High Frequency Electric Field
Dielectric heating arises when a high frequency electric field is established within a material which is a poor electrical conductor. The ease with which electrical energy can be transferred to a body as well as the distribution of that energy, depends on the dielectric properties of the various constituent materials which make up the substance. The main dielectric property of interest is the Loss Factor, which principally determines the amount of energy absorbed by the material for a given electric field.
It is important to realise that the dielectric loss factor is not constant but varies with a number of parameters such as frequency, moisture content and temperature. It is possible to find information in the literature giving values of loss factors for certain common materials. However, these are normally measured at ambient temperature and at equilibrium moisture content and should be regarded only as a guide, for a material at a given temperature and moisture content, there is usually a frequency (or resonance) which gives a maximum value of loss factor.
For example, water has a maximum dipolar loss factor at about 20GHz. However the loss factor at other frequencies is usually sufficient for an alternative to be satisfactory.
The power transferred to a dielectric is given by:
P = 2 π f εo εr tanδ E²
- P is the power of density in Watts/m³
- f is the frequency in Hertz
- εo is the permittivity of free space = 8.85 x 10-12 Farad/metre
- εr is the relative permittivity of the material to be heated
- tanδ is the loss tangent
- εr x tanδ = is the Loss Factor of the material being heated
- E is the RF field strength or voltage gradient within the material, in Volts/metre
The frequencies used for dielectric heating are not chosen because of any relationship with the resonance of water or other molecules but are specified by international regulation in order to minimise the risk of interference with telecommunications. The frequencies available include 13.56, 27.12 and 40.68 MHz in the radio frequency band and are usually referred to as “ISM” bands used exclusively for Industrial, Scientific and Medical applications.