OK, it’s kind of a trick question. Signal level isn’t really an important measurement of over-the-air digital television signals. Because the signals are digital, it only matters if you can receive them at all. But, because you came all this way to ask, I’ll give you a basic framework.
In general terms…
Antenna signals are most often measured in dBm. This is a measurement that describes how much stronger or weaker than a milliwatt a signal is. Numbers that are positive (which are hardly ever seen) mean the signal is stronger than one milliwatt. Numbers that are negative mean the signal is weaker than one milliwatt. The numbers are logarithmic, meaning that -10dBm is equal to one tenth of a milliwatt, while -20dBm is equal to one one-hundredth of a milliwatt, and so forth. There’s a whole tutorial here about dBm as it relates to satellite signals, but the same applies to antennas.
Keep in mind that one watt is enough to power a small LED flashlight, while it takes 180 watts to power a desktop computer. This should tell you that antenna signals are really really weak.
How weak? Tell me already.
As I said, raw power isn’t the most important thing. But in general, signals that are stronger than -35dBm (such as -30, -20, etc.) are so strong that they may overload your tuner. Signals that are weaker than about -70dBm may be so weak that your tuner might not pick them up. Anywhere in the middle is fine. Honestly. If your signal level is anywhere in that range, there’s no need to amplify it unless you’re running lots of cable or using lots of splitters.
So if great power isn’t the answer…
You know at this point I’m dying to quote a Spider-Man movie. But I won’t.
The best measure of antenna performance is signal-to-noise ratio. I go into great detail here, but signal-to-noise ratio, often abbreviated SNR, is the most important measurement of digital television signals. It describes the amount of signal you have compared to the noise that’s everywhere. A very noisy signal will be hard to decode, while one that is practically free of noise will be much easier. It’s the difference between your TV’s tuner seeing something like this:
and something like this:
One is really clear and easy to understand. The other takes some time and effort. Eventually there’s so much noise that the tuner just can’t figure out where the signal is.
The problem with noise
There are two problems, actually. The first is that everything you do adds noise. Distance adds noise. The electronics in the antenna add noise. Cables add noise. Eventually it all adds up. The other problem is that it’s impossible to get rid of noise unless you use fancy digital signal processing.
The only option if you have a high amount of noise is to try to amplify the signal. At some point, though, you either make the signal too strong, or you end up amplifying the noise as well as the signal.
Signal to noise numbers
According to Televes, who specialize in the science of antennas, the ideal signal-to-noise ratio is anything over 25. Anything under 20 probably won’t let your TV’s tuner work right. In fact, since signal-to-noise ratio is measured by tuning a signal in as if you were going to watch it, it’s practically impossible to measure SNR under 20. The meter just won’t lock on.
There’s really no maximum level of SNR. It’s possible for the signal to be so strong that it overloads the tuner, but you could theoretically have an SNR of 35 or more in extremely low-noise situations. But it’s a level of abundance you don’t need. Really anything over 25 is going to let your TV tune the channel properly.
How can you measure SNR?
The best way is with a real antenna meter. Don’t use your TV, unless you’re sure you’re actually getting true SNR. Many TVs, if they have any signal measurement tools at all, use made-up measurements like “Signal Quality” that run on a scale of zero to 100. This won’t do you any good.
You’ll want to use a signal meter like our SignalScout. This is a true digital signal meter that not only measures raw power (which isn’t useful) but actually provides the measurements you need. Yes, I get that it’s an expensive tool for one-time or occasional use, but it really is what you need.