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Subelement T0
SAFETY
Section T0C
RF hazards: radiation exposure, proximity to antennas, recognized safe power levels, radiation types, duty cycle
What type of radiation are radio signals?
  • Gamma radiation
  • Ionizing radiation
  • Alpha radiation
  • Correct Answer
    Non-ionizing radiation

Electromagnetic energy may be considered as ionizing and non-ionizing radiation. When radiation is 'ionizing,' it means that it can separate electrons from an atom to create ions. Lower frequency radiation, which includes all amateur radio frequencies, is non-ionizing. Regardless of the power of the signal, the frequecies of VHF and UHF signals are too low to dislodge the electrons. (Gamma and alpha radiation are both ionizing radiation.)

Last edited by jsharris1229. Register to edit

Tags: radio waves uhf vhf safety arrl chapter 9 arrl module 23

At which of the following frequencies does maximum permissible exposure have the lowest value?
  • 3.5 MHz
  • Correct Answer
    50 MHz
  • 440 MHz
  • 1296 MHz

Experimental data indicates the frequencies that are easiest for the human body to absorb are between 30 MHz and 300 MHz. This means that this range requires the lowest exposure or MPE. Since only 50 MHz falls in this range, it is the correct answer. All other frequencies are not within MPE limits.

Tricky question since easy absorption gives 30 - 300 MHz the HIGHEST value, or risk, of reaching a maximum exposure limit.

Additional information and a great chart can be found here.

Maximum Permissible Exposure (MPE) to radiofrequency electromagnetic fields.

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Tags: 6 meter arrl chapter 9 arrl module 23

How does the allowable power density for RF safety change if duty cycle changes from 100 percent to 50 percent?
  • It increases by a factor of 3
  • It decreases by 50 percent
  • Correct Answer
    It increases by a factor of 2
  • There is no adjustment allowed for lower duty cycle

Duty Cycle is the ratio of time a circuit is ON compared to the time it is OFF. See the Wikipedia article for a good visualization.

The short answer to this question is that we care how much radiation is absorbed, which is directly relational to how much is emitted. If you emit half as much per transmission time, you can be exposed to twice as many transmissions.

In more depth, the FCC regulates exposure based on Specific Absorbtion Rates, which will depend on factors in addition to the radio's output power, such as the antenna configuration, antenna gain, frequency, physical location relative to the antenna, and geometry of the object absorbing the radiation.

In the Specific Absorbtion Rate formula, the RMS power level of the Electric Field is in the numerator. Because the denominator won't change as long as we're measuring relative to the same object (such as a human), the RMS power level directly relates to the SAR.

How do we connect Duty Cycle to SAR? Well, Duty Cycle can also be written as

D = Average Power / Peak Power

We can then take the Duty Cycle and plug that into a version of the RMS formula for waves:

RMS = Vp√D

Assuming we know the other variables for calculating SAR, we can then calculate the value. Luckily, we don't have to find those values, as the FCC has chosen the standard values for us.

Since most of the values are fixed for amateur radio purposes, there are many online calculators, such as this one from Lake Washington Ham Club that can give you the relevant calculated outputs.

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What factors affect the RF exposure of people near an amateur station antenna?
  • Frequency and power level of the RF field
  • Distance from the antenna to a person
  • Radiation pattern of the antenna
  • Correct Answer
    All these choices are correct

Each of these answers affect a persons exposure to RF energy radiating from an antenna. The frequency of the signal determines how easily the body absorbs the RF energy. As the power level of the signal increases, more energy can be absorbed, increasing the exposure. The distance is important because the power level of the signal drops rapidly with distance. Finally, the radiation pattern indicates the areas where the energy is directed and how it is concentrated.

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Tags: safety radio waves transmit power frequencies antenna arrl chapter 9 arrl module 23

Why do exposure limits vary with frequency?
  • Lower frequency RF fields have more energy than higher frequency fields
  • Lower frequency RF fields do not penetrate the human body
  • Higher frequency RF fields are transient in nature
  • Correct Answer
    The human body absorbs more RF energy at some frequencies than at others

The body absorbs energy differently at different frequencies because different materials and parts of your body are resonant at different frequencies.

The other answers listed can also be seen to be incorrect: RF field energy does not depend on frequency, lower RF frequencies can penetrate the body, and higher frequency RF fields are not transient in nature any more than any other frequency.

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Tags: safety radio waves arrl chapter 9 arrl module 23

Which of the following is an acceptable method to determine whether 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 these are correct. In most cases, either the FCC OET Bulletin 65 (found at http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet65/oet65b.pdf) or computer modeling are sufficient.

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Tags: safety radio waves arrl chapter 9 arrl module 23

What hazard is created by touching an antenna during a transmission?
  • Electrocution
  • Correct Answer
    RF burn to skin
  • Radiation poisoning
  • All these choices are correct

The human body absorbs RF energy and acts like a resistor. A resistor turns electrical energy into heat energy. If the body absorbs too much RF energy, it means that it will create enough heat to damage tissue in the form of a burn, a painful RF burn. Touching an antenna will not cause radiation poisoning because RF is non-ionizing radiation and thus lacks the energy required to cause it.

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Tags: antenna safety arrl chapter 9 arrl module 23

Which of the following actions can reduce exposure to RF radiation?
  • Correct Answer
    Relocate antennas
  • Relocate the transmitter
  • Increase the duty cycle
  • All these choices are correct

RF radiation "radiates" outward from the antenna, therefore, it is the antenna that must be moved. Relocating the transmitter will have no effect, provided the transmitter is connected to the antenna correctly. Increasing the duty cycle would have the reverse effect - it would increase your exposure.

Duty cycle is the ratio of how much time the transceiver spends transmitting to how much it spends receiving; thus, the more you transmit, the higher your duty cycle.

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Tags: safety fcc operating procedures antenna arrl chapter 9 arrl module 23

How can you make sure your station stays in compliance with RF safety regulations?
  • By informing the FCC of any changes made in your station
  • Correct Answer
    By re-evaluating the station whenever an item in the transmitter or antenna system is changed
  • By making sure your antennas have low SWR
  • All these choices are correct

Any time you change your equipment, it's a good idea to re-evaluate your station to ensure compliance. You do not inform the FCC because the amateur service is designed to be self-policing. The SWR of your system will only ensure efficient power transfer, not compliance with safety regulations.

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Tags: safety radio operation arrl chapter 9 arrl module 23

Why is duty cycle one of the factors used to determine safe RF radiation exposure levels?
  • Correct Answer
    It affects the average exposure to radiation
  • It affects the peak exposure to radiation
  • It takes into account the antenna feed line loss
  • It takes into account the thermal effects of the final amplifier

Duty cycle affects average exposure level, which is what the Maximum Permissible Exposure (MPE) limits are based on. A person is more likely to tolerate a one- time, high peak exposure of very short duration than a lower exposure over a long period of time. This is where duty cycle is important because it is a measure of the transmit and non-transmit times, which determines the average exposure. Antenna feedline losses and amplifier thermal effects have nothing to do with RF exposure.

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Tags: safety radio operation arrl chapter 9 arrl module 23

What is the definition of duty cycle during the averaging time for RF exposure?
  • The difference between the lowest power output and the highest power output of a transmitter
  • The difference between the PEP and average power output of a transmitter
  • Correct Answer
    The percentage of time that a transmitter is transmitting
  • The percentage of time that a transmitter is not transmitting

The duty cycle is made up of 'on' times and 'off' times. It is defined as the ratio of on-air time to total operating time. A 40% duty cycle means that 40% of the time, the station will be transmitting.

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Tags: radio operation arrl chapter 9 arrl module 23

How does RF radiation differ from ionizing radiation (radioactivity)?
  • Correct Answer
    RF radiation does not have sufficient energy to cause chemical changes in cells and damage DNA
  • RF radiation can only be detected with an RF dosimeter
  • RF radiation is limited in range to a few feet
  • RF radiation is perfectly safe

RF energy cannot break chemical bonds, they don't have the energy required to cause genetic damage.

Source: According to John E. Moulder (Biological Effects of Power-frequency Fields as they Relate to Carcinogenesis, University of Wisconsin, 1995, p. 209:309-324), professor of Radiation Oncology, because non-ionizing electromagnetic (RF) energy cannot break chemical bonds, they don't have the energy required to cause genetic damage.

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Tags: arrl chapter 9 arrl module 23

Who is responsible for ensuring that no person is exposed to RF energy above the FCC exposure limits?
  • The FCC
  • Correct Answer
    The station licensee
  • Anyone who is near an antenna
  • The local zoning board

A good rule of thumb for Amateur Radio -- always -- is that you are responsible for your own emissions. Any time you are transmitting you are responsible for the consequences.

This holds true whether you are talking about RF exposure limits, avoiding interference with other stations or devices, or even having safe harmonics which don't exceed limits on other bands and cause interference there. You should be remembered when you choose a transceiver -- even if you don't know you are causing a problem it is still your responsibility.

Thus you (the station licensee) are responsible for preventing unsafe RF exposure from your station.

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