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Subelement T0
Electrical safety: AC and DC power circuits; antenna installation; RF hazards
Section T0C
RF hazards: radiation exposure; proximity to antennas; recognized safe power levels; exposure to others; radiation types; duty cycle
What type of radiation are VHF and UHF radio signals?
  • Gamma radiation
  • Ionizing radiation
  • Alpha radiation
  • 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.)

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Tags: radio waves uhf vhf safety arrl chapter 9 arrl module 18

Which of the following frequencies has the lowest value for Maximum Permissible Exposure limit?
  • 3.5 MHz
  • 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.

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Tags: 6 meter arrl chapter 9 arrl module 18

What is the maximum power level that an amateur radio station may use at VHF frequencies before an RF exposure evaluation is required?
  • 1500 watts PEP transmitter output
  • 1 watt forward power
  • 50 watts PEP at the antenna
  • 50 watts PEP reflected power

The FCC considers power levels less than 50 watts PEP as not being a significant risk to people and has issued rules excluding stations operating in the range from performing an evaluation. Obviously, this includes the smaller hand-held radios. Therefore, when a station reaches or exceeds 50 watts PEP at the antenna, an RF exposure evaluation is required.

Reflected power depends on what it's being reflected by, so that choice can be discarded.

Not many radios use less than 1 watt, so that can be discarded too.

RF exposure is measured at the antenna, not at the transmitter output.

For More information on PEP (peak envelope power) go to http://www.radio-electronics.com/info/t_and_m/rf-microwave-power-meter/peak-envelope-average-pulse-power.php

tl;dr, its the maximum power or spike during a cycle.

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Tags: safety vhf antenna transmit power arrl chapter 9 arrl module 18

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
  • All of 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 18

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
  • The human body absorbs more RF energy at some frequencies than at others

As noted in question C02, the body absorbs energy differently at different frequencies. This is because your body has a resonant frequency. (Actually several, depending on the body part.) The other answers listed are not correct. 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 18

Which of the following is an acceptable method to 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
  • All of 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 18

What could happen if a person accidentally touched your antenna while you were transmitting?
  • Touching the antenna could cause television interference
  • They might receive a painful RF burn
  • They might develop radiation poisoning
  • All of 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.

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Tags: antenna safety arrl chapter 9 arrl module 18

Which of the following actions might amateur operators take to prevent exposure to RF radiation in excess of FCC-supplied limits?
  • Relocate antennas
  • Relocate the transmitter
  • Increase the duty cycle
  • All of 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 18

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
  • By re-evaluating the station whenever an item of equipment is changed
  • By making sure your antennas have low SWR
  • All of 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 18

Why is duty cycle one of the factors used to determine safe RF radiation exposure levels?
  • It affects the average exposure of people to radiation
  • It affects the peak exposure of people 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 and it is the average exposure 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 18

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
  • 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 50% duty cycle means that 50% of the time, the station will be transmitting.

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Tags: radio operation arrl chapter 9 arrl module 18

How does RF radiation differ from ionizing radiation (radioactivity)?
  • RF radiation does not have sufficient energy to cause genetic damage
  • 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 18

If the averaging time for exposure is 6 minutes, how much power density is permitted if the signal is present for 3 minutes and absent for 3 minutes rather than being present for the entire 6 minutes?
  • 3 times as much
  • 1/2 as much
  • 2 times as much
  • There is no adjustment allowed for shorter exposure times

Exposure to radiation is measured by the amount of energy (E) absorbed from that radiation. Power density (PD) is the amount of energy absorbed per unit of time from a given distance squared, so

PD = E / time / r2, or E = PD X time X r2

For a six-minute average exposure,

E6 = PD6 X 6 minutes X r2= 6PD6r2

But if the same signal was applied for three minutes on and then three minutes off from the same distance,

E3 = PD3 X 3 minutes X r2 + 0 X 3 minutes X r2= 3PD3r2.

If the same exposure (maximum permitted) is experienced in both cases,

E6 = E3, or substituting,

6PD6r2 = 3PD3r2, so that

PD3 = 2PD6

Therefore, twice as much power density would be permitted for the three-minute-on-then-off case as with the six-minute-full-on case, so the answer is 2 times as much.

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