how to find the focal point of a parabolic dish?preamp circ.

parabolic dish

hello

could someone tell me how to find the focal point of a parabolic dish. also pls post me a preamp circuit for the parabolic dish microphone.

Re: parabolic dish

deepak007 said:

hello

could someone tell me how to find the focal point of a parabolic dish. also pls post me a preamp circuit for the parabolic dish microphone.

Click to expand...

This link might help you:

**broken link removed**

Focal Point: If its somewhat Reflective with light, Shine a light, directly onto it and at the same time move a piece of paper in front of it, until you get a small spot of light showing on the paper.

Re: parabolic dish

Aren't depth and diameter of the dish enough for getting the focal distance?

EDIT: like this https://www.satsig.net/focal-length-parabolic-dish.htm

Re: parabolic dish

rfmw said:

Aren't depth and diameter of the dish enough for getting the focal distance?

EDIT: like this https://www.satsig.net/focal-length-parabolic-dish.htm

Click to expand...

Yes that Works.

But May be a Problem for parabolics that are Offset Focal points.
(Focal point Not on Center Axis as shown there.)
Like the ones used on newer Satellite dishes.

I Have one of these types here, so I'll check it out later today and let you know.

EDIT:
I tested this formula on two parabolics I have.

1) Yes it works perfectly on a Symetrical, on axis parabolic.
(Focal point in Exact center of diameter)
The Mirrored glass one that I show on my site.

2) It does not calculate the correct focal point on Non-Symetrical dishes.
This is a Satellite dish for TV reception, also shown on my site.
Actual focal point is 14.75 inches.
Calculated focal point was 19 inches.

So using the light and paper can find either focal point, assuming the dish is light reflective.
Or use a constant frequency sound source, Microphone/Voltmeter if not light reflective surface.

If you have a dish and want to know its focus, the easiest way has been described above- use the Sun and piece of thin paper. Measure the distance from dish center to the smallest light spot.
If your dish does not reflect, use aluminum stickers on the rim and half-way to the center- this will give you the same focus like a shiny dish.
For offset dishes, this will not help. Try first to get the original LNB mount (possibly with a LNB)- this will directly show you the focal point. I used a Ku-band LNB radio telescope to find the focus of an offset dish by receiving solar noise, but it is usually a quite difficult procedure. I prefer symmetrical dishes to offsets.