"a" wrote in message
...
a wrote:
I'm looking at making a parabolic reflector for a wifi link.

I'm trying to work out the distance required between the radiating
element and the reflector.
Should it be an odd number of quarter wavelengths, or an even number?

(An odd number of quarter wavelengths would be required if there is a 180
degree phase change on reflection, and
and even number would b required if there is no phase change).

So... is there a 180 degree phase change at the reflector?

TIA




Thanks for the replies.

I agree that the radiating element must be placed at the parabola focus to
give minimal beamwidth, and that this condition is met when the radiating
element is placed at the focus (which is given by D^2/16d).

The point remains that I can still choose the parabola parameters to set
the focal length to whatever is desired.
Should I choose them so that the focal length is an odd or even number of
quarter wavelengths?

What I really had in mind was a uniformly radiating element (ie a simple
whip) with a parabolic reflector behind it, like this:-
( x
reflector radiating element

To get the right-going signal from the reflector in phase with the
right-going signal from the radiating element I need to choose the
reflector distance correctly.

I have a feeling that there WILL be a phase inversion at the reflector but
I'm not certain.

The reason that I think that there might be a phase inversion is that the
(radiator plus reflector) could be considered to be a (radiator and its
image). At the (perfectly conducting) reflector the voltage will be zero
and the current will be infinite, which implies that, at the reflector,
the reflected wave must be phase inverted wrt the radiated wave.

Any thoughts?

Hi "a"

Just a thought -- I think you are concerned with the radiation from the
illuminator adding to (or canceling) the radiation from the reflector.
Normally, the illuminator is designed to establish a pattern that aims only
toward the reflector. But, since you dont wish to shape the radiation
pattern from the illuminator, you might want to consider that the total
pattern from both the illuminator will depend on how big the reflector is
(in wavelengths).
If the parabola is large, the half of the illuminator's radiation in the
"wrong" direction wont seriously effect the total pattern, and can be
ignored. You might assume the radiation from the reflector is concentrated
to be within a 20 degree wide sector (20 dB gain antenna). The radiation
from the illuminator in that 20 degree sector is so weak that it will not be
noticeable.

The fact is - There are many factors that a designer of parabolic
antennas considers. Google

Jerry