How does noise affect radio signals? Man-made noise can be so strong, that it can mask even the strongest radio signal in your radio receiver. Dealing with man-made noise is not covered in this section, but you can explore it at the following links that deals with man-made noise:

• AM Radio noise
  
• INTERFERENCE - What is it?
  
• Medium Wave Interference
  
• Polarization of man-made noise

Noise - A real issue for radio communication effectiveness

As we begin our look at atmospheric noise, it is most useful to look at the problem as an issue of radio signal effectiveness. Often, when people talk about radio reception, signal strength is touted as the most useful factor in the effort of getting a signal from the transmitter to the receiver. However, since the problem of reception is more complex than a simple power issue (just pump more watts into the antenna), the better way to get a handle on the problem is to use the Signal-to-Noise Ratio (SNR) measurement of a radio circuit. (The radio circuit is the path between, and including, the transmitter and receiver). The SNR is a real measure of effectiveness. With it, we can better understand how effectively a signal can get from point A to point B.

Take a look at the four following sample radio circuit analysis graphs. These are all modeled with isotropic antennas on both ends of the radio circuit, and the transmitter is running 100 watts. Each graph shows the signal to noise ratio (SNR) in dB, on each of the standard Amateur Radio HF bands, at 2000 UT, for February 2006, between Washington state and Alaska. These were created using ACE-HF PRO, Version 2.05.



This first graph (above) models the circuit with a Man-Made (MM) noise level of "remote" at the receiver (164 -dBW-Hz, for 1 Hertz bandwidth at 3 MHz). Note that "propagation" is possible on a number of bands. The green color for a frequency band indicates that the circuit reliability is at least eighty percent. This means that for eighty percent of the month, statistically, the signal will be reliably received on this circuit.



The graph, above, shows how an increase of the MM noise by 10 dB causes a slight degradation on the bands. The same bands are mostly usable, but the effectiveness of the signal has decreased.



This graph (above) is for a city environment. Increase noise by another 10 dB, and you can note a significant degradation in circuit quality on most bands. The same signal that worked in the remote and rural environments, is not nearly as effective in the city's noise levels.



Finally, increasing the noise level a full 24 dB over the remote level as shown in the first graph, above, and you can see severely limited circuit usefulness on any band. In all four cases, the antenna was the same, the signal strength was the same, and the propagational parameters (sunspots, and so on) are the same. But, the effectiveness decreases with the increase of noise.

This means that with all parameters except noise staying the same (power, antenna, solar activity, azimuth, time-of-day), MM noise makes a very large difference in the quality of a circuit. Be careful not to generalize from that finding, as different circuits and different seasons would yield different data, I'm sure. The noise factor in these examples was based on Man-Made sources. The other source of noise also plays the same way in a circuit's usefulness.

After dealing with man-made noise, we are left dealing with two non-man-made sources of noise:

1. Cosmic. Cosmic noise, which originates at points outside of the Earth's atmosphere, doesn't contribute much to the problem of radio signal reception.
   
   
2. Atmospheric. Atmospheric noise has a significant impact on the reception of a radio signal.

Atmospheric Noise

All atmospheric noise is created by weather. More specifically, this noise comes from lightning flashes, with most of the noise caused by cloud-to-ground flashes because the currents in those strokes are much stronger than those of cloud-to-cloud flashes. However, some energy from horizontal flashes gets converted into vertically polarized energy and adds to the total at the ground receiver. (Horizontally polarized energy doesn't propagate well to the surface, but is an important factor with airborne radio reception, such as used in trans-oceanic flights.)

Worldwide, more than eight million lightning flashes occur daily. That's roughly 100 lightning flashes (with their resulting pulse of radio noise) per second. If your receiver is very far away from most of the storm centers, you'll only experience what is sometimes called, "white noise." Atmospheric noise is impulsive, though, and is not evenly distributed as is true white noise. White noise, when viewed on a 'scope is pretty well evenly distributed, as would arise from cosmic "background" noise. A Gaussian distribution of most parameters usually follows a "normal" (or Gaussian) probability curve--often called a bell-shaped curve. But Impulsive noise is just that--impulsive. If you view it on a 'scope, it looks like short-lived pulses rising out of an even bed of background noise.

Atmospheric noise, then, is the combination of many, many lightning flashes. Radio scientists model each thunderstorm center as a radio transmitter, usually called an "Equivalent Noise Transmitter" or ENT. Such energies then propagate around the world just as do International Broadcast radio transmissions. At a receiver we can then add up all of those energies propagated from worldwide storm centers. We find that the amount of that power-sum varies with seasons and with the nearness of the major storm centers.

Starting in the 1960s and continuing through the 1980s, a worldwide effort was made to measure all of this. The result was the CCIR 322 publication, which has been updated several times. The latest version is the CCIR 322-3, which summarizes the vast amounts of raw data on noise. A reader of the publication will quickly note that frequency plays a great part in HF communication from a noise standpoint. Lightning creates a broad-spectrum emission, but in the high frequency range, it is frequency-dependent, with noise power decreasing as frequency increases. In VLF work, atmospheric noise dominates nearly completely (assuming an electromagnetic interference-clean local environment and EMI-clean radio components). At HF, however, man-made noise is a large part of the total energy in the high bands.

When the question is asked, "When will good propagation occur?" the reader should look at more factors than just concentrating on the space-weather disturbed environment. The other factors that affect propagation are radio circuit path length and orientation, frequency, diurnal effects, as well as the transmitter power and antenna gain, and the parameters of the receiving station. Space weather and geophysical (weather, geomagnetic field, location) factors are not changeable by the average radio hobbyist (but, if you were God, perhaps you could tweak conditions). The rest of these factors are the parts you can control.

The principal effect is always propagation itself, which is the result of ionospheric profiles that vary over the world as the day-night terminator sweeps through -- and that can not be controlled by the radio operator. One might start by running propagation analysis tools like ACE-HF Pro version 2.05 for Amateur Radio and SWL operators to see how different the ionosphere is between steady-state daytime and nighttime, and how that affects reception on simple circuits. (ACE-HF defines the most reliable mode at every time of the prediction.)

ACE-HF will sort out the best frequencies to use, regardless of environmental conditions. Up until now, ACE-HF was targeted primarily at the Amateur Radio operator. This new version (version 2.05) now includes very powerful Shortwave Listening tools and features. Using ACE-HF, a selected radio circuit can be defined, and then an analysis of the affect of noise can be made. Change out different antenna models, and see what that does to your reception. After you begin to understand the way these factors influence radio propagation, then you can begin playing with the differences caused by the range of Smoothed Sunspot Numbers (SSN), the month, and so on. Using the powerful modeling tools of ACE-HF (like the animated maps that show the hour-by-hour coverage of a transmitted signal) one can quickly see that generalized "rules of thumb" about sunspots are often overly simplified. While low SSNs are usually worse, some frequencies favor lower SSNs while others favor higher SSNs. It all depends on time-of-day, season, circuit position, and so on.

ACE-HF has worked closely with the IONCAP/VOACAP software engineers and scientists, and together these folks have calibrated the IONCAP engine through measurements made during a wide range of environmental conditions. This has resulted in SNR distributions implicitly including the effects of a range of disturbed conditions. The range of environmental effects is built into the model, and shows up in the statistical factors. Since VOACAP was validated through so many years of testing, and is generally acknowledged to be the "gold standard" of propagation models, it's a relief to know how easy it is to use with confidence. From a radio hobbyist standpoint, it is much easier to use tools based on VOACAP, like ACE-HF, than other models where such factors must be laboriously worked out and inputted.

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