The radiation boundary of Antenna
I would like to know any difference between using different shape (Cylinder, Box, Sphere etc) for defining radiation boundary? Any concerns about the shape of boundary?
Best Regards,
Le :o
For an omni-directional antenna such as the "dipole", it is more accurate to have the shape of the air box as "cylindrical". While for a planar antenna, the radiation is mainly in boresight direction, hence an rectangular air box is adequate.
To further clarify things, take the rectangular airbox for example, which radiation boundaries are defined on all faces of the airbox. When the wave hit one of the face normal to it, the wave doesn't reflect back, which is an ideal situation for the radiation boundary. But if the wave hit the face at angle almost 90 degree but not quite, the wave is reflected with a reduced magnitude. This could effect the on-coming wave and could destroy the ideal radiation pattern of the antenna.
Going back with dipole antenna, if this was placed inside a rectangular box, the percentage of wave arriving at an angle on to the radiation boundary is quite high, worst case is when arriving at the edge of the box which most wave landed in high magnitudes gets reflected back. Having the cylindrical box, the matter is reversed, the wave actually arrive normal to the boundary and they get absorbed rather than reflected.
If rectangular airbox is a must in your structure, then one should consider using the Perfectly Matched Layered (PML) boundary, as this is fairly efficient even for the wave arriving at an angle. Moreover, the airbox size can be reduced, the boundary can be down to Lamda/10 away from the nearest radiating object compare to Lamda/4 with the Radiation boundary.
Hope this help!