Module 21 - RADIO FREQUENCY EXPOSURE
- READING ASSIGNMENT
- NON-IONIZATING RADIATION
- RF EXPOSURE
- RF INTENSITY
- DUTY CYCLE
- EXPOSURE EVALUATION
- QUIZ 21
- RECOMMENDED READING
Read Chapter 9, Pages 9-5 to 9-10 in your text before you continue.
"Ionization" is a process by which electrons are stripped from atoms and molecules. This process can produce molecular changes that can lead to damage in biological tissue, including effects on DNA, the genetic material of living organisms. This process requires interaction with high levels of electromagnetic energy.
Those types of electromagnetic radiation with enough energy to ionize biological material include X-radiation and gamma radiation. Therefore, X-rays and gamma rays are examples of ionizing radiation.
The energy levels associated with RF and microwave radiation, on the other hand, are not great enough to cause the ionization of atoms and molecules, and RF energy is, therefore, is a type of non-ionizing radiation. Other types of non-ionizing radiation include visible and infrared light.
Often the term "radiation" is used, colloquially, to imply that ionizing radiation (radioactivity), such as that associated with nuclear power plants, is present. Ionizing radiation should not be confused with the lower-energy, non-ionizing radiation with respect to possible biological effects, since the mechanisms of action are quite different.
Ham radio RF is non-ionization radiation.
We are only concerned with exposure caused by an amateur transmitter. Receiver emissions are so tiny they are not even considered an issue.
Exposure to high levels of RF can cause problems. If precautions are taken, RF exposure is minimal and not dangerous. The problem is RF energy can heat body tissues. Heating depends on the RF intensity and frequency.
Exposure standards for radiofrequency energy have been developed by various organizations and countries. These standards recommend safe levels of exposure for both the general public and for workers. In the United States, the FCC has adopted and used recognized safety guidelines for evaluating RF environmental exposure since 1985.
The exposure guidelines are based on thresholds for known adverse effects, and they incorporate prudent margins of safety. In adopting the most recent RF exposure guidelines, the FCC consulted with the EPA, FDA, OSHA and NIOSH, and obtained their support for the guidelines that the FCC is using.
There are hundreds of thousands of amateur radio operators ("hams") worldwide. Amateur radio operators in the United States are licensed by the FCC. The Amateur Radio Service provides its members with the opportunity to communicate with persons all over the world and to provide valuable public service functions, such as making communications services available during disasters and emergencies. Like all FCC licensees, amateur radio operators are required to comply with the FCC's guidelines for safe human exposure to RF fields. Under the FCC's rules, amateur operators can transmit with power levels of up to 1500 watts. However, most operators use considerably less power than this maximum. Studies by the FCC and others have shown that most amateur radio transmitters would not normally expose persons to RF levels in excess of safety limits. This is primarily due to the relatively low operating powers used by most amateurs, the intermittent transmission characteristics typically used and the relative inaccessibility of most amateur antennas. As long as appropriate distances are maintained from amateur antennas, exposure of nearby persons should be well below safety limits.
The strength of RF fields is greatest at its source, and diminishes quickly with distance. Access near base station antennas is restricted where RF signals may exceed international exposure limits. Recent surveys have indicated that RF exposures from base stations and wireless technologies in publicly accessible areas (including schools and hospitals) are normally thousands of times below international standards.
The strength9-9 of RF fields vary depending upon transmitter power & frequency, antenna gain, antenna pattern, and proximity & Mode duty cycle. The more time the power output is at high level, the higher the intensity.
How close someone is to the antenna is a large factor in determining exposure. There are two environment classes used in determining allowable power levels of your transmitter.
Controlled Environment: You know where people are standing in relation to your antenna and you can do something about it. More power is allowed because you can make adjustments if needed.
Uncontrolled Environment: You have no idea, or have no control of people near your antenna. Less power is allowed because you have to assume the worse case scenario.
In addition, guidelines for maximum permissible exposure are different for different transmitting frequencies. This is due to the finding (discussed above) that whole-body human absorption of RF energy varies with the frequency of the RF signal. The most restrictive limits on whole-body exposure are in the frequency range of 30-300 MHz where the human body absorbs RF energy most efficiently when the whole body is exposed. For devices that only expose part of the body, such as mobile phones, different exposure limits are specified (see below). The exposure limits used by the FCC are expressed in terms of SAR, electric and magnetic field strength and power density for transmitters operating at frequencies from 300 kHz to 100 GHz
When body parts act like antennas, those parts absorb RF energy at certain frequencies (wavelengths) more efficiently and increase risk. Therefore, RF exposure risk varies with frequency. More caution is dictated at some frequencies more than other frequencies.
Duty cycle is the ratio of the transmitted signalâ€™s on-the-air time to the total operating time during the measurement period.
Since the effects of RF exposure are related to heating and take place over many seconds, the maximum permissible exposure limits are based on averages, not peak exposure. This allows exposure to be averaged over fixed time intervals.
The averaging period is 6 minutes for controlled environments & 30 minutes for uncontrolled environments. The difference in averaging periods reflects the difference in how long people are expected to be present and exposed.
During the averaging period, a transmitter may only be generating RF for a fraction of the time. For most amateur contacts, the transmitter is keyed no more than 50% of the time and usually much less. This pattern lowers the duty cycle of the emissions. Since duty cycle affects the average power level of transmissions, it must be considered when evaluating exposure. So really, since the time spent transmitting on the air is low, this will reduce average exposure. Some modes also have lower average power than others.
All fixed stations must perform an exposure evaluation. Several methods are available to do this: By calculation based on FCC OET Bulletin 65; By calculation based on computer modeling; By measurement of field strength using calibrated equipment.
At lower power levels, no evaluation is required. This low level varies with frequency - example: below 50 W at VHF. Consult the table on the right to see if you even need to do an evaluation.
If you do have to do a calculation, an easy caculator is available on line.
You do need to have some information available before you can use the calculator. For example: Transmitter Power; Frequency Band(s) used; Antenna Gain (dipole is already set in program); Antenna Height (for controlled area); Distance from Antenna to Uncontrolled area; Feedline Type & Length; any other losses such as tuner or filters.
The calculator is very versatal and covers every necessary calculation to satisfy the FCC requirements.
A free download of the calculator and instructions are available by clicking here.
Relocating antennas is one way to reduce RF exposure. Also, regardless of the exposure evaluation results, make sure that people cannot come into contact with your antennas - RF burns are painful
Your quiz will tell you what information you need to know. Pass the quizzes - pass the exam!
Tips on how to remember the correct answer are included.
You can take the quiz as often as you wish.
No one but you will see the quiz results.
A copy of the FCC regs on RF Exposure is available by clicking here.
Review the Term Glossary in Chapter 10 of your Ham Radio License Manual
Practice Exams are available at www.arrl.org/exam-practice and there are links to many other test practice sites also.