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?