As mentioned in another thread, I'm building LNBF / scalar support arms, so I measured my dish with the "string test". I found fishing line works really well for this purpose. I put down 4 "strings", every 45 degrees around the dish. This was with the dish assembled on the ground, not attached to the mounting ring.
What I found was a bit odd. Two of the strings at 90 degrees to one another lined up, and the other two at 90 degrees to one another lined up, but each pair was about 1/4 of an inch apart where they crossed. I thought that maybe I could swap the placement of two the petals to improve things, so I did that, but it didn't help! I measured and found that the edges of the petals are a slightly different diameter than the mid-points of the petals. It's a used dish, so I can't expect it to be perfect any more, I guess.
So, I've been using an average of the two points while calculating my focal length and subsequent placement of the scalar ring and LNBF.
My question is to anyone who has done a string test - what did you find with your dish? Was your dish absolutely perfect or was it out a bit, like mine? Did you only use two strings, or four, or more? If the strings were off by a small amount where they crossed (like mine, 1/4 inch), did you find it greatly affected the performance of the dish?
sgs
What I found was a bit odd. Two of the strings at 90 degrees to one another lined up, and the other two at 90 degrees to one another lined up, but each pair was about 1/4 of an inch apart where they crossed. I thought that maybe I could swap the placement of two the petals to improve things, so I did that, but it didn't help! I measured and found that the edges of the petals are a slightly different diameter than the mid-points of the petals. It's a used dish, so I can't expect it to be perfect any more, I guess.
So, I've been using an average of the two points while calculating my focal length and subsequent placement of the scalar ring and LNBF.
My question is to anyone who has done a string test - what did you find with your dish? Was your dish absolutely perfect or was it out a bit, like mine? Did you only use two strings, or four, or more? If the strings were off by a small amount where they crossed (like mine, 1/4 inch), did you find it greatly affected the performance of the dish?
sgs
