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Subel. G0
12 questions
Subelement G0

ELECTRICAL AND RF SAFETY

Section G0A

RF safety principles, rules, and guidelines; routine station evaluation

What is one way that RF energy can affect human body tissue?

  • Correct Answer
    It heats body tissue
  • It causes radiation poisoning
  • It causes the blood count to reach a dangerously low level
  • It cools body tissue

(A). RF energy can heat body tissue and cause burns. It is important to use proper precautions against excessive exposure to high amounts of RF energy. Just remember that Microwave ovens use RF energy to cook food!

If RF radiation is absorbed by the body in large enough amounts, it can produce heat. This can lead to burns and body tissue damage. Although RF radiation is not thought to cause cancer by damaging the DNA in cells the way ionizing radiation does, there has been concern that in some circumstances, some forms of non-ionizing radiation might still have other effects on cells that might somehow result in cancer.

American Cancer Society: Radiofrequency (RF) Radiation

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Which of the following is used to determine RF exposure from a transmitted signal?

  • Its duty cycle
  • Its frequency
  • Its power density
  • Correct Answer
    All these choices are correct

All of the choices listed (duty cycle, power density, and signal frequency) are factors in estimating whether an RF signal will exceed the maximum permissible exposure (MPE).

Maximum Permissible Exposure (MPE) is the maximum level of radiation to which a person may be exposed without hazardous effects or biological changes in the eye or skin. The MPE is determined by the wavelength, the energy involved, and the duration of the exposure.

MPE is according to field strength and power density limits based on whole-body Specific Absorption Rate (SAR) averaging concepts that are used for evaluating the exposure conditions of fixed and mobile exposure conditions. SAR compliance is with respect to both whole-body and partial body SAR limits.

Think of defrosting chicken breasts in a microwave oven (which uses RF energy to cook food): below the MPE the chicken thaws, above the MPE the chicken COOKS!

FCC: § 1.1310 Radiofrequency radiation exposure limits.

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How can you determine that your station complies with FCC RF exposure regulations?

  • By calculation based on FCC OET Bulletin 65
  • By calculation based on computer modeling
  • By measurement of field strength using calibrated equipment
  • Correct Answer
    All these choices are correct

All of the methods listed are good ways to make sure that your station complies with FCC RF exposure regulations.

Computer resources are available to make the job easier. It is important to follow good engineering and electrical safety practices to make sure your station does not put you at risk of RF exposure in excess of the maximum permissible exposure (MPE)

To see these guidelines, here is the link to the FCC for downloading the FCC OET Bulletin No. 65 (August 1997): Evaluating Compliance With FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields

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What does “time averaging” mean when evaluating RF radiation exposure?

  • The average amount of power developed by the transmitter over a specific 24-hour period
  • The average time it takes RF radiation to have any long-term effect on the body
  • The total time of the exposure
  • Correct Answer
    The total RF exposure averaged over a certain period

Because the amount of time that a person is subjected to sources of RF radiation is important in determining the safe amount of RF exposure (the greater the exposure time, the less RF you should be exposed to over that period), the FCC uses a "time averaging" method for calculating the limits for Maximum Permissible Exposure (MPE), or looking at the total RF exposure averaged over a certain time.

They look at the different factors that affect RF dose and set limits based on either a 6 or 30 minute time-averaged exposure period. As an example of how voltage affects RF dose over time, a microwave oven (which uses RF energy to cook food) can safely thaw a pound of chicken at 10% power for 5 minutes and not be completely thawed, however, those same 5 minutes at 100% power will burn your popcorn!

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What must you do if an evaluation of your station shows that the RF energy radiated by your station exceeds permissible limits for possible human absorption?

  • Correct Answer
    Take action to prevent human exposure to the excessive RF fields
  • File an Environmental Impact Statement (EIS-97) with the FCC
  • Secure written permission from your neighbors to operate above the controlled MPE limits
  • All these choices are correct

If you find that your station shows RF energy in excess of permissible limits, you must take action to prevent human exposure to the excessive RF fields, which may cause tissue damage and RF burns.

Reducing the total RF exposure may be accomplished by reducing any or a combination of factors:

  • Reduce power
  • Move your antenna
  • Move the RF source farther away from possible contact
  • Improve shielding or insulation.

All of these can help

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What must you do if your station fails to meet the FCC RF exposure exemption criteria?

  • Correct Answer
    Perform an RF Exposure Evaluation in accordance with FCC OET Bulletin 65
  • Contact the FCC for permission to transmit
  • Perform an RF exposure evaluation in accordance with World Meteorological Organization guidelines
  • Use an FCC-approved band-pass filter

The standard requirement is that it is mandatory for ALL transmitting stations, with timed exposure greater than 1 milliwatt, to conduct an RF Exposure Evaluation on their stations. The FCC does however give a few exemptions to this rule, if you meet that criteria the RF Exposure Evaluation is not required.

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What is the effect of modulation duty cycle on RF exposure?

  • Correct Answer
    A lower duty cycle permits greater power levels to be transmitted
  • A higher duty cycle permits greater power levels to be transmitted
  • Low duty cycle transmitters are exempt from RF exposure evaluation requirements
  • High duty cycle transmitters are exempt from RF exposure requirements

The duty cycle refers to the amount of time you are actually transmitting. No transmitter is exempt from RF exposure requirements.

CW and phone operations only use intermittent power (you are sending part of the time, pausing to listen or receive), so these are lower duty cycle transmissions.

Data transmissions such as RTTY and packet use much closer to 100% power duty cycles.

Because the lower duty cycle transmissions use less RF energy over time (e.g. 20-60% instead of 100%), they permit a greater short-term exposure level.

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Which of the following steps must an amateur operator take to ensure compliance with RF safety regulations?

  • Post a copy of FCC Part 97.13 in the station
  • Notify neighbors within a 100-foot radius of the antenna of the existence of the station and power levels
  • Correct Answer
    Perform a routine RF exposure evaluation and prevent access to any identified high exposure areas
  • All these choices are correct

Routine RF exposure tests must be performed to make sure that our amateur station does not produce RF doses higher than the maximum permitted exposure (MPE). You need to know that you are not going to expose yourself or others who may be near your station.

FCC: 47 CFR § 97.13 - Restrictions on station location.

Here are links to FCC OET Bulletin 65 which outline testing for RF exposure:

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What type of instrument can be used to accurately measure an RF field strength?

  • A receiver with digital signal processing (DSP) noise reduction
  • Correct Answer
    A calibrated field strength meter with a calibrated antenna
  • An SWR meter with a peak-reading function
  • An oscilloscope with a high-stability crystal marker generator

The most cost-effective and accurate way to measure an RF field around your equipment is to use a field-strength meter and antenna, both of which have been properly calibrated for the range of RF energy being measured.

Hint: The words "field strength" are in both the question and the answer.

Wikipedia: Field strength meter

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What should be done if evaluation shows that a neighbor might experience more than the allowable limit of RF exposure from the main lobe of a directional antenna?

  • Change to a non-polarized antenna with higher gain
  • Use an antenna with a higher front-to-back ratio
  • Correct Answer
    Take precautions to ensure that the antenna cannot be pointed in their direction when they are present
  • All these choices are correct

You should always try to avoid pointing a directional antenna directly toward nearby people. Steps must be taken to make sure that the antenna cannot be directly pointed in their direction, such as marking the conflict points on your antenna rotator control, so that you do not use that directional heading.

Title 97 §13(c)(2) states:

If the routine environmental evaluation indicates that the RF electromagnetic fields could exceed the limits contained in §1.1310 of this chapter in accessible areas, the licensee must take action to prevent human exposure to such RF electromagnetic fields.

ARRL: RF Radiation and Electromagnetic Field Safety

FCC: Radio Frequency Safety

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What precaution should be taken if you install an indoor transmitting antenna?

  • Locate the antenna close to your operating position to minimize feed-line radiation
  • Position the antenna along the edge of a wall to reduce parasitic radiation
  • Correct Answer
    Make sure that MPE limits are not exceeded in occupied areas
  • Make sure the antenna is properly shielded

You should use testing equipment (calibrated field-strength meter and calibrated antenna) to make sure that the maximum permissible exposure (MPE) limits are not exceeded.

ARRL: RF Radiation and Electromagnetic Field Safety

FCC: Radio Frequency Safety

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What stations are subject to the FCC rules on RF exposure?

  • All commercial stations; amateur radio stations are exempt
  • Only stations with antennas lower than one wavelength above the ground
  • Only stations transmitting more than 500 watts PEP
  • Correct Answer
    All stations with a time-averaged transmission of more than one milliwatt

Determination of exemption.

(i) For single RF sources (i.e., any single fixed RF source, mobile device, or portable device, as defined in paragraph (b)(2) of this section): A single RF source is exempt if:

(A) The available maximum time-averaged power is no more than 1 mW, regardless of separation distance.

https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-1/subpart-I/section-1.1307#p-1.1307(b)

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