I don't want to make it look like I'm PIMping a company, but Onpoint has one of the best primers for tower crews and installers. It's written for the person who is scratching their head regarding what PIM is all about. Link It doesn't have all of the charts and stuff some people have that goes far beyond an introductory article that loses you in the first four paragraphs. Here is another one from Forward Link. It's called Understand PIM 101, but it may be a little bit too much to digest if you're just starting out unless you can easily understand this: F4 = F2 + (F2 - F1) = 2F2 - F1 . It's more like 101 with a year or so of intermod knowledge under your belt.