[Microwave] Parabolas.

[J. Forster]

KD7JYK wrote:

> This is being sent to two groups that may have in interest...
>
> In a few months I will be acquiring a machine that can make parabolas up to 7"
> or 10" diameter, not sure on the exact size yet.  What frequencies would this
> be good for in the microwave range?

>From my Collins Space Systems Slide Rule, 12" diameter has about 31 dB gain at
15 GHz with about 4.5 degrees beam width The surface needs to be better than
.03" rms for 1 db gain reduction. These seemk to be useful numbers.

> For the laser fellows, the parabolas can be as small as you want, and I am
> guessing from what I have seen they can be polished like a mirror.  This would
> be first surface.  They could easily be gold plated for IR though for the time
> you'd have to do this yourself unless there is enough demand to warrant
> purchasing the equipment to do it here.

Surface figure is very important, not just finish. If the figure is poor, one
part of the dish will cancel out another part.  Typically you want the figure to
be better than (wavelength) / 4 which says 0.25 micron for 1 micron wavelength
(near IR). Can your proposed machine do that well?

> Either way these parabolas or dishes as some call them would be machined from
> a solid billet of material and would be extremely stable!!

When you machine things, you sometimes relieve stresses which results in overall
distortion. This may well not be as simple as it seems.

Good luck,
John

> Kurt
>
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