Radiation and Scattering

The radiation and scattering calculations deliver the following data:

NTF post-processing performs the near-to-far field transformation in the frequency domain to calculate the radiation patterns of a radiating structure, at frequencies specified by the user.

The far fields are available for any directions in space, with Reference Axis, Reference Point, angles and gain scaling options available for user's modifications. The far field patterns are calculated in a spherical coordinate system with one Reference Axis and two angles: elevation (Theta) and azimuthal (Phi).

It is worth noting at this point that QuickWave enables calculation of the scattering patterns of periodic structures, analysed using periodic boundary conditions. For this kind of problems the software automatically calculates the directions of higher diffraction order beams.

There are, however, cases when we may wish to calculate radiation patterns using only some of the NTF walls (pickup walls). A practical application would be to directly compare simulated and measured results, with near-field measurements taken over a single aperture. Another case is more FDTD-specific: due to numerical dispersion and the necessity to average either electric or magnetic field components across each pickup wall, a wave that physically propagates in one direction may numerically generate a non-physical backward lobe. A simple way to see whether a particular backlobe is physical or a numerical artefact is to disconnect the pickup surface "looking" in its direction from NTF transform and to check how the results change.

To exemplify this approach we consider the patch antenna example. The |Etheta| curve at 5.525 GHz has main beams around ±30^{o} but also two weaker beams around ±160^{o}. The user may wonder whether these are the actual downward propagating waves or numerical images of the upward ones.

To check whether EM energy is really radiated downwards by the considered patch, we exclude the bottom (–Z) wall from NTF transformation (the green curve). The ±160^{o} lobes are reduced, compared to the complete NTF transform, and the green curve is practically monotone for large angles. This indicates that the patch does radiate through the –Z pickup wall. By comparing radiation efficiency and radiated power in the two cases we conclude that 2.34% of the total power radiated by the antenna exits the NTF box through the –Z wall.

To check whether EM energy is really radiated downwards by the considered patch, we exclude the bottom (–Z) wall from NTF transformation (the green curve). The ±160

The region in which the radiating object (and thus the NTF box) is placed must be homogeneous and may be filled with a medium of arbitrary permittivity ε

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Directive – it refers to directive gain calculated with respect to the power radiated by the antenna. It is given as a unitless value.

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Power – it equals to Directive gain multiplied by radiation efficiency in quadratic scale (i.e., in linear scale, Power gain equals to Directive gain multiplied by a square root of antenna efficiency).

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Absolute_1port - it equals to Power gain multiplied by the coefficient of reflection loss (1-|S11|^{2}) in quadratic scale (i.e., a square root of this coefficient in linear scale).

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Absolute – it is calculated similarly to the Directive gain with this difference that here the power available from the source is taken as a reference.

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Relative – it is calculated through normalisation to the highest value of the displayed pattern (except for cross-polarisation patterns normalised to the highest values of the corresponding co-polarisation patterns).

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Fields at 1m – it refers to radiated field in the far zone scaled to 1m distance from the reference point using exp(-α*r)/r far field dependence.

In QW-V2D, for linear polarisation such results as Copl (co- polarisation) and Cxpl (cross polarisation) are additionally available.

If the analysed antenna is linearly polarised (one of Etheta or Ephi components is obviously dominant and the other is zero), the left- and right-handed polarisation curves practically overlap.

If the Etheta and Ephi curves are equal in magnitude and phase-shifted by 90 degrees, upon switching to circular polarisation circularly polarised wave will be observed with Eleft or Eright component being dominant.