NEAR ELECTROMAGNETIC FIELD OF HIGH FREQUENCY TRANSMITTING CURTAIN ANTENNAS BY JÜRGEN REICHE
Today's level of radiated power in the high frequency range has called for studies of harmful effects on the human body. An accurate determination of the near field-strength, electric as well as magnetic, in the vicinity of high power transmitting antennas is necessary to allow administrations to access any possible radiation hazard.
Based on many theoretical and experimental research data, recommendations have been proposed for the safety limits of human exposure to electromagnetic fields. This limits may differ from country to country. Basically the field levels are given for so-called controlled environment (locations where only experienced staff may be affected) and uncontrolled environment (locations where general population is living). There is an ongoing incentive to unify this limits on an international platform.
For planning purposes most countries apply today the IRPA recommendations for the maximum allowed field levels in order to establish safety zones within the surrounding area of the antennas. This zones are established based on a numerical calculations of near field contours as shown below, applying the grid method, i.e. the area near ground level around an antenna for example is split up in patches with a constant but selectable size in both directions of a rectangular co-ordinate system.
In case near-field contours are to be calculated for one antenna only, the center point of this system coincides with the antenna axis of symmetry produced to the level of this area above ground.
Electric NEAR-FIELD Contours around a curtain antenna
Calculated with Numerical Electromagnetic Code for the analysis of wire antennas and scatterers
Grid constant 100m
Summary contour (overlay) of three neighboured curtain antennas
All calculations are based on method of moment techniques, i.e. the Numerical Electromagnetic Code. This software is a user-oriented computer code for the analysis of the electromagnetic response of antennas, scatteres and other metal structures. It is built around the numerical solution of integral equation for the currents induced on the structure by sources or incident fields. This approach avoids many of the simplifying assumptions required by other solution methods and provides a highly accurate and versatile tool for electromagnetic analysis.
The code comprehends an integral equation specialised to wires to provide for convenient and accurate modelling of a wide range of structures. A model may include perfect or imperfect conductors, and lumped element loading.
The excitation may be either voltage sources on the structure or an incident plane wave of linear or elliptic polarisation. The output may include induced currents and charges, near electric or magnetic fields and radiated fields.
Hence the software is suited to either antenna analysis or scattering and EMC studies.
Antenna Model
Dipoles, towers and screen are modelled by wires of respective equivalent diameter and length.
Feeder lines are considered by TEM theory.
The potential hazard areas surrounding the transmitting antennas are established form near-field calculations, whereas the desired contours are extrapolated from electric and magnetic field samples which are obtained at rectangular or square grid locations by vector addition of the field components.
As a single curtain antenna may be operated over up to six adjacent short-wave frequency bands and up to 5 slew positions per frequency, and as state-of-the-art short-wave transmitters may be operated with DCC (dynamic carrier control) such that the transmitter power varies and causes additional calculations, there is only way to manage this flood of data, i.e. to focus on what can be seen as a worst case scenario. Also an overlay of contours may be required, which result from the fact that several antennas are operated at the same time.
From many hazard studies which we have established in the latest past we learnt that we need to focus on the magnetic near-field contour rather than the electric near-field contour, because the antenna is of horizontal polarisation and as a consequence the (tangential) electric field component is zero close to perfect ground and of negligible quantity if a good ground conductivity can be assumed.
Moreover we learnt that suspension height of the antenna above ground is as important as the frequency range of operation with focus on the higher bands.
This is illustrated comparing the spreading of the electric field contours shown above with the magnetic field contours shown below.
Magnetic NEAR-FIELD Contours around of a curtain antenna
Calculated with Numerical Electromagnetic Code for the analysis of wire antennas and scatterers
Grid constant 100m
Summary contour (overlay) of three neighboured curtain antennas
