Now more than ever, noise limits performance and enjoyment of shortwave radio listening. Signal strength is one thing, noise is another.
Everyone should think of a radio receiver as a device to distinguish signals from noise. Too often, people look at sensitivity as a really important receiver specification. However, your ability to hear weak signals depends more on noise than sensitivity. Here’s why.
To distinguish a radio signal, you need to achieve a signal to noise ratio of a certain level, perhaps 3 dB for data or 10 dB for voice. On the higher frequencies, you can focus on amplifying a weak signal more than you amplify noise. But throughout the HF bands, the problem is noise itself.
When you sit down at your receiving system (radio and antenna) you face three kinds of noise: internal radio noise, atmospheric and galactic noise, and man-made noise. Let’s consider each.
Internal radio noise is typically not a problem. Every radio generates noise as it amplifies and mixes signals. The noise figure of your receiver is usually around 15 dB, lower if your preamplifier is on. If you disconnect your antenna, your radio will show a noise floor of around -130 dBm, which is the amount of noise above thermal noise generated inside your radio.
Atmospheric noise is a challenge on lower HF frequencies, and galactic noise can cause problems on the higher ones. Look at the picture above. A well calibrated S-Meter will normally read about S2-S3 based on atmospheric noise alone, increasing to S5-S7 on lower frequencies as a result of nearby or distant thunderstorms.
Man-Made Noise Limits Performance a Lot
But for most of us, man-made noise is the real killer for two reasons. First, you can see in the graphic above that man-made noise is usually stronger than atmospheric noise. Second, unlike the largely white noise of atmospherics, man-made noise contains modulation – artificial signals that add to the mayhem. In effect, you will find additional signals that jam the signals you are trying to hear. For example, the distinctive buzz of 120 Hz harmonics from power-related noise or the complex whooshes from plasma television.