I'm doing research on electromagnetic radiation pollution emitted by a radio tower and I need to measure the power that reaches in each point of the space that surrounds the tower.
Hi Ramiro,
in the vicinity of antennas, the electromagnetic (near) field can be split (theoretically) into a part whose energy oscillates away and back toward the antenna, and a part (radiation) whose energy leaves the antenna without returning. Beyond a certain distance from the antenna (usually of the order of magnitude of the wavelength) the latter part dominates.
As long as the radiation propagates without attenuation (e. g. through dry air), the total power flow through a spherical shell has to be independent of the radius of the shell (conservation of energy). Intensity is the power flow per unit area, so the product of intensity and the area of the (imaginary) spherical shell has to be constant. Since the surface area of a sphere is proportional to the square of its radius r, the intensity is proportional to 1 / r2.
The same ist true for a hemisphere above ground.
While some antennas, like a simple vertical monopole, show no azimuthal directivity, i. e. the intensity does not depend of the azimuthal angle, others are highly directional up to beam-like radiation fields; i. e. the intensity depends on the azimuthal angle as well as on the elevation angle. If you are interested in effects on the ground and you have to deal with beams, it is possible that in some directions the intensity near the ground is very low near the radio tower but increases further away. (Imagine the beam of a lighthouse in a clear night: Near the lighthouse it's quite dark but in some distance you can read a newspaper while the beam sweeps over you.)
In order to apply the 1 / r2 rule in such cases, you had to measure the intensity at different points on a straight line touching the center of the antenna.
While the kind of modulation might have an impact on irradiated biological systems , the 1 / r2 law is independent of the way the carrier is modulated.
Dear Joerg Fricke, Thank you very much for the answer to my question, in the future of the investigation I will take advantage of these topics related to the calculations of intensity depending on the distance
- Badges
- Science topic