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).



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:

  • Interaction of probes and personnel in the near-field, frequency response of testing equipment,and probe orientation can all affect field strength when using a detector to take measurements.
  • Nearby conductive materials (metal objects) can add or subtract from the field strength measurements that you are taking.
  • Electric field strength is measured in volts per meter and magnetic field strength is measured in amperes per meter.  Power density of RF signals are measured in milliwatts per square centimeter.

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.



The key principles:

  • Always operate on low power when possible.  The lower the power level transmitted, the lower the dose of radiofrequency radiation that is delivered.  It's good practice to keep low power whenever possible to reduce the chance of interference.
  • There is more risk associated with higher frequencies (VHF and UHF) than lower ones.   The smaller wavelengths characteristically are able to better penetrate tissue than lower frequencies.
  • High frequency radiation can also cause cataracts if a signal is transmitted towards your head.
  • In the near field, distance away from the source contributes to field strength, depending entirely upon what antenna is being used.  The field strength decays with distance in the far field region.  The factors which determine the boundaries between the near and far fields are wavelength and physical size of the antenna.
  • The duty cycle (the time that you are actually transmitting at full power during a single transmission) is a contributing factor in exposure. 

The primary applications:

  • Never touch a beam antenna while operating on high power.  In fact, don't even stand close, and definitely don't look down the being during transmission.  Direct contact with tissue with a high power-emitting antenna can burn you severely, and high frequency exposure will literally cook your tissue to some extent - just like a microwave does.  That is where the name "microwave" comes from - it works on the same principle.
  • For the same reason mentioned above, keep the antennas of your handheld VHF-UHF units as far away from your head as possible.  The power levels are relatively low, but the risk is still there.
  • The best place for your ground-plane mobile antenna is on top of your car roof; that way, you have a buffer between you and your passengers and the antenna while transmitting.  This is important because mobile units are capable of generating relatively high power.
  • Keep as much distance between your base antennas and you and your neighbors (elevate antennas high off of the ground, if possible).
  • Construct obstacles around your antennas (such as fences) to keep both the unsuspecting minors and what I like to call the "blatantly mentally deficient" from coming into direct contact with your transmitting equipment.  I use the quoted term for the ones that really know better, but for some unknown reason, like to challenge the principles of physics laid out by our Creator.  They eventually learn.  The hard way.  Ouch.
  • Keep the duty cycle as short as possible.  The FCC guidelines are based on a 6-minute time for the controlled exposure population, while the uncontrolled time is 30-minutes (longer because they should be farther away from the source).  The exposure that the operator is actually receiving should be time-weighted to correctly evaluate to account for that time which he is not transmitting.  Any reduction in the allowed duty cycle yields a reduction in the compliance distance.  Out of all available modes, SSB communications tend to generate the shortest duty cycles.



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.



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:

Table NTO-1

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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