The term “radiation” simply refers to the emission and propagation of energy in the form of waves or particles.
Electromagnetic (EM) radiation is a radiation consisting of particles called photons that act in some respects like a wave. It is caused by an oscillating electric charge.
EM is characterised by the frequency of the wave. The frequency is the number of full waves that travel past a point in a given time – this is the number of crests or equivalently the number of troughs. When the unit of time is a second the frequency is expressed in Hertz (abbreviation: Hz). Therefore a frequency of 434 MHz indicates a wave in which 434 million waves travel past a point in each second.
All EM waves travel at the same speed. This is a universal constant known as the velocity of light (c) and is approximately 300,000 km / sec (3 x 108 m / s) – the phase velocity.
The frequency and wavelength of a wave are inversely proportional to one another, to calculate the speed of the wave; one is divided by the other. For example, in the case of 434 MHz this implies an in vacuo wavelength of 69.1 cm (less inside matter such as a patient’s body).
It includes the range of the EM spectrum: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays. The divisions between these parts of the spectrum reflect differences in how the energy is created or detected rather than specific phenomena. For example, gamma rays (those produced by a LINAC) are basically high energy x-rays.
EM radiation carries energy – each photon is an indivisible packet of energy (E) and the amount of this energy is proportional to its frequency). Photons have no mass but they still carry momentum (p), that is to say, they push things that they strike, albeit a very small push.
EM radiation is classified into ionising and non-ionising radiation, based on whether it is capable of ionising atoms and breaking chemical bonds. Radiation with energies in the ultraviolet (UV) band or higher is ionising.
Radiowaves are not ionising – the only known biological effect they have on the human body is heating.
Radio frequency (RF) waves
Radiofrequency (RF) radiation is a portion of the electromagnetic (EM) spectrum between frequencies of 10 kHz up to infra-red (less than 300 GHz). It is a non-ionising form of radiation.
Ultra-high frequency (UHF) is a band of the radio spectrum designated to be between the frequencies of 300 MHz and 3 GHz (3,000 MHz). The frequency of 434 MHz used in radiowave therapy is located towards the lower end of this band.
Radiowaves and humans
The amount of RF energy that is absorbed (deposited) into human body tissue and the rate at which the energy decreases with the depth of penetration depends on both the type of tissue that the energy passes through and on the frequency of the incident radiation. Radio frequencies below 4 MHz the body is essentially transparent to the energy. As the frequency is increased more energy is absorbed by the human body. At high frequencies (around 1 GHz or more) there is scattering. Increasing the frequency of electromagnetic energy still further the point is reached when the body reflects radiation (visible light).
When electromagnetic radiation passes from one medium to another, the direction, velocity and wavelength of the wave changes.
Safety of radiowaves
When radiofrequency waves enter the human body a portion of the energy is absorbed and causes molecules in the body to vibrate. On a macroscopic level these vibrations are what we experience and measure as heat (NIOSH 1987): The extent of heating is primarily dependent on the water content of the tissue and the intensity and duration of the RF energy. Most parts of the body have sufficient blood supply to dissipate heat resulting from absorption of RF radiation.