You have two threads. Let's focus here.
First, please repost the link to your TVFool report. Use the URL exactly as it shows in your browser. The following is based on the image you posted...
All but one of your local signals are low-power stations that can broadcast original programming or retransmit a distant station. You can tell by the call sign listed Knnxx, where nn represents the channel the signal is broadcast on. Unlike full-power stations, low-power stations were not required to cease analog broadcasting in 2009, which is why analog signals are showing up in your report (the pink ones).
Thanks to issues with the FCC's database extracts, it's hard for sites like TVFool to keep track of the status of low-power stations accurately. That's why you see these stations listed twice (as digital and as analog) often. From what you're describing, I bet you are getting some in digital and some in analog.
Why? Again, just guessing since we don't know what you're using as a tuner, the channels where SNR is listed are digital, as that, or "signal quality" is something that most every digital TV tuner provides. The stations with a terrible picture and 0 SNR are likely analog, as you should be getting NO picture with a 0 SNR on a digital tuner. Can you tell us the channels that are working and those that are not?
As far as your homemade db4 goes, you should use a design that is documented
here. Designs that feature 7" whiskers that you will find documented elsewhere are not optimum for the entire UHF band.
