Originally Posted by PeninsulaMark
How many bars out of ten should I get for good viewing, viz what is the minimum ? What SNR is good ?
These questions haven't been answered for you yet, so let me explain the difference between the "bars" reading and "SNR".
Signal indication by bars or, on many TV receivers, by a 0 to 100 scale, have no standard reference. Each manufacturer has their own idea of what value should be shown for a certain signal, and it varies greatly. If you put the exact same signal into five different tuners, you would probably see five different readings. You might see two bars on one, 5 bars on another: 20 on one 0-100 scale and 60 on another. You can't compare the reading on one TV to that of another TV using the bars or 0-100 scale. These scales can only be used to show the differences in signals on that particular TV. Also bars and 0-100 scales can show signal strength or they can show signal quality. Unless it's specified in the manual that came with the TV, you have no idea what they're actually showing.
I have several tuners and on one I can get a picture with 1 bar, but on another you need 5 bars to produce a picture. Another will show 10 on the 0-100 scale when a picture shows up, while on another you need a signal of 50 or better to get a picture.
The HD Home Run receivers show both signal strength and signal quality on their readouts. (See the examples above that Stephen Fischer posted.) With these receivers you need a signal strength AND a signal quality reading of 50 or higher each to get a picture. You can have a signal strength of 100 and still not get a picture if the signal quality is below 50. Multipath reflections cause the signal quality to vary greatly as you turn your antenna with a rotor or switch from one antenna to another as I do. I think that's why we here in the Bay Area have so many reception problems.
While bars and 0-100 scale readings have no standard, the SNR (Signal to Noise) reading in dB is a standard. Using it you know exactly how strong the signal actually is. Most TV receivers don't show the SNR reading, though. For those that do, you can use this reading and compare it to another SNR reading from a different receiver. Using the same example used above where you put the exact same signal into five different tuners, all should show the exact same SNR dB reading.
You need a signal strength of 15.2 dB SNR to produce a picture. Since SNR is a standard, this is the same on every tuner. Some call this the "cliff edge". Below 15.2 you get no picture; above 15.2 you do. The higher the dB reading is, the better the signal is. 15.2 to 16 dB will produce a picture, but you'll still see pixelation and occasional break up. You really need 17 dB to get rid of the pixelation and get a solid, clear picture. For reliable viewing, you should have a nominal signal of at least 20 dB from a station to allow for signal variations. Signals can vary several dB up and down due to changes in atmospheric conditions. If you can't get a signal of at least 20 dB on average, you can expect occasional drop outs, or times when the signal will disappear completely for a while.
Some tuners show a maximum reading of 27 dB SNR, while others show higher readings. The highest SNR reading I've seen on any TV is 33 dB.