Sky Waves

You could also receive sky waves. Sky waves travel toward the sky, rather than hang out on the ground. You would not be able to hear the sky-wave signals, except for the ionosphere. The ionosphere is many miles above the earth, where the air is "thin"--containing few molecules. Here, the ionosphere is bombarded by x-rays, ultraviolet rays, and other forms of high-frequency radiation. The energy from the sun ionizes this layer by stripping electrons from the atoms.

When a sky-wave signal reaches the ionosphere, it will either pass through it or the layer will refract the signal, bending it back to earth. The signal can be heard in that area where the signal reaches the earth, but depending on a number of variables, there might be an area where no signal from that particular transmitter is audible between the ground wave and where the sky wave landed. This area is the skip zone. After the sky-wave signal bounces on the earth, it will return toward the sky again.

Skipping Around the World.

Again, the signal will be refracted by the ionosphere and return to the earth. If the HF signals all bent and bounced off the ionosphere with no loss in signal strength, HF stations around the world would be heard across the earth with perfect signals (something like if a "super ball" was sent bouncing in a frictionless room). Whenever radio signals are refracted by the ionosphere or bounce from the earth, some of the energy is changed into heat, causing absorption of the signal. As a result, the signal at the first skip is stronger than the signal at the second skip, and so on. After several skips, typical HF signals will dissipate.

The skip and ground waves can be remarkably close together. It is not unusual for one station to receive a booming signal while a nearby station cannot hear a trace of the sending station even though using a better receiver with a better antenna. The first station was receiving either the ground wave or the first skip and the other station was located somewhere between these two

Angles of Radiation.

If the HF users only had skip to contend with, the theories and uses of the HF spectrum would be simple. But several other factors also come into play. The critical angle of radiation is the steepest angle at which a radio signal can be refracted by the ionosphere. The critical angle depends on such factors as the frequency that is being used, the time of year, the time of day. Sometimes a signal that shoots straight up from the antenna will be refracted by the ionosphere. In this case, the critical angle would be 0 degrees. In another case, the signal might slice through the ionosphere and continue into space. From this signal, you would not be able to determine the critical angle; you would only know that the sky-wave signal was above the critical angle.

Natural Cycles Affect Propagation.

Aside from the critical angle, the frequency used can also affect whether the signal will be passed through or refracted by the ionosphere. When a signal penetrates through the ionosphere without being refracted, the signal is said to operate above the Maximum Usable Frequency (MUF). The MUF is not a set frequency; it varies greatly, depending on the time of day and the part of the world that you are attempting to contact. Nearly the opposite of the MUF is the lowest usable frequency (LUF). However, the LUF has nothing to do with whether or not the signal will be refracted by the ionosphere; instead, it is the lowest frequency that you can use to reach a particular region (using a base standard amount of power).

In the daylight hours, the MUF is highest; in night hours, it is lower. There is also some seasonality, too. In the winter, with longer hours of darkness, the MUF is generally lower than the summer when the MUF is higher. Likewise, during the hours of darkness, when the ionosphere is less ionized, the LUF is lower, and during the daylight hours, it is much higher. The MUF and the LUF provide the boundaries between which you should operate the transceiver in order to make your contacts.

Cycles that Affect Propagation.

Propagation is affected by cyclical environmental conditions. The shortest of these conditions is the day/night cycle. In general, the transmitting and receiving conditions are by far the best in the nighttime hours. During the daytime, the MUF and LUF both rise -- in order to talk across great distances, less reliable (because of the very long skip) higher frequencies must be used. The season of the year also affects propagation The winter/summer cycles are somewhat like the day/night cycles, except having a lesser influence. In general, the MUF and LUF will both be higher in the summer and lower in the winter. Also, the noise from thunder storms and other natural phenomena is much higher during the summer. In fact, except for local transmissions, communications in the 1700- to 3000 kHz range during the summertime are of limited regular use.

The longest environmental cycle that affects propagation is the sunspot cycle. Before the age of radio, it was noticed that the number of solar storms (sun spots) varies from year to year. Also, the number of sunspots per year was not entirely random. The number of solar storms during a good propagational month exceeds 150 and the number during a weak month is often fewer than 30. The sunspot cycle reaches its peak approximately every 11 years, cycles that have a great impact on radio propagation.

Between these peaks are several years with very low sunspot activity. During years with high sunspot activity, the MUF dramatically increases and long-distance communications across much of the HF band is possible. During the peak of the last sunspot cycle, in 1989, the MUF was often above 30 MHz! When the cycle is at its low point, the MUF decreases and much less of the HF band is usable for long-range communications. Generally, the frequencies above 10,000 kHz dramatically improve during the peak years of the sunspot cycle, and the frequencies below 10,000 kHz are much less affected.

HF Interference.

Although the long distances that HF radio signals can be received is amazing, in comparison to the other radio bands, several types of distance-related interference can ruin reception or make listening unpleasant. The most widespread type of interference fits under the broad heading of noise. Noise consists of natural and man-made noise. Natural noise is produced by everything from thunder storms to planets (hence, radio telescopes).

Thunder storms are the worst because they cause very loud crashes; because of the long distances that shortwave signal travel, the noise produced by thunderstorms is also likely to travel hundreds of miles (or further). Even if the weather is clear (you should never operate HF equipment during a local thunderstorm), a distant thunderstorm could ruin your reception of a weak station that would otherwise be audible at your location.

Man-Made Interference

Man-made interference can arrive from a vast variety of sources. If nothing else, at least most man-made interference is limited in its range; most is limited to the building that the radio equipment is located in or to a several-block surrounding area. One of the worst causes of man-made interference is fluorescent lights, which create a medium-strength buzz across the HF range, although it is often at its worst on the lower frequencies. In fact, fluorescent lights near an antenna can drown a normally receivable signal. If your radio is located near computers, it will probably receive a light buzz across the bands and much stronger "bleeps."

Adjacent-channel interference is a special type of man-made interference where a station from a nearby frequency is "washing over" or "splattering across" another. A somewhat similar type of interference is co-channel interference, where the interfering station is on the same frequency. A good example of co-channel interference is the 1400- to 1500 kHz "graveyard" region of the AM broadcast band in the evening hours, where dozens of signals are all "fighting" to be heard.

Propagational Interference

Other types of HF interference cause signal distortion from propagational effects. One of the most interesting effects is polar echo, which occurs when one component of a radio signal takes an East-West path and another arrives over one of the poles of the Earth. Most every morning, one can tune into one of the BBC broadcast transmitters and hear the effect of polar echo. Because the signals take different paths, they arrive at different times, creating an echo on the audio signal. During the lightest effects, the voices sound a bit "boomy;" at worst, the delay is so long that the programming is difficult to understand. A related phenomenon is polar flutter, where the signal passes over one of the poles and quickly fades up and down in strength, creating a "fluttery" sound.

Fading is the most common and damaging form of propagational interference. The two most common types of fading are selective fading and multipath fading. With selective fading, the ionosphere changes orientation quickly and the reception is altered (somewhat like a ripple passing through the signal). FM and AM signals are especially prone to selective fading, SSB is slightly affected, and the CW mode is almost free from selective fading. The other type, multipath fading, occurs when signals take different paths to arrive at the same location. Multipath fading is a variation of polar echo; instead of the signals creating an echo effect, the phase of the signals are altered as they as refracted by the atmosphere. As a result, the received signal fades in and out.

Sun Spots

The last major propagational effect does not actually cause interference to a signal; it absorbs it. Although sun spots are beneficial to propagation as a whole, solar flares destroy communications. During a solar storm, communications across a wide frequency range can suddenly be cut off. Many listeners have thought that their receivers either weren't working or that the exterior antenna had come down because virtually no signals were audible. Instead, they had turned on their radios during a major solar flare. On the other hand, other listeners had thought they were listening during a solar flare, but actually didn't have their antenna connected or they had tuned their radio above the MUF or below the LUF.

Signal paths

Signals take various routes to travel to a receiver from the transmitter. The problems that can result from signal paths include polar flutter and echo, and multipath fading. The signal path is also important when attempting to contact or receive signals from a particular area. When you receive a signal, you can typically assume that it took the shortest path to reach you (i.e. you could connect the points between the transmitting and receiving locations with a line on a globe). This is known as short-path reception. Exceptions to this rule occur when two or more different paths are nearly the same distance (such as the BBC example of polar flutter, where the north-south path isn't much longer than the east-west path).

Long Path

The other major signal path is the long path. The long-path radio signal travels the opposite direction from the short-path signal. For example, the long-path signal from the BBC transmitter (mentioned earlier) would be east: across Europe, Asia, the Pacific Ocean, most of North America, finally arriving in Pennsylvania. Signals received via long path are often very weak--especially if the long path was very long and the frequency is low.

On the other hand, if the station is on the other side of the world and there is little difference between the long path and the short path, you could be receiving either or both. This case occurred recently to a listener on the east coast of the USA who was listening to a small, private broadcast station from New Zealand -- 12 time zones away. At the same time he was listening to it, it was also being heard throughout North America and in Germany. Because the signals were generally a bit better in the West and Midwest, we can assume that he heard the Pacific Ocean-to-Western North America route, rather than the one that passed through Asia and Europe.

One of the most intriguing propagational anomalies is the effect of the grey line on HF radio transmissions. The grey line region is the part of the world that is neither in darkness nor in daylight. Because two grey-line stripes move constantly around the earth, the propagational alterations are brief (usually only about an hour or so in length). Many amateurs and hard core radio listeners actively scour the bands at sunrise or sunset. The ionosphere is highly efficient at these times, so listeners can often pull in some amazing signals. Grey-line propagation is probably of far less interest to those who use the radio bands in conjunction with their occupation. If you are one of these users, chances are that grey-line propagation will be either a curiosity or a nuisance, as more stations that could cause interference to your signal become audible."