Andy Lee made a post on the AVS Forum with two very good Noise Margin diagrams, one for an indoor antenna without a preamp and the other for an outdoor antenna with a preamp.
TV Fool Discussion Thread
http://www.avsforum.com/forum/25-hdt...l#post15700679
should be post #397 in that thread
You have to log in to see the images in his post, but you should be able to recover the images from the attachments there or here.
I added some notes to his diagrams to help me understand the Noise Margin concept when I was learning about it.