For microwave. Is it any insulator? Rubber will work?
A typical parabolic antenna consists of a METAL parabolic reflector.The reflector can be of sheet metal, metal screen, or wire grill construction, and it can be either a circular "dish" or various other shapes to create different beam shapes. A metal screen reflects radio waves as well as a solid metal surface as long as the holes are smaller than one-tenth of a wavelength
There was a discussion on PF about it some time ago:
Thanks for the link jedishrfu. Adjent (or another): are you sure that the shape of the reflector changes 'beam shape'? What are the benefits of various beam shapes? I think the shape just satisfies practical physical constraints (ability to pack it up into a small container, for example), after a certain multiple of wavelength.
The reflector concentrates the incoming parallel EM from a distant source onto the receive antenna structure at the focul point of the dish. The focal point can either be coaxial (in the middle of the parabolic structure), or off-center.
Off center for casegranian I think. Maybe I misspelled that. But, what does the beam have to do with the shape of the dish?
Parabolic reflectors are usually used for collimated-to-focused conversion of EM waves. So for an RX antenna, usually you are focusing parallel incoming EM waves from a distant source onto the focus where your RX antenna (or input wave guide) is. For TX, you are taking the feedpoint EM and using the parabolic antenna surface to send them out as a tight beam toward the distant RX antenna.
The only time you would move the Feedpoint out of the focus would be if you want to spread the beam for some reason -- like maybe widening the beam of a swivelling radar antenna. And the same would apply to an RX parabolic dish -- if you needed a wider incoming aperture with lower gain for some reason, or maybe even an adjustable beamwidth and gain...
Usually you want the highest gain and directivity, though, when using a parabolic antenna.
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