I was getting my local HD channels from Comcast QAM, but then they started encrypting everything, so I went back to my roof antenna.
For many years the roof antenna was working great, picking up all the channels without issue. Then one specific channel dropped out, channel 13 in the Eugene, OR market. I scanned and re-scanned, re-aimed the antenna, emailed the engineer at the station, no resolution to the problem. I knew it had something to do with either the antenna or the coax cable as I could plug in an indoor Silver Sensor antenna and be able to at least pick up the channel. With the roof I had zilch.
Now switching back to the roof antenna I was more determined to figure out what was wrong. Finally, after going over everything as I did before, I dug through my box of random RF gear and pulled out a 75-ohm / 300-ohm combiner that had a UHF hoop antenna attached to the 300-ohm terminals. I did this mainly to see what a more omnidirectional UHF antenna would do. And on a lark, I went ahead and screwed in the coax from the roof antenna to the combiner's 75-ohm jack.
With that, all channels, including 13 got picked up. That makes sense in terms of the combiner combining the signals from the roof and hoop antennae, but then why would 13 only show up when getting fed through the combiner? At first I thought the UHF hoop was picking up 13, so I removed the 75-ohm connection, only to see both 9 and 13 (the two stations in the VHF segment) drop out and all the UHF stations remain.
So it appears that the combiner is doing something to the roof antenna signal, but I can't fathom what as I don't know what's inside the combiner.
Here's the type of combiner I'm using: http://www.radioshack.com/product/index.jsp?productId=2062059&locale=en_US