The old f/d question

[PANHANDLER]

I am getting closer to putting up my 10' solid prodelin.
Originaly this dish was a button hook instalation. I am converting it to a four suport arm type.What i need to do is figure out the length of each arm to fabricate.From the calculations below i can see the dimention from the center of the dish to the lnbf is 35-15/16" but what i don't understand is the f/d of .300,,, what does this mean ?


D = diameter
d = depth

D x D = 120.5 x 120.5 = 14520.25
16 x d = 16 x 25.25 = 404
D x D/16 x d = 14520.25/404 = 35.941 ( 35-15/16" )
Therefore focal distance f = 35.941 inches ( ok i undersdtand )

f / D = 35.941/ 120.5 = .300
f /D = .300 ( ? )

 

[greenguy]

the f/d ratio are the numbers you see on the lnb where you line up your f/d with the scalar ring.

in your case you will line it up with the .30

hxxp://www.wsidigital.com/DMX241-BOTTOM2.jpg

 

[PANHANDLER]

Thank you greenguy .

 

[PANHANDLER]

While i'm on the topic f/d . I am restoring two dishes a 10' prodelin solid and a 8' Perfect 10 mesh. All is going pretty good except i was hoping to have them both up by now . Before i took the perfect 10 down i measured the distance to the lnb and it was 33-5/8" . When i use the above fomula it tells me it is supose to be 32-1/2" . Make me wonder if the formula is close enough sense i am building new support arms for the Prodelin . It would be a shame if i made the arms to the length that i determind from the formula and it was off enough where i wouldn't be able to peek . Any sugestions ?

D=90"
d = 15"
f/d =32.4

 

[Anole]

I won't comment on the measurement. But others will.

Let me just suggest this:
- make the legs a little too long, placing the feed & scalar a bit further from the dish.
- then float the scalar toward the dish on long screws.

I've seen the idea used several times by members to fine-tune or correct for a new feed, and it seems to work well.
Stogie5150 and ACradio have both posted pix showing the idea.

edit: added links

- ACradio: start here and look at next two images.
- Stogie: thread on mod for Paraclipse

 

[greenguy]

the focal distance will really come into play if you are tryin for ku band, i find it dont affect the c band at all. . .

my focal distance should 50" but im off like 3". . .47" with no probs on c band.

also, the arms should be able to move (will look as if they are tryin to spin the scalar ring) so it can adjust the focal distance a bit.