wireless networking - Wifi poor SNR, reasons and solutions:

I'd like to know technically how to diagnose whats causing poor signals and how to improve: Now I have a D-Link DIR-655 and its signal level is set to full, but the router model should not be the primary thing. I also have a custom antenna, specifically:

http://www.newegg.com/Product/Product.aspx?Item=N82E16833998052

Here are my SNR calculated with KISMAC at different distances:

-1 Feet: 47

-20 Feet: 15

Now at 15 it seems signal is low, the connection bad, so I'd like to know:

1. 
   

   What is a good SNR, IE: what should be your SNR to get a great connection.

   
   


2. 
   

   How to diagnose if anything is causing a bad SNR.

   
   


3. 
   

   How to improve it, wifi router settings, antennas.