Calculating focal depth

[joshrr]

Ok, I know how to calculate focal depth but what is F/d. I need to fix my focal depth so I read up on it. Every place I read says how to calculate then 2nd you calculate your F/d. My F/d is .39... now what do I do with that number??? There are number markings on the side of my dmx741 but they are not in decimal format. They are like... 2 then 4 then 6 then 8 then they go back down again. Reading on another forum your f/d is always going to be between .3 and .4 so how would I correlate .39 to the numbers on the side of my dmx741. The manual does not say. Thanks for any help!

 

[merkin]

Wondered the same thing, so i used some calculus and chaparels chart on their site to make a spreadsheet which does all the work. Welcome To Chaparral Communications
? about f/d ratio and more - Ricks Satellite Wildfeed and Backhaul Forum

download book2.zip from above.

 

[Lak7]

This may help...
Inches -- f/D
.12 ------ .42
.32 ------ .40
.52 ------ .38
.72 ------ .36
.92 ------ .34
1.12 ---- .32

So for you, the Feedhorn mouth should be about 1/2" in front of the face of Scalarring.

 

[1beertap]

The spacing between the numbers on the dmx741 equals 0.25 or 1/4 inch. Hope this helps.

 

[joshrr]

Wow you guys rock. Thanks a ton. Its pouring tonight but I will adjust tomorrow. I'm a bit off and I think its messing up my ku reception

 

[joshrr]

OH, and Lak7. The numbers I was talking about on my dmx741 are 2,0,8,6,4,2. They chopped off the first number for each f/d. So based on your chart since my f/d is 39 I would line up the back of the scalar ring in the middle of the 0 and the 8 which would be (0 would be .40 and 8 would be .38. Thanks again.

 

[crossbow]

OH, and Lak7. The numbers I was talking about on my dmx741 are 2,0,8,6,4,2. They chopped off the first number for each f/d. So based on your chart since my f/d is 39 I would line up the back of the scalar ring in the middle of the 0 and the 8 which would be (0 would be .40 and 8 would be .38. Thanks again.

This adjustment sets the feed illumination of the dish as if it were a transmitter but by reciprocity theory applies equally to receiving. But it is only half of the battle. The mouth of the waveguide needs to be set to the focal point of the dish measured from the center to the waveguide edge. Multiply the dish diameter by the F/D to get this distance, so a 72 inch with a F/D of .39 would be 28 inch. This is a usually only a good starting point. A bunch of adjusting in/out on both plus skew will find the sweet spot, takes time and patience.