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Re: Dish aiming

In discussing the beam size and pointing accuracy of a dish, here is
a quick rule-of-thumb:

   The beamwidth of a 2.5 foot (i.e. ~75 cm) diameter dish
   in degrees is equal to the wavelength in centimeters.

If the dish is bigger than 2.5 feet, the beamwidth gets smaller.
So a 5' dish at 13 cm has a beamwidth ~6.5 degrees, while a 
2.5' dish at 23 cm has a beamwidth ~23 degrees.

I like to use the rule that your pointing system (the rotor motor
and the encoder) should be able to point the antenna to about 0.1
times the size of the beam. So for a 1 meter dish at 3 cm with
a beamwidth ~0.7 degrees, I would want to see a pointing system
good/smooth at levels better than a tenth of a degree. If you
are using a digital shat encoder, it needs to have at least 12 
bits of resolution. If you use a 10 kohm potentiometer, you have
to measure the resistance to about 3 ohms. Yes, pointing a dish
at microwave frequencies is not easy!

73, Tom
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