Safe Distance From Hf Antenna

[Takako Dito]

SoI have the misfortune of being an apartment dweller. I know that 50 watts is pretty safe as it is what first responders use on 2 meters. Am I really going to have to run around with an analyzer checking safety? That is likely to cause some probably unwarranted alarm. I can't use more than 200 watts anyway as my gear isn't rated for higher nor does it make sense to purchase such with other human beings in such close proximity. My radio itself won't do more than 100 watts without an amp.

The FCC has published a bulletin OET 65B that has a table showing for VHF you don't need to evaluate the RF hazard if your effective power is 50 watts or below. For 70 cm UHF the limit is 70 watts. This does not mean it is not safe above those limits, it just means you must do an evaluation.

Another table shows safe distances from antennas. It distinguishes between uncontrolled access (outside) and controlled (in your house or fenced in yard). For VHF with a 0 gain antenna in a uncontrolled environment you need a separation of 3.2 meters. So if the antenna is on a 12 foot pole you are ok.

The FCC is amending part 97 with regards to exposure limits. This will require radio operators to perform RF exposure evaluations regardless of power levels. Previous low power stations were not required to do this. The ARRL is working with the FCC to develop easy to use online RF calculators to make this task easier.

As I recall, you're under no obligation to do RF field safety checks for radiated power up to 100 W. The only significant safety concern (assuming you're running solid state, as opposed to old vacuum tube equipment) is RF burns from actually touching the antenna other radiating element.

This is a concern, but it's worth noting that an antenna radiates just as well if made from insulated wire as it does in bare metal, so the touch exposure can be limited to connectors and cut wire ends (which can all be enclosed in heat shrink or otherwise insulated).

FWIW, if you have an antenna that's halfway efficient and reasonably well matched, your main contact range limitation on 20 and 40 meters is band conditions (which suck just now, early 2020, with the solar minimum). Even so, either of those bands should be able to reach hundreds of kilometers with 100 W and an omni antenna, so long as you aren't sending almost all of your power upward.

Note that as of May 3, 2021 the FCC dropped the minimum power rule, and now all services, including amateur radio and with no lower power limit, are required to do a environmental RF exposure study. This rule change is after most of the answers in this question, so it invalidates them in part.

As far as radiated energy, you're safe. At 100W on 40m, if you touch the antenna wire, you run the risk of RF burns (ask me how I know), and perhaps similar on 2m. But as far as problems from the RF floating around the room, it's a non-issue. The other thing that might come up is RFI, your signal getting into their TVs and stereos and such. But that's just an annoyance, not a safety issue.

I am putting a 40m antenna in my attic and I was wondering would it be safe to have a shack in the room right below the attic (10ft of coax or less) or should I have my shack in the basement/ground level (50ft coax).

Various organizations have issued guidelines on maximum permissible exposure. Below a certain power, the station can be assumed harmless. Above that power, some more careful design is prudent. Your station is mostly below that threshold, except possibly on the upper HF bands.

As you say you intend to operate 100 watts on HF, and 50 watts on VHF, you are below these thresholds except on 12 and 10 meters. So if you want to be safe, reduce power a little bit on these bands and there should be no issue at all having the shack near the antenna.

The FCC regulates RF exposure for safety and health. RF exposure must be evaluated on a station-by-station basis. As a free public service, the American Radio Relay League (ARRL) makes available a free download of RF Exposure and You by staff member Ed Hare, W1RFI. Chapter 5 contains the references needed to complete the RF Exposure Station Evaluation and Exemption Worksheets. The League has published numerous works over the years, including some that can help you interact with neighbors on the subject, if needed.

The RF waves emanating from an antenna are generated by the movement of electrical charges in the antenna. Electromagnetic waves can be characterized by a wavelength and a frequency. The wavelength is the distance covered by one complete cycle of the electromagnetic wave, while the frequency is the number of electromagnetic waves passing a given point in one second. The frequency of an RF signal is usually expressed in terms of a unit called the "hertz" (abbreviated "Hz"). One Hz equals one cycle per second. One megahertz MHz equals one million cycles per second.

Different forms of electromagnetic energy are categorized by their wavelengths and frequencies. The RF part of the electromagnetic spectrum is generally defined as that part of the spectrum where electromagnetic waves have frequencies in the range of about 3 kilohertz (3 kHz) to 300 gigahertz (300 GHz). Microwaves are a specific category of radio waves that can be loosely defined as radiofrequency energy at frequencies ranging from about 1 GHz to 30 GHz. (Back to Index)

"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. (Back to Index)

The most important use for RF energy is in providing telecommunications services. Radio and television broadcasting, cellular telephones, personal communications services (PCS), pagers, cordless telephones, business radio, radio communications for police and fire departments, amateur radio, microwave point-to-point links and satellite communications are just a few of the many telecommunications applications of RF energy. Microwave ovens are an example of a non-telecommunication use of RF energy. Radiofrequency radiation, especially at microwave frequencies, can transfer energy to water molecules. High levels of microwave energy will generate heat in water-rich materials such as most foods. This efficient absorption of microwave energy via water molecules results in rapid heating throughout an object, thus allowing food to be cooked more quickly in a microwave oven than in a conventional oven. Other important non-telecommunication uses of RF energy include radar and industrial heating and sealing. Radar is a valuable tool used in many applications range from traffic speed enforcement to air traffic control and military surveillance. Industrial heaters and sealers generate intense levels of RF radiation that rapidly heats the material being processed in the same way that a microwave oven cooks food. These devices have many uses in industry, including molding plastic materials, gluing wood products, sealing items such as shoes and pocketbooks, and processing food products. There are also a number of medical applications of RF energy, such as diathermy and magnetic resonance imaging (MRI). (Back to Index)

An RF electromagnetic wave has both an electric and a magnetic component (electric field and magnetic field), and it is often convenient to express the intensity of the RF environment at a given location in terms of units specific to each component. For example, the unit "volts per meter" (V/m) is used to express the strength of the electric field (electric "field strength"), and the unit "amperes per meter" (A/m) is used to express the strength of the magnetic field (magnetic "field strength"). Another commonly used unit for characterizing the total electromagnetic field is "power density." Power density is most appropriately used when the point of measurement is far enough away from an antenna to be located in the "far-field" zone of the antenna.

The quantity used to measure the rate at which RF energy is actually absorbed in a body is called the "Specific Absorption Rate" or "SAR." It is usually expressed in units of watts per kilogram (W/kg) or milliwatts per gram (mW/g). In the case of exposure of the whole body, a standing ungrounded human adult absorbs RF energy at a maximum rate when the frequency of the RF radiation is in the range of about 70 MHz. This means that the "whole-body" SAR is at a maximum under these conditions. Because of this "resonance" phenomenon and consideration of children and grounded adults, RF safety standards are generally most restrictive in the frequency range of about 30 to 300 MHz. For exposure of parts of the body, such as the exposure from hand-held mobile phones, "partial-body" SAR limits are used in the safety standards to control absorption of RF energy (see later questions on mobile phones). (Back to Index)

Biological effects can result from exposure to RF energy. Biological effects that result from heating of tissue by RF energy are often referred to as "thermal" effects. It has been known for many years that exposure to very high levels of RF radiation can be harmful due to the ability of RF energy to heat biological tissue rapidly. This is the principle by which microwave ovens cook food. Exposure to very high RF intensities can result in heating of biological tissue and an increase in body temperature. Tissue damage in humans could occur during exposure to high RF levels because of the body's inability to cope with or dissipate the excessive heat that could be generated. Two areas of the body, the eyes and the testes, are particularly vulnerable to RF heating because of the relative lack of available blood flow to dissipate the excess heat load.

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