The simplified method, and in particular the system base-RF-power determination used in the simplified method equation, only applies to far-field data. Why? Well, as an example, the radiation polar-plot provided by antenna suppliers is far-field data and in no way represents how the field-strength varies with angle close to the antenna.
So where exactly is the near-field / far-field divide? That is, where does the simplified method no longer apply? We could talk about this until the cows come home and get nowhere, because basically one man’s near-field is another man’s far-field, with both parties eager to prove how the dividing line was derived quoting various permutations of wavelength and Pi as the precise answer.
The Near-Field / Far-Field Decree
So for the purposes of this blog-thread we are going to create a dictate. As regards RF waves surrounding a radiating antenna, when a wave is an independent entity the wave is in the far-field. By independent we mean the excitation signal feeding the source can do a backward-summersault for all the wave cares, it has already escaped the influence of the source and will carry on along it’s merry way at the speed of light (in vacuo).
Conversely, if a change in the excitation signal causes a change in a surrounding RF wave, that wave is in the near-field. For example, if the excitation source to a resonating dipole was suddenly removed the voltage and current standing waves present across the resonating structure would die away almost instantaneously.
We complete the dictate by stating that the near-field / far-field dividing line is at 3 wavelengths.
Effect on the Simplified Method
The new decree means the lowest frequency at which the simplified method is valid is 300MHz. The determination is easy enough:-
Our test distance is 3 meters. Obeying the decree means that in order for the calibration plane to just be in the far-field, the 3 meter distance must represent 3 wavelengths. Therefore the limiting wavelength is 1 meter. Using c = f/Lambda, this corresponds to a lowest frequency of use of 300MHz.
The Antenna Suppliers’ Get-Around
Given antenna suppliers sell antennas starting at 80MHz (wavelength 3.75 meters) into this 3m RF immunity test market, how do they get around that the supplied gain and radiation pattern data only applies at far-field distances? (This of course is where the “yes it is, no it isn’t in the near-field” brigade kick in and try to claim that 3m distance is far-field even at 80MHz. And this of course is why we ‘cut them off at the pass’ by announcing the decree).
The supplier get-around is to provide data on how much RF power is required to produce a particular test-field at a particular test-distance. The antenna supplier is not forthcoming with (or doesn’t know) how much of the test-field is created by field type, that is how much of the measured field is contributed by the far-field and how much by the near-field. In fact you will find that the only data supplied is boresight data. That is the data represents the field we would measure with a field-probe mounted at the center of the calibration plane. No guarantee is given regarding the field achieved elsewhere across the calibration plane. Why does this matter? Answer – we are designing the test system under ideal conditions (perfect, very large fully-anechoic chamber). The premise being that if we are unable to generate a compliant field across the calibration plane under these ideal conditions, we do not stand ‘a snowflake’s chance in hell’ of achieving one under real 3 meter semi-anechoic chamber conditions. For instance, what if the measured field across the plane was to vary due to multiple peaks and nulls in the near-field radiation pattern?
To be continued…