Scalar F/D Setting

[TSchafer]

I recently purchased my first C Band dish, along with a B1SAT (bandstacked) LNBF. Unfortunately, the B1SAT doesn't have f/D markings, so I'm not sure how far the tip of the feedhorn should be from the front of the scalar.

I've found two tables for this distance online. The first one (from Chaparral) is as follows:

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

This works out to y = -10x + 4.32, where x is the f/D. So, for my my dish (f/D of 0.385), the distance between the front of the scalar and the tip of the feedhorn should be 0.47 inches.

On the other hand, bigdish.info has a different table as follows:

F/D .42 = .00 inches
F/D .40 = .20 inches
F/D .38 = .40 inches
F/D .36 = .60 inches
F/D .34 = .80 inches
F/D .32 = 1.0 inches

According to this table ( y = -10x + 4.2 ) the distance between the tip of the feedhorn and the front of the scalar should be 0.35 inches.

So... which table should I use? Are the Chaparral feedhorns slightly different from everyone else's (hence the .12" offset)... or is one of the tables wrong? Or, is 0.12" within the "margin of error" for the f/D setting?

Thanks for your help, both now and over the past year or so (I've been lurking on the forum for a while, and you've all helped me greatly even though you don't know it).

--TS

 

[Pixl]

It would seem to me that it would vary from one manufacturer to the next since there are differences in the outer diameter of the scaler ring and some difference in the thickness. So I would think you need the manufacturer to publish data or put markings on the body of the lnb. My DMX-741 came with no instructions and has marks of 2,4,6,8,0,2 ?? what ever that means.

Jim