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RE: Dish questions for 2.4GHz and 24GHz





> -----Original Message-----
> From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
> Behalf Of Don Woodward
> Sent: Thu, Sep 20, 2001 22:03
> To: amsat-bb
> Subject: [amsat-bb] Dish questions for 2.4GHz and 24GHz
>
>
> Would the dish below from DH Satellite be suitable for 2.4GHz now and then
> be adaptable to 24GHz later provided substitution of two correct
> feed mounts
> and feeds?
>
> Don Woodward
> KD4APP
>

They don't say anything about the quality of the dish surface. To have good
efficiency, a dish
should have an RMS surface accuracy (both the general shape of the parabola,
and the roughness of the surface) "smooth" at levels of 1/20 wavelength.
Since 24 GHz = 1.3 cm, this means that the quality needs to be at the 600
micron level.

And a 1.2 m dish operating at 1.3 cm has 120/13 = 92 wavelengths in its
aperture. Therefore the beamwidth (full width at half max) will be ~1/92
radians (the fact that the aperture is circular actually ~1.22/92 = .013
radians = .76 degrees = 45 arc minutes. To use such a dish you will need to
somehow point it to a fraction of the size of the beam, i.e. something in
the range of tens of arc minutes. Just to put this in perspective, the hour
hand of an ordinary 12 hour analog clock moves thru 360/12 = 30 degrees per
hour. at 24 GHz, you would have to be able to point this dish to an accuracy
better than the angle that the hour hand moves in a minute of time in two
dimensions.

So the dish might work at 24 GHz, but I'm not sure that you would find it
very useful!

73 de Tom, W3IWI

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