### Re: How can EM radiation be concentrated on a specific section of an antenna?

Date: Sat Jan 29 00:20:58 2000
Posted By: Yaxun Liu, Grad student, Electrical Engineering, National University of Singapore
Area of science: Physics
ID: 949043401.Ph
Message:
```
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.

```

Current Queue | Current Queue for Physics | Physics archives