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antenna reflector
"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 |
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