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RF Exposure and Safety Limits

This module discusses the effects of RF energy on the human body and the safety limits to prevent excessive exposure. Topics include power density, absorption rates, averaging and duty cycle, controlled and uncontrolled environments, antenna systems, and station evaluation.

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RF Exposure and Safety Limits

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  1. General License CourseChapter 9 Lesson Plan Module 38 – RF Exposure

  2. RF Exposure • Exposure to RF at low levels is not hazardous • Factors to consider: power level or density, frequency, average exposure time, and duty cycle of the transmission • Two primary factors that determine how much RF the body will absorb are power density and frequency 2015 General License Course

  3. Power Density • Heating from exposure to RF signals is caused by the body tissue absorbing RF energy • Power density is the intensity of the RF energy and it is measured in mW/cm2 (milliwatts per square centimeter) • Example: power density is 10 mW/cm2 and area is 75 cm2 then total power is: • 10 mW/cm2 x 75 cm2 = 750 mW 2015 General License Course

  4. Power Density • Power density: • Highest near antennas and in the directions in which antennas have the most gain • Increasing transmitter power increases power density • Increasing distance from an antenna lowers power density 2015 General License Course

  5. Absorption & Limits • Specific absorption rate (SAR) is the rate at which energy is absorbed from the power to which the body is exposed • Safe levels of SAR, called Maximum Permissible Exposure (MPE), vary with frequency as established by the FCC • SAR varies with frequency, power density, average amount of exposure and duty cycle • Body parts absorb RF best when dimensions are similar to the wavelength of the energy 2015 General License Course

  6. Averaging & Duty Cycle • RF energy is averaged over fixed time intervals because the body responds differently to heating for short and long duration exposures • Controlled environment • Uncontrolled environment • Time-averaging evaluates the total RF exposure over a fixed time interval 2015 General License Course

  7. Controlled & Uncontrolled • People in controlled environments are considered to be aware of their exposure (family members) • They can minimize their exposure so a shorter exposure is considered • People in uncontrolled environments are not aware of their exposure (neighbors) • Longer exposures are evaluated 2015 General License Course

  8. Duty Cycle • Duty cycle is the ratio of the time the transmitter is on to the total time during the exposure • 50% duty cycle is half off and half on • Lower transmission duty cycle permits greater short-term exposure levels for a given average exposure 2015 General License Course

  9. Duty Cycle • Different modes themselves have different emission duty cycles • The likely pattern of use during contacts using that mode 2015 General License Course

  10. Antenna System • When estimating exposure, include the following considerations: • The amount of gain provided by your antenna • High gain antennas increase a signal’s average power considerably • Losses in the feed line 2015 General License Course

  11. Estimating Exposure & Station Evaluation • All fixed amateur stations must evaluate their capability to cause RF exposure, no matter whether they use high or low power • A routine evaluation must then be performed if the transmitter PEP and frequency are within the FCC rule limits 2015 General License Course

  12. Station Evaluation • You are required to perform the RF exposure evaluation only if your transmitter output power exceeds the levels shown for any band 2015 General License Course

  13. Station Evaluation • You can perform the evaluation by actually measuring the RF field strength with calibrated field strength meters and calibrated antennas • You can also use computer modeling to determine the exposure levels • It’s easiest for most hams to use an online calculator listed on the ARRL website 2015 General License Course

  14. Station Evaluation • You must re-evaluate the RF exposure from your station if you: • Change to a higher power • Increase antenna gain • Change antenna height • You need not re-evaluate RF exposure if you reduce the output power of a station already in compliance 2015 General License Course

  15. Exposure Safety Limits • If the evaluation shows that your transmissions exceed MPE limits: • Reduce antenna gain • Reduce average power level • Move or rotate antenna away from people • Take measures to prevent excessive power from being radiated in your neighbors’ direction 2015 General License Course

  16. Exposure Safety Limits • If you use a stealth or indoor antenna, make sure MPE limits are not exceeded in your home’s living quarters • You are much closer to the antenna than stations with external antennas • Place mobile antennas on the vehicle roof or trunk to minimize exposure to passengers. Use a speaker-mike to reduce your exposure. • Use a dummy load with testing a transmitter 2015 General License Course

  17. Practice Questions 2015 General License Course

  18. What is one way that RF energy can affect human body tissue? A. It heats body tissue B. It causes radiation poisoning C. It causes the blood count to reach a dangerously low level D. It cools body tissue G0A01 2015 General License Course

  19. What is one way that RF energy can affect human body tissue? A. It heats body tissue B. It causes radiation poisoning C. It causes the blood count to reach a dangerously low level D. It cools body tissue G0A01 2015 General License Course

  20. Which of the following properties is important in estimating whether an RF signal exceeds the maximum permissible exposure (MPE)? A. Its duty cycle B. Its frequency C. Its power density D. All of these choices are correct G0A02 2015 General License Course

  21. Which of the following properties is important in estimating whether an RF signal exceeds the maximum permissible exposure (MPE)? A. Its duty cycle B. Its frequency C. Its power density D. All of these choices are correct G0A02 2015 General License Course

  22. How can you determine that your station complies with FCC RF exposure regulations? A. By calculation based on FCC OET Bulletin 65 B. By calculation based on computer modeling C. By measurement of field strength using calibrated equipment D. All of these choices are correct FCC Rule: [97.13(c)(1)]G0A03 2015 General License Course

  23. How can you determine that your station complies with FCC RF exposure regulations? A. By calculation based on FCC OET Bulletin 65 B. By calculation based on computer modeling C. By measurement of field strength using calibrated equipment D. All of these choices are correct FCC Rule: [97.13(c)(1)]G0A03 2015 General License Course

  24. What does “time averaging” mean in reference to RF radiation exposure? A. The average amount of power developed by the transmitter over a specific 24 hour period B. The average time it takes RF radiation to have any long-term effect on the body C. The total time of the exposure D. The total RF exposure averaged over a certain time G0A04 2015 General License Course

  25. What does “time averaging” mean in reference to RF radiation exposure? A. The average amount of power developed by the transmitter over a specific 24 hour period B. The average time it takes RF radiation to have any long-term effect on the body C. The total time of the exposure D. The total RF exposure averaged over a certain time G0A04 2015 General License Course

  26. What must you do if an evaluation of your station shows RF energy radiated from your station exceeds permissible limits? A. Take action to prevent human exposure to the excessive RF fields B. File an Environmental Impact Statement (EIS-97) with the FCC C. Secure written permission from your neighbors to operate above the controlled MPE limits D. All of these choices are correct G0A05 2015 General License Course

  27. What must you do if an evaluation of your station shows RF energy radiated from your station exceeds permissible limits? A. Take action to prevent human exposure to the excessive RF fields B. File an Environmental Impact Statement (EIS-97) with the FCC C. Secure written permission from your neighbors to operate above the controlled MPE limits D. All of these choices are correct G0A05 2015 General License Course

  28. What precaution should be taken when installing a ground-mounted antenna? A. It should not be installed higher than you can reach B. It should not be installed in a wet area C. It should limited to 10 feet in height D. It should be installed such that it is protected against unauthorized access G0A06 2015 General License Course

  29. What precaution should be taken when installing a ground-mounted antenna? A. It should not be installed higher than you can reach B. It should not be installed in a wet area C. It should limited to 10 feet in height D. It should be installed such that it is protected against unauthorized access G0A06 2015 General License Course

  30. What effect does transmitter duty cycle have when evaluating RF exposure? A. A lower transmitter duty cycle permits greater short-term exposure levels B. A higher transmitter duty cycle permits greater short-term exposure levels C. Low duty cycle transmitters are exempt from RF exposure evaluation requirements D. High duty cycle transmitters are exempt from RF exposure requirements G0A07 2015 General License Course

  31. What effect does transmitter duty cycle have when evaluating RF exposure? A. A lower transmitter duty cycle permits greater short-term exposure levels B. A higher transmitter duty cycle permits greater short-term exposure levels C. Low duty cycle transmitters are exempt from RF exposure evaluation requirements D. High duty cycle transmitters are exempt from RF exposure requirements G0A07 2015 General License Course

  32. Which of the following steps must an amateur operator take to ensure compliance with RF safety regulations when transmitter power exceeds levels specified in FCC Part 97.13? A. Post a copy of FCC Part 97.13 in the station B. Post a copy of OET Bulletin 65 in the station C. Perform a routine RF exposure evaluation D. All of these choices are correct G0A08 2015 General License Course

  33. Which of the following steps must an amateur operator take to ensure compliance with RF safety regulations when transmitter power exceeds levels specified in FCC Part 97.13? A. Post a copy of FCC Part 97.13 in the station B. Post a copy of OET Bulletin 65 in the station C. Perform a routine RF exposure evaluation D. All of these choices are correct G0A08 2015 General License Course

  34. What type of instrument can be used to accurately measure an RF field? A. A receiver with an S meter B. A calibrated field strength meter with a calibrated antenna C. An SWR meter with a peak-reading function D. An oscilloscope with a high-stability crystal marker generator G0A09 2015 General License Course

  35. What type of instrument can be used to accurately measure an RF field? A. A receiver with an S meter B. A calibrated field strength meter with a calibrated antenna C. An SWR meter with a peak-reading function D. An oscilloscope with a high-stability crystal marker generator G0A09 2015 General License Course

  36. What is one thing that can be done if evaluation shows that a neighbor might receive more than the allowable limit of RF exposure from the main lobe of a directional antenna? A. Change to a non-polarized antenna with higher gain B. Post a warning sign that is clearly visible to the neighbor C. Use an antenna with a higher front-to-back ratio D. Take precautions to ensure that the antenna cannot be pointed in their direction G0A10 2015 General License Course

  37. What is one thing that can be done if evaluation shows that a neighbor might receive more than the allowable limit of RF exposure from the main lobe of a directional antenna? A. Change to a non-polarized antenna with higher gain B. Post a warning sign that is clearly visible to the neighbor C. Use an antenna with a higher front-to-back ratio D. Take precautions to ensure that the antenna cannot be pointed in their direction G0A10 2015 General License Course

  38. What precaution should you take if you install an indoor transmitting antenna? A. Locate the antenna close to your operating position to minimize feed line radiation B. Position the antenna along the edge of a wall to reduce parasitic radiation C. Make sure that MPE limits are not exceeded in occupied areas D. Make sure the antenna is properly shielded G0A11 2015 General License Course

  39. What precaution should you take if you install an indoor transmitting antenna? A. Locate the antenna close to your operating position to minimize feed line radiation B. Position the antenna along the edge of a wall to reduce parasitic radiation C. Make sure that MPE limits are not exceeded in occupied areas D. Make sure the antenna is properly shielded G0A11 2015 General License Course

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