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Making a Parabolic Reflector

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Bob Scott

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There are lots of uses for parabolic reflectors like reflecting telescopes, "big ear" type directional microphones, spot lamps and microwave antennas. You should be able to make your own by spinning a liquid like mercury in a pot on a rotating turntable. This method was discovered as early as 1935, when Popular Science had an article about it.

I have derived an easy formula for the focal length of the produced parabola as a function of the required RPM. If anyone is interested, I can post the derivation:

RPM = (45.6 * g/f) ^ 0.5 or putting it another way:
RPM = SQRT((45.6*g)/f)

where g is the acceleration due to gravity expressed as 32 ft/sec/sec or as 9.8 m/sec/sec
and f is the required focal length in ft or metres. The equation works in the FPS and MKS and any other systems.

Does anyone have any suggestions for a material to make the parabola? I was thinking of a thin layer of wax floating on water, but wax shrinks when it solidifies. I was thinking of portland cement or plaster of paris.

If I use sand instead of a liquid, I'll have to rig up some kind of vibrator to coax it into shape.
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How about mixable liquid acrylic, or, even easier, fibreglas. Spray a pie or cake pan with some lubricant, mix up a batch, pour it in and spin. Add cloth to thin subsequent layers for strength.

Given the number of obsoleted satellite dishes, you'd need special dimensions to want to build.
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