How To Find My Parabolic Dish Focal Point

[ThaiLife]

I need to find my Parabolic Dish's Focal Point , ive had a go with a few tables but I'm not sure if I'm doing it right , can some one here tell me the focal point distance please,

:jap:

[paulfr]

Odd as it may seem, the 14 inch and 1.5 inch measurements are

irrelevant to the location of the focus.

The focal point of a parabola has several properties and definitions

but the one most useful here is the property that ....

All lines of energy [acoustic or electromagnetic] coming in parallel

to the axis of symmetry [line connecting the focus to the vertex, the bottom

most point when laid on its curved side] will reflect to the focus.

This is where the receiver needs to be to capture the incoming energy.

So drop a ping pong ball on one side of the dish and see along what

line it bounces. The do it again on the other side. The intersection of

these two lines will be the focus.

A crude and not very accurate method I agree, but it will work.

Then again you can just set up the system and move the receiver

around to see at what point you get maximum signal denoted by

clarity of picture or sound.

[Tywais]

Finding the focal point .pdf

Also Analyze math

Downloadable calculator

[sfokevin]

Finding the focal point .pdf

Also Analyze math

Downloadable calculator

I would prefer to see the ping pong show...

[ThaiLife]

Thanks guy's for the advice, its appreciated , I downloaded the Calculator and its given me a focal length of 6.13 inches

:jap:

[paulfr]

I stand corrected.

Those measurements define a point on the parabola from

which "a" can be determined and this will give the focal point coordinates.

The result from the equation at this site

http://www.radio-astronomy.org/library/Parabolic%20Focal%20Point.pdf

gives F = (14)^2 / 16 x 1.5 = 8.16

and that is consistent with what this site predicts

http://www.pbs.org/wgbh/nova/teachers/activities/pdf/3406_solar_03.pdf

f = x^2 / 4 a = 7^2 / 4 x 1.5 = 49/6 = 8.16

[jombom]

Thanks guy's for the advice, its appreciated , I downloaded the Calculator and its given me a focal length of 6.13 inches

:jap:

This answer is basically impossible. It appears you have a mirror image of the Parabola.

The correct answer will probably be a little more than 8.5 inches (based on the info you gave).

My best guess is between 8.5 and 9 inches.

Trial and error should get it sorted for you.

[Semper]

Finding the focal point .pdf

Also Analyze math

Downloadable calculator

I would prefer to see the ping pong show...

[bangkockney]

Odd as it may seem, the 14 inch and 1.5 inch measurements are

irrelevant to the location of the focus.

The focal point of a parabola has several properties and definitions

but the one most useful here is the property that ....

All lines of energy [acoustic or electromagnetic] coming in parallel

to the axis of symmetry [line connecting the focus to the vertex, the bottom

most point when laid on its curved side] will reflect to the focus.

This is where the receiver needs to be to capture the incoming energy.

So drop a ping pong ball on one side of the dish and see along what

line it bounces. The do it again on the other side. The intersection of

these two lines will be the focus.

A crude and not very accurate method I agree, but it will work.

Then again you can just set up the system and move the receiver

around to see at what point you get maximum signal denoted by

clarity of picture or sound.

In fact the two measurements provided are all that's needed.

F = diameter squared divided by 16 times the depth.

[Paulme]

Cover the dish in foil and put it out in the son and then focus on a piece of paper - the focal point should soon become clear! Keep your eyes well out of the potential focus point area to avoid instant blindness!

[Tywais]

There is a problem with that calculator. They require input values to be integer, that is no fractions. It rounds up the 1.5" to 2" thus the error. Thought that a bit odd.

D=Diameter

d=depth

f=focal point

f = D² / 16d

f = 14²/(16*1.5)

f = 8.166"

[ThaiLife]

Thanks for the follow up advice, its much appreciated, so have I now got this correct ..

The focal point is ... 8.16 inches