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A how to of what for, by Mike(aka Billy)Graham, NE1V

Evaluating RF Exposure Part 2

In Part 1 of this series published in the January, 1998 issue of the NARC Bulletin, we discussed the background and general information criteria regarding Evaluating Compliance with FCC Guidelines for Human Exposure to Radio Frequency Electromagnetic Fields as defined in FCC OET Bulletin 65 and Supplement B thereto. This month we'll discuss specific requirements and evaluation criteria. In Part 3, we'll perform an actual evaluation, using a fictitious amateur radio station. It is also strongly recommended that each of you download the contents of OET Bulletin 65 and Supplement B, and retain copies in your files for future reference. These guidelines apply to applications for FCC Station Licenses filed on or after January 1, 1998. The Commission has also adopted a date certain of 1 September, 2000, by which time all existing facilities and devices must be in compliance with the new guidelines. Both OET Bulletin 65 and Supplement B thereto may be downloaded from: http://www.fcc.gov/oet/

In Part 1 a table of Power Thresholds for routine evaluation was published. This equates to Table 1 of Supplement B. If a station's transmitted power exceeds the guidelines specified, an evaluation is required. The evaluation should include estimates for both occupational/controlled and general population/uncontrolled exposure limits. Table 1 values were created using time and spatial averaging. Spatially averaged RF field power levels relate to estimating the whole-body averaged specific absorption rate that will result from the expose and the MPEs specified in Table 1. These levels are measured in milliwatts/cm(squared) of body surface. Time averaging is set at six minute measurement intervals for occupational/controlled environments and thirty minute measurement intervals for population/uncontrolled environments. Because not all RF transmissions from amateur transmitters are 100% duty cycle, a duty factor may be applied when calculating the average power. For example, assume a transmitter is operating at 1000 Watts PEP SSB with NO speech processing and transmits ("worst case") three minutes on and three minutes off. From Table 2 you find that the "duty factor" for SSB with NO speech processing is 20%. The formula is:

P(watts) X "duty factor" X % of time transmitting.

In the case of controlled exposure, the effective power would be:

1000 X (.2) X (.5 (3 of 6 min)) = 100 Watts

In the case of general/uncontrolled exposure, the effective power would be:

1000 X (.2) X (.5 (15 of 30)) = 100 Watts

The values derived may then be used with various tables for differing types of antennas to determine if the station is in compliance with the guidelines.

Supplement B contains two tables which take most of the math work out of determining compliance. Table 4a for the MF/HF bands was developed by Fred Maia, W5YI Group, working in cooperation with the ARRL. The table is designed for the absolute "worst case" scenario. The table vertically lists bands and typical antenna gains for those bands. Horizontally, it lists columns of transmitting power at 100% duty cycle for both controlled and uncontrolled environments. For example, given a station transmitting with 1500 Watts on the 40- meter band using a dipole (3 dBi), the controlled safe distance is beyond 1.9 meters (or 6.23 feet) and uncontrolled safe distance is beyond 4.2 meters (or 13.78 feet). Note: multiply distance in meters by 3.28 to determine distance in feet; multiply the distance by .707 if the duty cycle method to determine distance is used and is calculated at say 50%. The actual distance to the antenna is calculated using the formula for determining the hypotenuse of a right triangle, h2=a2 + b2, where h = distance to antenna, a = height of antenna above ground, and b = distance from directly beneath the antenna to the observer. In the case of our dipole, if the dipole is 30 feet above ground on average, and the observer is 40 feet away from directly beneath the closest point of the antenna, the distance from observer to antenna is 50 feet. The same applies to antennas mounted on towers. In the case of ground-mounted vertical antennas, the distance from antenna to observer is just the straight-line distance from observer to antenna.

Table 4b, also developed by Fred Maia of the W5YI Group, provides similar data for the VHF/UHF bands. A cursory glance at both tables quickly shows that safe distance values to antennas "peak" in the 2 Meter band. For power levels not shown on the tables, interpretation is required, or Bulletin 65 should be consulted for alternative methods of determining compliance (e.g., calculations, measurements, etc.).

Appendix B to Supplement B provides an optional worksheet and record of compliance which the operator may use to methodically proceed with calculations and record the results. If properly accomplished, recorded, dated and signed, this becomes a station's permanent record of evaluation and fulfills the FCC's requirements now stated on FCC Form 610. This form is not mandatory, and others may be used if technically sound.

If, after an evaluation is performed, it is determined that a potential problem exists, Section 4 of Bulletin 65 should be consulted for a discussion of recommended methods for reducing or controlling exposure. Such methods could include one or more of the following:

  1. Restricting access to high RF-field areas.
  2. Operating at reduced power when people are present in high RF-field areas.
  3. Transmitting at times when people are not present in high RF-field areas.
  4. Considering duty factor of transmissions.
  5. Time-averaging exposure.
  6. Relocating antennas or raising antenna height.
  7. Incorporating shielding techniques.
  8. Using monitoring or protective devices.
  9. Erecting warning/notification Signage. Limiting access may be the easiest method to reduce exposure (fenced-in yards, etc.). Reducing transmitting power can also significantly reduce exposure levels. Output power has a linear relationship with the power density exposure level. Reducing power 20% reduces power density by 20%. Utilizing the inherent duty cycle of the transmission from an amateur station is extremely easy to use as was demonstrated above and is a valid method of determining ERP. The worst-case duty cycle factor, 100%, occurs during continuous or "key down" transmissions. However, a great deal of amateur service transmissions are more likely to be the "key on, key off" type, resulting in a more typical duty cycle factor of 50% or so.

    In all cases, it is the FCC's position that they rely on the skills and demonstrated abilities of amateurs to comply with its technical rules, and will continue to do so. That having been said, it is obvious that it is contingent upon us all to conscientiously apply the guidelines, thus resulting in an improved and safe amateur service that will benefit both amateur licensees and those individuals residing or working near amateur facilities.

    For now, 73 es best DX ……… de NE1V

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