# Re: f/d ratio on dish

• Subject: Re: [amsat-bb] f/d ratio on dish
• From: "Bill Magnusson" <williamb@xxxxxxxxxxx>
• Date: Mon, 30 Jul 2001 08:22:50 +1000

```> Can someone give me  a quick lesson? The answer is probably right in front
> of me, I just can't find it.
> What is the f/d ratio & how is it correctly determined?  I have a 30 year
> old 2.5 meter dish that I wish to place a Patch feed on, however the dish
> must be in the window 0.3 - 0.5 f/d.
> Gunther Meisse

Hi Gunther,

The focal length of a (normal prime focus) dish
can be calculated from the diameter and depth of the dish surface.

f (focal length) = D squared divided by 16 times C
or D x D / 16 x C
where D is the diameter of the dish surface and C is the depth.

Find the depth by placing a long straight stick or rod across the face of the dish and
measuring down to the centre of the dish. Then do the above calculations.
The f/d ratio (focal length to diameter ratio) is just that. Divide D into f  from the
formula and it should come out to 0.3 - 0.4 - 0.5 or whatever. The importance of this
figure is in the way the feed apparatus illuminates the dish surface. The larger the f/d
ratio, the further away the feed will be and the characteristics of the feed will need to
be changed accordingly so as to correctly illuminate the dish with not too much
"over-spray" as this can degrade the noise figure of the system as a whole. Similarly, a
feed with too narrow a beam will waste some of the dish surface due to under-illumination.
A feed with a lobe which is 10db down at the edge of the dish is considered to be about
right. You can see how the f/d ratio will affect this figure.
Hope this helps.

Regards, Bill...vk3jt

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