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Method of determination of the electromagnetic field energy stream density

Method of determination of the electromagnetic field energy stream density in the area of action of high-frequency radiations.

Oleg Kapral.

On frequencies more than 300 MHz the level of radiation from the transmitting devices is determined by the electromagnetic field energy stream density and its legitimate values W, W/м², or мкW/см² (1 W/м² = 100 мкW/см²).

W = P/S,

where P – power, W, (selected, or dispersed on the area S, м².)

Error at the measuring of energy stream density of high-frequency radiations is taken to exactness of determination of amplification factor aerial and exactness at determination of standing-wave ratio, SWR (or travelling-wave factor TWF, TWF = 1/ SWR ).

A method allows to conduct estimation of radiant high-frequency energy and define possible norms and safe areas of people staying. If in the area two or more sources are found, the calculations must be conducted for every source, and possible norms are determined accordant ratified methods.

For computation we apply isotropic aerial with the amplification factor  D = 1, The effective area of antenna is determined by a formula:

where  – wave-length of high-frequency signal, m/sec. – speed of light in an environment,  – frequency on which is produced measuring, Hz), if SWR = 1.  Fig. 1. Effective area of the antenna for wave-length .

Fig. 1. Effective area of the antenna for wave-length .

The formula for determination of electromagnetic field high-frequency radiations energy stream density:

, or:

Power of transmitter, antenna’s amplification factor, directional pattern, and distance to the antenna are basic data for determination of electromagnetic field energy stream density. In this case the power P from a radiant will determine the energy stream density on definite distance from the transmitter. The error of method will be taken to the error of determination of aerial’s amplification factor of passing device, by determination (or measuring) of its directional pattern and radiation efficiency which depends on SWR (SWR - standing-wave ratio)  (if SWR = 1.2, the error from none concordance does not exceed ± 3.5 %).

In same queue power from a source on definite distance relies on attenuation of high-frequency signal in the atmosphere and determined by a formula:

where – damping factor of electromagnetic energy in an environment, r – distance to the radiant.

During conducting of computations it is necessary to pay attention with what exactness set, or measured parameters such as: power of transmitter, directional pattern of aerial of transmitter and damping factor, for power 3 dB - 100%, 2 dB - 60%, 1 dB - 25%, 0.5 dB - 12% and etc.

added 15.11.08 01:04:30 | looked over 6632 times