Coincidentally, when I changed over from my old analog TV to a digital plasma TV, I also began to have a problem receiving all the channels. I thought it had to do with the antenna. After all, the new digital channels do call for antennae that are designed for the different digital frequency bands. I did start to look for a replacement antenna but fortunately one shop assistant suggested that I try one of the Monster TV cables. Well, to tell you the truth, I was not about to spend $35 just to see if that would work. A good new antenna would cost only twice that much.
However, by chance I came across a shielded TV cable in Bunnings that was described as "virtually noise free". For $10.50, it seemed like a safe bet. Sure enough, that did the trick. It has saved me the job of climbing to my roof, plus money and effort to replace the antenna. And then, that might not have solved the problem at all. So if you are having problems getting consistently good reception on your TV, you might want to look no further than the simple cable connecting your TV to the antenna socket.
(My Maths professor in college used to remind us not to bring out the big cannons before we have tried the simple ways to solve problems. This is one of those examples)