The 5 green dots which we all get after a ‘Rescan’ really don’t tell us much of anything. In theory they tell us that during the scan the signal quality for that channel was ‘good’. But they don’t tell us anything about that channel’s signal quality 5 minutes later.
But it gets worse. What does that ‘good’ quality during a rescan actually mean? How good is ‘good’. Signal quality for DTV does not slowly decrease. It suddenly plummets. (Thus the references to ‘cliff-effect’.)
I wanted to know how good was ‘good’, but I do not own a spectrum analyzer. So I have developed a test using about $20 worth of attenuators (pads).
I chose a clear day without any storms in my extended viewing area. Initially I chose my strongest channel for testing.
I attenuated the RF signal down in steps, first in large steps and then in smaller steps. (Rescanning the Tablo after each change.) The 5 green dots were displayed until I had inserted 35db of attenuation. When I put in one more 1db pad and rescanned, my strongest channel was NOT detected at all. When I took out that 1 db pad and rescanned, I had 5 green dots again.
When I repeated the test on weaker channels, after reaching the threshold - adding a 1db pad and rescanning the Tablo caused 2 or 3 orange dots to be displayed and adding one more 1db pad would cause the channel to go completely undetected during a rescan.
My weakest channel is about 55 miles away and it sometimes drops out during a storm. I could pad it down 9db and still get 5 green dots. But adding one more 1db pad caused 2 orange dots and adding one more 1db pad caused it to go undetected during a rescan.
An RF signal moving through the air is subject to varying amounts of loss. The losses are dependent on weather conditions and we can not do anything about that. Modern television receivers include RF automatic gain control circuitry to adjust the incoming signal to within some acceptable range. (Increasing gain for weak signals and decreasing gain for strong signals.) But we still need a little reserve signal level on good days, so that it doesn’t degrade completely on bad days. (Fade margin)
So there are 2 good lessons here.
Never judge your received signal based on those 5 green dots. ALWAYS check your signal strength, not signal quality. SIGNAL QUALITY WILL DEGRADE SUDDENLY AND COMPLETELY.
Want to know what your fade margin might be? My experience shows that 10db is not enough. Add 20db of attenuation to the input of your Tablo and watch your strongest channel and then your weaker channels. Don’t forget to remove the pad after testing!