body to RF fields that exceed the RFPG for sufficiently long times represents noncompliance with the Standard.

The third qualification of the limiting values of RF exposure given in fig. 2 is that they represent the time averaged  value over any six –minute period of time. Thus, the limits given in the ANSI RFPG are applicable to long-term exposure of indefinite extent.  But, when the exposure duration can be controled to periods shorter than six minutes, then higher levels are permitted.  This means that RF fields higher than the values shown now  fig. 2  are acceptable,  providing that the exposure time is controlled in such a way as to keep the average value over any six minute window  of  time to no more than that given by the graph. 

During the remaining three minutes of the six minutes period. However, no exposure would be permited.  This provisión is helpful in those instances where transient exposures may occur as in passing momentarily through elevated field áreas,  In practice, determining the actual time- averaged exposure leve lis complicated because of having to keep track  of both the RF field level and the time duration of exposure to each level.

National  Council on Radiation Protection and Measurements.

(NCRP) is a congressionality chartered, nonprofit corporation established in 1964 to collect,  analyze, develop and disseminate in the public interest information and recomendations about radiation and radiation measurement, particularly those concerned with radiation protection.  The NCRP has developed exposure criteria for RF  fields but has distinguised máximum recomended RF levels for those occupationaly exposed from levels that are applicable to the  general public, a distinction not contained in the present ANSI RFPG.  The concpt introduced by NCRP to recommend more stringent limits  for the general public is based on a concern that a greater margin of safety is appropiate since a wider range of healt conditions may exist among the public (i.e. there may be individuals that are more susceptible due to age or other factors) and those working with RF fields are more likely to be aware of the potencial for hazards and can take preventive measures to limit their exposure.

The NCRP criteria for ocupacional exposures are the as the ANSI criteria, however, the criteria for the general public are equivalent to one-fifth of these values.  In the VHF band ( 30  -  300 MHz), for example, the NCRP general public exposure limit is 0. 2m W/cm²  However, the averaging time associated with the NCRP general public criteria is 30 minutes rather than the six minutes used in the ocupacional exposure criteria and the occupational exposure criteria and the ANSI RFPG.  By selecting a 30 minute averaging time,   the total  energy absirption  (equal to the product of power density and time) is the same for both the general public and workers.

Both the occupational and general public expose limits recommended by IRPA employ an averaging time of six minutes during a 24 hour day.  In this sense,  the IRPA limits are more restrictive than the NCRP criteria since relatively high but momentary exposures will be more curtailed according to the shorter time over wich they can be averaged.  The IRPA employ an averaging time of six minutes during a 24 hour day.  In this sense,  the IRPA limits are more restrictive than the NCRP criteria since relatively high but momentary exposures will be more curtailed according to the shorter time over which they can be averaged.  The IRPA guidelines also contain an exclusión clause, similar to ANSI,  permitting the use of low powered transmitting devices which opérate with powers no greater than 7 W that may produce RF fields exceding the specified field strength limits.

“Proposed Alternatives for Controlling Public Exposure to Radiofrequency Radiation)  contained four alternative  appproaches to limit the publics exposure to RF radiation.  Three of these options were regulatory and saggested posible numerical  exposure limits for RF fields.  The fourth option was nonregulatory ; information and technical assistance programs will be conducted in lieu of adopting federal guidance.

The EPA  office of Radiation Programs has tasked the agency´s Human Health Assessment Group  ( formerly called the Cancer Assessment Group) to perform a comprehensive review of the cáncer literatura.

Local Standards.

Several states, countries, and cities have enacted, or attemped to enact, standard or ordinances which set upper limits on exposure of the public to RF fields.  Some of these standars have relied on the ANSI standard and others have been developed in a  manner more similar to the NCRP exposure criteria :  examples;  Arizona, Connecticut, Massachusetts, New Jersey, and Texas.  Counties and cities active in this local regularory área include  Avon, Connecticut;  Onondaga, New York; Multnomah  County and City of Portland, Oregon; Jefferson and Boulder Counties, Colorado; New York City; and King County and the City of Seattle, Washington.   The local codes vary from location to location, as well as within a community over time, which means that the interested broadcaster will need to research the particular restrictions that apply to him.

 Consequenly,  more proteccion  of the pubic is not warranted since lower level exposures are not cumulative in causing effects .  RF field biological effects are believed to be principally dose rate phenomena, i.e. They depend on the rate at which energy absorbed as is the case with nuclear radiation.  This rate concept is demostred is demostrated in the time –averaging provisions of the standards.

The use of  induced and conduced body currents as surrogates for SAR is envolving.  Currents as surrogates for SAR is evolving.  Currents in tissues of  the body can be related to the SAR in the tissue, and thus, can be used to evaluate conformance of  RF exposures with the exposure  criteria upon  which many  protection  guide are based,  Often, currents, either induced from the field itself, current, either induced from the field itself or as a consequence of contacting  an object immersed in a RF field, can offer a more meaningful  indication of ponential hazard than the fields themselves.  Measurement of contact currents can, in many instances, show that the SAR  which may result  from touching an object which exhibits strong Surface RF fields that exceed the field strength limits of various proteccion  guide does not, in fact, exceed the SAR limits of  the guid

CONSIDERATIONS SPECIFIC TO BROADCAST SERVICES

FCC rules implementing the National Environmental Policy Act of 1.969 encampass much more than RF exposure, the subject matter of this Handbook chapter however, this discussion will relate only to the environmental aspects of RF exposure.

To avoid the requirement for an Environmental Assessment, the broadcaster must be prepared to  demonstrate by calculation or measurement that their existing or proposed operation subject to results in RF exposures not in excess of the protection guide set forth in ANSI C95.1 – 1982.  Compliance with the protection guide (and with the other aspects of environmental considerations) establishes that  Commission actions granting the license renewal or construction permit for new or changed facilities would “have no significant effect on the quality of the human environment and are categorically excluded from environmental processing.  Broadcast facilities sites should be selected,  or the equipment designed, to achive “categorical excusión”.  Failure to meet the RF protection guide is virtually certain to result in refusal by the Commission of a requested construction permito  license renewal.

TV.  STATIONS

Television antennas are, more often than not, no tall towers or  relatively tall structures on building tops.  Furthermore, the antennas used have relatively Little radiations downward.  Consequently, even though substantial power is generally involved, particularly in the UHF band, radiation levels near ground level are usually well below protection criteria.  The result is that the major consideration may be related to tower and antenna maintenance rather than ground-level exposure.

Preferably by measurement, the limits of areas conforming to the protection guide should be defined for different power levels of the facility.  As described later in this section, safe work rules should be adopted based on those determinations.

SITUATIONS

HIGH RF LEVELS ARE PRODUCED AT ONE OR MORE LOCATIONS ABOVE GROUND LEVEL ON AN APPLICANT´S TOWER,   APPROPRIATE WARNING SIGNS, EFECT ON THE HUMAN BODY?

HIGH RF LEVELS ARE PRODUCED IN OCCUPIED STRUCTURES, ON BALCONIES OR ON ROOFTOPS USED FOR RECREATIONAL OR COMMERCIAL PURPOSES.

William F Hammethh, PE.
Hammet & Edison, Inc., San francisco California

Jules Cohen, P.E.
Jules Cohen & Associates, P.C., Washington, District of Columbia

Richard  A.  Tell
Richard Tell Associates, inc, Las Vegas, Nevada.