Radiofrequency and Microwave Radiation
Electromagnetic radiation consists of waves of electric and magnetic energy moving together (i.e., radiating) through space at the speed of light. Taken together, all forms of electromagnetic energy are referred to as the electromagnetic “spectrum.” Radio waves and microwaves emitted by transmitting antennas are one form of electromagnetic energy. They are collectively referred to as “radiofrequency” or “RF” energy or radiation. Note that the term “radiation” does not mean “radioactive.” Often, the terms “electromagnetic field” or “radiofrequency field” are used to indicate the presence of electromagnetic or RF energy.
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.
Radiation having a wide range of energies forms the electromagnetic spectrum and has two major divisions:
Radiation that has enough energy to move atoms in a molecule around or cause them to vibrate, but not enough to remove electrons, is referred to as “non-ionizing radiation.” The energy levels associated with RF and microwave radiation are not great enough to cause the ionization of atoms and molecules, and RF energy is, therefore, is a type of non-ionizing radiation.
Radio waves broadcasting
Radiation that has Radio waves broadcasting enough energy to remove tightly bound electrons from atoms, thus creating ions, is referred to as “ionizing radiation.” “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.
X-ray and Gamma ray.
How is Radiofrequency Radiation Measured?
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.
Power density is defined as power flow per unit area. For example, power density is commonly expressed in terms of watts per square meter (W/m2), milliwatts per square centimeter (mW/cm2), or microwatts per square centimeter (µW/cm2). One mW/cm2equals 10 W/m2, and 100 µW/cm2 equal one W/m2. With respect to frequencies in the microwave range, power density is usually used to express intensity of exposure.
Quantity for measurement
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).
Specific Absorption Rate (SAR)
A SAR value is a measure of the maximum energy absorbed by a unit of mass of exposed tissue of a person using a mobile phone, over a given time or more simply the power absorbed per unit mass. SAR values are usually expressed in units of watts per kilogram (W/kg) in either 1g or 10g of tissue.
where σ is the tissue conductivity (S/m), E is the rms electric field strength induced in the tissue (V/m) and ρ is the mass density (kg/m3).
The Exposure standards
The Exposure standards set safety limits for the public and workers that are intended to provide protection against all established health hazards. They usually provide basic restrictions, the maximum allowable RF energy deposited in the body, and reference levels, external field levels that are more easily measured for compliance purposes.
Most countries around the world require or recognize RF exposure limits based on guidelines established by the International Commission on Non-Ionizing Radiation Protection (ICNIRP).Both the World Health Organization (WHO) and the International Telecommunications Union (ITU) recommend the adoption of ICNIRP guidelines as national exposure standards.
ICNIRP Reference Levels
RF Exposure from Cellular Base Stations
Base station antennas transmit RF electromagnetic fields (also called radio waves or EMF) in patterns that are typically very narrow in the vertical direction (height) but quite broad in the horizontal direction (width). The RF field intensity generally decreases rapidly the greater the distance from the antenna, but because of the narrow vertical spread of the beam, the RF field intensity on the ground directly below the antenna is also extremely low. RF measurements near base station sites typically show public exposures to radio waves that are hundreds or even thousands of times below the ICNIRP exposure limits.
NIRP Exposure Limits for General Public:
According to the World Health Organization (WHO):
Recent surveys have shown that the RF exposures from base stations range from 0.002% to 2% of the levels of international exposure guidelines, depending on a variety of factors such as the proximity to the antenna and the surrounding environment.
This is lower or comparable to RF exposures from radio or television broadcast transmitters.
Fig: Spectrum plot of typical radio communications signal levels in a community
When a mobile phone is switched on, it listens for specific control signals from nearby base stations. When it has found the most suitable (usually the nearest) base station in the network to which it subscribes, it initiates a connection. The phone will then remain dormant, just occasionally updating with the network with information such as location, until the user wishes to make a call or is called.
Mobile phones use Adaptive Power Control as a means of reducing the transmitted power to the minimum possible whilst maintaining good call quality. This reduces interference between mobile phone calls and also prolongs battery life and, hence, extends talk time. The output power of mobile phones is very low. During a call, and depending on whether it is a 3G or 2G handset, the output power can vary between a minimum level of less than 1 μW up to a peak level of 2 W. The maximum average power of a handset is however less than 0.25 W.
No adverse health effects are expected from continuous exposure to the RF radiation emitted by the antennas on mobile and phone base station towers.