A new group of questions has recently surfaced in the exam question pools involving radiofrequency (RF) safety. The hazardous effect of RF exposure is one reason why you have heard so much during the past decade about the damage that cellular phones can do to your "gray matter" during signal transmission. RF radiation exposure risk is not folklore or urban mythology - it is a well-studied and proven fact. RF radiation can do damage to you by heating tissue via direct contact with transmitting sources or by being in close proximity to antennas that produce high frequency (VHF and UHF) radiation. The FCC has given us guidelines to use for protection against non-ionizing radiation exposure. Non-ionizing radiation is that in which the frequency is too low for there to be enough photon energy to ionize atoms (like with X- or gamma rays).
EXPOSURE ASSESSMENT REQUIREMENTS
To responsibly set up you equipment, you can use Tables NTO-1 RF Exposure Limits to determine how far an antenna should be located to the "controlled" exposure population (you and your family who know that you are generating RF) and the "uncontrolled" exposure population (this might include your neighbors and anyone else around your shack who may not know that you are transmitting RF). Specific Absorption Rate (SAR) is the rate at which RF energy is absorbed into the human body. The maximum permissible exposure limits are based on whole-body specific absorption rates. This factor is important because the body absorbs some frequencies (the higher ones) more than others.
The tables, which were derived from information contained in FCC OET Bulletin No. 65, can be used as sufficient resources for hams to use in performing routine evaluations of their stations. No computer modeling or sophisticated calculations are required, even though these means can also be utilized. Gain and directivity are the most important contributing factors in performing these calculations.
Detectors, such as a calibrated field-strength meter, can also be used to meet the intent of the evaluation requirements. Make sure that the detector is used according to the manufacturer's specifications and remember the following:
Computer models must take into account ground interactions. Emitting antennas for other radio services in close proximity to the ham station must be integrated into the evaluation so that there effects to the overall radiation exposure can be realized.
LET'S KEEP IT SIMPLE . . .
The key principles:
The primary applications:
NEAR FIELD AND FAR FIELD CALCULATIONS
Because the near field boundary is highly dependant on antenna type, it is difficult to accurately evaluate the effects of radiation exposure. Generally speaking, the dimensions of the near field can be defined by multiplying the square of the antenna length by 2 and then dividing by the wavelength of the signal.
In the far field, exposure diminishes by distance with the inverse square law: the power density is proportional to the inverse square of the distance. There is an relationship for equating the electrical field with the magnetic field in the far field region.
377 ohms = E (the electric field in volts/meter) / H (the magnetic field in amps/meter)
All hams must perform an initial evaluation to see if their stations are meeting the radiation exposure guidelines of Part 97. But only a few are required to perform routine evaluations. HT's and mobiles are categorically exempt, and so are base stations operating below 50 watts PEP. But if your station operates at or above the 50 watts PEP cut-off limits, then you are accountable for performing these evaluations periodically. Documentation is not required, but is advised.
USING THE TABLES AND FIGURES
There will be questions on the exam involving interpretation of the tables and figures (to be provided to you by the VE's during the test), so be familiar with their use. A question will ask what the acceptable distance of separation between the RF source (antenna type and power output specified) and the controlled or uncontrolled exposures should be, at a minimum. No problem - all you have to do is be familiar with the use of the tables.
RF Exposure Limits: Figures and Tables:
Figure NTO-1 Controlled and Uncontrolled Exposure Limits
Figure NTO-2 Main Beam Exposure (With Reflection) This figure can be used to quantify worst case radiation exposure from a given station.
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