Accuracy of string test

[sgs]

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

 

[. Raine]

I did a string test on my 9' Radio Shack dish when I first put it together and it was pretty much spot on with four strings. On that dish, I have the manual and there is a specific procedure for tightening the bolts and aligning all the petals, which is why I'd bothered to do the string test on that one when I was done, because the manual stated that if it wasn't done correctly, it could warp the dish.

 

[primestar31]

My "Perfect Ten" 10 foot dish won't lay perfect UNLESS it's on the mount, and bolted together in a very specific manner laid out in the instructions (which are posted on this site). First dish out of many that I had, that required being installed on the mount a certain way before it would align correctly and pass the string test.