|MadSci Network: Physics|
When antennas are used for transmitting electromagnetic signals, generally there is quite a large distance between the transmitter and the receiver, compared either with the wavelength or the size of the antennas. Therefore, from the viewpoint of the transmitter, the receiver is like a point with no size. The transmitter can only control the direction of transmitting. From the viewpoint of the receiver, the incident wave is like planar wave, which is evenly distributed across the direction of propagation. If the wavelength is not very large (in meters), you may consider using a parabolic reflector behind the receiving antenna. Just put the specific point of your receiving antenna at the focus of the parabolic reflector. However, remember that you can not eliminate the electromagnetic fields at other parts of your receiving antenna. It is hard to accurately control or "synthesize" the spatial distribution of electromagnetic fields. The electromagnetic fields depend on the conductive currents in metals and equivalent currents in dielectrics. We are able to control the amplitude and phase of the currents at one or several feeding points of the transmitting antenna, but we can not control the induced currents in metals and dielectrics. These currents are determined by the Maxwell's equations. Given the currents at all feeding points and the geometry and electromagnetic properties of the materials, we can calculate the electromagnetic fields by using numerical methods. By adjusting the position of the feeding points, the relative amplitude and phase of the currents at these feeding points, and the shape and material of surrounding structures we can gain some kind of controls of the electromagnetic fields, but we can not predict the results until we do a numerical computation.
Try the links in the MadSci Library for more information on Physics.