RADIO PROPAGATION. How does that radio wave travel half-way around the world to your antenna? How come I can hear radio stations farther away at night? Why is it that I can hear AM radio stations hundreds of miles away, while FM stations fifty miles away are inaudible? These are some of the first questions asked by my students in my Propagation Block at the school I teach at. In this file I'll be attempting to give you a simple primer on Radio Propagation which you can use to make better decisions on when and where to listen to enhance your listening skills. If you are worried about your lack of knowledge, put your mind at ease. I'll be covering working knowledge, not esoteric theory. With that said, lets get started. What is a Radio wave? Simply put, it is a combination of electric and Magnetic fields that were originally generated at the transmitting antenna by passing current through a conductor. These fields are at right angles to each other, an effect caused by a simple law of electricity, known to electronic technicians around the world as the 'Left-Hand Rule'. When looking at a Dipole antenna, the Electric field is parallel to the plane of the conductor, while the Magnetic field is at right angles to the conductor. ^ -> -> -> -> -> -> -> ^ -> -> -> electric field. ---------------------^------------------ Wire ^ ^ ^ Magnetic Field So Radio waves, like Light, has polarity. For best reception one must arrange the receiving antenna's wire to be in the same plane as the transmitting antenna. By doing this you arrange for the magnetic field to induce maximum current in the antenna wire. How important is 'Polarity'? For Ground waves, a receiving antenna that is at right angles to the transmitting antenna will suffer a 6dB power loss, a difference that is definitely audible. Now that the radio wave has left the transmitters antenna, it will travel through space until it is completely absorbed or attenuated to nothingness by distance. Radio waves act differently depending on a combination of frequency and the media it is passing through. Since those waves we will be interested in generally travel through the atmosphere, we will break down the propagation effects into frequency bands. VLF; Very Low Frequency is that band of frequency's that range from 0 to 150 kHz. Frequency's this low are propagated entirely by Groundwave, that is, the radio waves travel close to the earth. In fact, frequency's this low will actually follow the curvature of the earth, completely circling the globe. Two properties stand out at these frequency's which make them uniquely useful. First, since they follow the curvature of the earth without being reflected from anything, there is only a single path the radio waves can take from the transmitter to the receiver. The distance of this path is easily calculated, being the Great Circle distance from the transmitter to the receiver. By measuring the time difference between the transmission of a radio pulse and its reception, or the time difference between the reception of pulses from three different VLF transmitters whose exact location is known, you can determine the distance that your receiver is from the transmitters, and by drawing arcs at those distances from the transmitters, find your location. This is the basis of the LORAN C Radionavigation system, one of the three prominent services in this band. The second prominent service on this band also rely on VLF's easily calculated propagation delay. These are Time Signal/Frequency Standard Stations, which allow the highly accurate setting and calibration of Atomic Clocks and Frequency Standards in remote locations. The third major service relays on the second major property of VLF frequency's. Unlike all higher radio frequency's, VLF signals penetrate the earth and water to substantial distances. This makes these frequency's uniquely useful to the military by allowing them to be able to communicate to submarines as deep as four hundred feet beneath the waves. The U.S. Navy has several transmitters between the frequency's 17.8 and 26.1 kHz. LF; Low Frequency is that band running from 150 kHz to 500 kHz. Like VLF, signals at this frequency propagate mainly by ground-wave. However, they do not follow the curvature of the earth as far, only about a thousand miles or so. Prior to 1930 this band was packed with radio services, such as Ship-to-Ship and Ship-to-Shore stations, as well as International Broadcasters. At the time it was a well known fact that the farther you wanted to transmit, the lower your radio frequency had to be. Until a fellow named Heaviside found a Joker in the Propagation deck. Now it is mainly a dead band, with only the scattered remains of its former glory evident in a few endangered Marine and Aeronautical Radio Direction-finding beacons and a couple of die-hard European Broadcasters. MF; The Medium wave band is the most familiar to laymen. It spans the range of 500 kHz to 3000 kHz. The lower half, 500 kHz to 1600 kHz, contains the AM Broadcast band, while the upper half is used by the Tropical Broadcast Band, the old LORAN A radionavigation system, and Ship communications. Propagation is mainly limited to ground-waves with a range of a hundred miles or so, with some highly attenuated single-hop skywave propagation at night adding about 600 more miles of range (a subject we will get into deeper in the Shortwave frequency range). The AM Broadcast band is used worldwide for domestic broadcasting, except in the Tropics, where atmospheric effects and high noise make it useless. In the Tropics two higher bands are used, 2300-2495 kHz and 3200-3400 kHz, giving these two bands their nickname of the Tropical Bands. HF; The Shortwave or High Frequency Band spans the range of 3000 kHz to 30,000 kHz. Prior to 1930 frequency's above 3 MHz (3,000 kHz) were thought to be totally useless for long-range radio communication. Propagation was limited to just slightly greater than line-of-sight, less than 100 miles. Then in 1926 Radio Amateurs discovered that there was a Joker in the deck. Banished to these useless frequency's, they discovered that they were suddenly able to do something that had eluded them on the lower frequencies. They could cross the Atlantic! Unfortunately, secrets that good are hard to keep, and before long it was general knowledge that there was some kind of radio mirror in the heavens that reflected these short waves back to earth several thousand miles away. The mirror was the Ionosphere, or the Ozone layer that has been so prominent in the news lately. This effect introduced a new propagation mode, called Sky-wave Propagation. As the Sun hits the Earths atmosphere, the Ultraviolet radiation strips the oxygen atoms apart in the upper atmosphere. This forms an ionized layer in the upper atmosphere. To frequency's below a certain frequency, the LUF (Lowest Usable Frequency, a frequency which changes from hour to hour, day to day), radio signals penetrating the Ionosphere are mainly absorbed, the lower the frequency, the greater the absorption. The little power that is left is either refracted back to earth, or into space (which explains why Medium wave frequencies, which are nearly always below the LUF manage to get reflected back to earth at night, although greatly attenuated.). As the radio waves frequency increases, the attenuation is reduced, but the Ionosphere progressively looses its ability to refract the signal back to earth. Finely a point is reached where there is not enough signal refracted back to earth to be considered useful. The frequency at which this occurs is called the Maximum Usable Frequency, or MUF. At this point most of the signal exits the other side of the Ionosphere and continues out to space. Between these two frequency's radio signals are refracted back to earth hundreds to thousands of miles from the transmitter with little attenuation. Often a radio signal may 'bounce' from the Ionosphere to earth and back to the Ionosphere to be refracted back to earth again. Sometimes a radio signal may 'bounce' up to six times before being attenuated into uselessness. This effect is what makes Shortwave frequencies so effective for worldwide communications. As the Ionosphere plays such an important part in our hobby, lets delve deeper into its workings. The Ionosphere displays two basic forms. The first is during the Daytime, when energy is constantly pouring into the Ionosphere from the Sun. This energy input causes the Ionosphere to split into four separate layers, From bottom to top they are generally referred to as the 'D' layer, the 'E' layer, the 'F1' layer, and the 'F2' layer. The 'D' layer, being only 40 to 60 miles up, is in a relatively thick section of the atmosphere. The Ionized atoms are very volatile as other atoms are always nearby to recombine with. Because of this the 'D' layer forms just after sunrise, reaches its peak density at noon, then quickly disappears at sunset, when the energy source is removed. As far as radio propagation is concerned, the 'D' layer mainly acts to absorb radio frequencies below 14 MHz, making the lower frequencies useless during most of the day. Also, it never is really thick enough to effectively refract radio waves at any frequency, so it is just a general pain in the preamp. The `E` layer, about 65 miles up, is much the same as the 'D' layer. It also quickly forms after sunrise, peaks at noon, and quickly disappears at sunset. Although this layer can refract radio waves in the range of 14 to 50 MHz, this is relatively rare, and it generally just absorbs frequencies below 14 MHz. The next layer, the 'F1' layer, is a relatively weak layer that splits off of the next higher layer, the 'F2' layer, during the daylight hours. It is about 100 miles up, and generally has little effect on radio wave propagation. The highest, thickest, and most useful layer is the 'F2' layer. It is about 100 to 300 miles high ( its height varies, depending on the season, latitude, time of day, and how the Cubs are doing this year.). At this altitude the atmosphere is so rarefied that recombination of ionized atoms occurs quite slowly. In fact this altitude is quite popular for spy satellites which need to remain up for only a week or so. As the Sun comes up, the ionization level increases until it reaches a peak about 14:00 local time. Since recombination takes place so slowly, the ionization level doesn't reach a minimum until shortly before sunrise. As the level of ionization increases, this layer becomes capable of refracting higher and higher frequencies, sometimes as high as 70 MHz. After sunset, the strength of this layer begins to decrease, and the frequency it can successfully refract back to earth goes down. However, the lower layers, which only act to attenuate the radio signal, disappear. So, on balance, Sky-wave propagation is best in the early evening. Many factors affect the stability and strength of this `F` layer, and thus its ability to refract back radio waves. The most prominent is the local time of day at the point the radio wave is being refracted at. As we discussed before, during the daylight hours the maximum frequency it can refract back goes up to about 25-50 MHz. After sunset, it starts to de-ionize, and the maximum frequency goes down, reaching a minimum of about 7 MHz just before sunrise. Another factor is the stability of the Sun. Sunspots, and the resulting outpouring of Solar wind, disturbs the thickness and stability of the 'F' layer. This can cause the 'F' layer to loose its ability to reflect radio waves from periods ranging from minutes to days. Magnetic Storms have the same effects. The third and more subtle effect is the so-called Solar Cycle. The average MUF increases and decreases on an eleven year cycle. During the trough years the MUF may only rarely exceed 15 MHz. 1986-1987 are good examples. During peak years ( to which we are heading now) the MUF may reliably exceed 50 MHz, going as high as 70 MHz on many days. There is also growing evidence of an even longer cycle, about 33 years long, which, if true, means that this coming peak may equal the amazing peak of the 1950s. To sum it up, you can use the following rules to determine which bands are probably open to 'Skip'. During the Daylight hours, listen high, above about 14 MHz. In late afternoon, skip frequencies began to decrease from the east, passing west during the early evening. So the higher frequency's from Europe fade out before sunset, while signals from the Pacific stay high into the early evening. As the evening continues, the 25 meter band will fade first, followed by the 31 meter band. By midnight, only the 41 and 49 meter bands will still receive skip. In the morning, start listening for Europe on the higher bands, while the Pacific will remain dead until 11:00 AM or so. Seasonal changes also occur, although this is more an effect of thunderstorms increasing background noise than anything else. So the background noise during the Summer months requires a stronger signal to overcome it than in Winter. VHF; The Very High Frequency Band ranges from 30 MHz to 300 MHz. At these high frequencies the Ionosphere can no longer refract the radio waves back, and there is no appreciable Ground-wave action. Propagation is limited to Line-of-Sight only. In other words, if you can't 'see' them, you can't hear them. This band, along with higher ones, are populated with local broadcasts, such as TV stations, FM stations, Aircraft, police, Delivery trucks, Taxi Services, Railroads, Military, etc.. Range is rarely more than 50 miles. As with all rules there are exceptions which extend the range of these signals far in excess of normal. The most common is the effect called 'Ducting'. This is where a dry layer of air is sandwiched between two layers of air with a higher humidity. Under these conditions, radio waves get trapped between them and can travel many hundreds of miles before exiting. This effect is quite common along the Gulf Coast, and along the Atlantic and Pacific Coasts at the lower latitudes. When I was a child in Texas, my Father was the first person in the block to get a TV set, in fact the first person on the entire Air Force Base. Although there was not a TV station within 200 miles, Ducting was so common that we could watch the TV station in Atlanta Georgia nearly every day! Other esoteric modes are Troposcatter (where very high power transmitters beam the signal up into the Atmosphere so that an over-the-horizon receiver can pick up the minute amount of signal scattered back from the Troposphere boundary). and Meteor Scatter (where the signal is reflected off the ionized trail of entering meteorites). UHF; Ultrahigh frequencies comprise the range from 300 MHz to 1000 MHz (or 1 GHz). Propagation at these frequencies is directly Line-of-Sight, no If's, And's, or But's about it. In fact, most communications at these frequencies are Point-to-Point rather than broadcast in all directions. At these frequencies the terrain becomes very important as even small hills between the transmitter and receiver can block reception. This concludes our little lesson on Propagation. In the interest of simplicity, I have told a few white lies, but the scope of this file was to give a layman a general feel on how radio waves propagate over different frequencies. In that I feel that I have succeeded.