Guide to Antennas:

.Anyone who does any listening to radio receivers at all - whether as a ham operator, a short-wave listener, or scanner enthusiast - notices rather quickly that radio signal propagation varies with time and something mysterious usually called 'conditions.' The rules of radio signal propagation are well known (the general outlines were understood in the late 1920s), and some predictions can be made (at least in general terms). Listen to almost any band, and propagation changes can be seen. Today, one can find propagation predictions in radio magazines, or make them yourself using any of several computer programs offered in radio magazine advertisements. Two very popular programs are any of several versions of IONCAP, and a Microsoft Windows program written by the Voice of America engineering staff called VOACAP.

Some odd things occur on the air. For example, one of my favorite local AM broadcast stations broadcasts on 630 kHz. During the day, I get interference-free reception. But after the Sun goes down, the situation changes radically. Even though the station transmits the same power level, it fades into the background din as stations to the west and south of us start skip ping into my area. The desired station still operates at the same power level, but is barely audible even though it’s only 20 miles (30 km) away.

Another easily seen example is the 3-30 MHz short-wave bands. Indeed, even those bands behave very differently from one another. The lower frequency bands are basically ground wave bands during the day, and become long-distance 'sky wave' bands at night (similar to the AM broad cast band (BCB)). Higher short-wave bands act just the opposite: during the day they are long-distance 'skip' bands, but some time after sunset, become ground wave bands only.

The very high-frequency/ultra high-frequency (VHF/UHF) scanner bands are somewhat more consistent than the lower-frequency bands.

But even in those bands sporadic-E skip, meteor scatter, and a number of other phenomena cause propagation anomalies. In the scanner bands there are summer and winter differences in heavily vegetated regions that are attributed to the absorptive properties of the foliage. I believe I experienced that phenomenon using my 2m ham radio rig in the simplex mode (repeater operation can obscure the effect due to antenna and location height).

THE EARTH'S ATMOSPHERE

Electromagnetic waves don’t need an atmosphere in order to propagate, as you will undoubtedly realize from the fact that space vehicles can transmit radio signals back to Earth in a near vacuum. But when a radio wave does propagate in the Earth's atmosphere, it interacts with the atmosphere, and its path of propagation is altered. A number of factors affect the interaction, but it’s possible to break the atmosphere into several different regions according to their respective effects on radio signals.

The atmosphere, which consists largely of oxygen (O2) and nitrogen (N2) gases, is broken into three major zones: the troposphere, stratosphere, and ionosphere ( FIG. 1). The boundaries between these regions are not very well defined, and change both diurnally (i.e. over the course of a day) and seasonally.

The troposphere occupies the space between the Earth's surface and an altitude of 6-11 km. The temperature of the air in the troposphere varies with altitude, becoming considerably lower at high altitude compared with ground temperature. For example, a 108C surface temperature could reduce to _558C at the upper edges of the troposphere.

The stratosphere begins at the upper boundary of the troposphere (6-11 km), and extends up to the ionosphere (_50 km). The stratosphere is called an isothermal region because the temperature in this region is relatively constant despite altitude changes.

The ionosphere begins at an altitude of about 50 km and extends up to 500 km or so. The ionosphere is a region of very thin atmosphere. Cosmic rays, electromagnetic radiation of various types (including ultraviolet light from the Sun), and atomic particle radiation from space (most of it from the Sun), has sufficient energy to strip electrons away from the gas molecules of the atmosphere. The O2 and N2 molecules that lost electrons are called positive ions. Because the density of the air is so low at those altitudes, the ions and electrons can travel long distances before neutralizing each other day they are long-distance 'skip' bands, but some time after sunset, become ground wave bands only.

The very high-frequency/ultra high-frequency (VHF/UHF) scanner bands are somewhat more consistent than the lower-frequency bands.

But even in those bands sporadic-E skip, meteor scatter, and a number of other phenomena cause propagation anomalies. In the scanner bands there are summer and winter differences in heavily vegetated regions that are attributed to the absorptive properties of the foliage. I believe I experienced that phenomenon using my 2m ham radio rig in the simplex mode (repeater operation can obscure the effect due to antenna and location height).

THE EARTH'S ATMOSPHERE

Electromagnetic waves don’t need an atmosphere in order to propagate, as you will undoubtedly realize from the fact that space vehicles can transmit radio signals back to Earth in a near vacuum. But when a radio wave does propagate in the Earth's atmosphere, it interacts with the atmosphere, and its path of propagation is altered. A number of factors affect the interaction, but it’s possible to break the atmosphere into several different regions according to their respective effects on radio signals.

The atmosphere, which consists largely of oxygen (O2) and nitrogen (N2) gases, is broken into three major zones: the troposphere, stratosphere, and ionosphere ( FIG. 1). The boundaries between these regions are not very well defined, and change both diurnally (i.e. over the course of a day) and seasonally.

The troposphere occupies the space between the Earth's surface and an altitude of 6-11 km. The temperature of the air in the troposphere varies with altitude, becoming considerably lower at high altitude compared with ground temperature. For example, a 108C surface temperature could reduce to _558C at the upper edges of the troposphere.

The stratosphere begins at the upper boundary of the troposphere (6-11 km), and extends up to the ionosphere (_50 km). The stratosphere is called an isothermal region because the temperature in this region is relatively constant despite altitude changes.

The ionosphere begins at an altitude of about 50 km and extends up to 500 km or so. The ionosphere is a region of very thin atmosphere. Cosmic rays, electromagnetic radiation of various types (including ultraviolet light from the Sun), and atomic particle radiation from space (most of it from the Sun), has sufficient energy to strip electrons away from the gas molecules of the atmosphere. The O2 and N2 molecules that lost electrons are called positive ions. Because the density of the air is so low at those altitudes, the ions and electrons can travel long distances before neutralizing each other by recombining. Radio propagation on some bands varies markedly between daytime and night-time because the Sun keeps the level of ionization high during daylight hours, but the ionization begins to fall off rapidly after sunset, altering the radio propagation characteristics after dark. The ionization does not occur at lower altitudes because the air density is such that the positive ions and free electrons are numerous and close together, so recombination occurs rapidly.

FIG. 1

PROPAGATION PATHS

There are four major propagation paths: surface wave, space wave, tropospheric, and ionospheric. The ionospheric path is important to medium-wave and HF propagation, but is not important to VHF, UHF, or microwave propagation. The space wave and surface wave are both ground waves, but behave differently. The surface wave travels in direct contact with the Earth's surface, and it suffers a severe frequency-dependent attenuation due to absorption into the ground.

FIG. 2

The space wave is also a ground wave phenomenon, but is radiated from an antenna many wavelengths above the surface. No part of the space wave normally travels in contact with the surface; VHF, UHF, and microwave signals are usually space waves. There are, however, two components of the space wave in many cases: direct and reflected ( FIG. 2).

The ionosphere is the region of the Earth's atmosphere that is between the stratosphere and outer space. The peculiar feature of the ionosphere is that molecules of atmospheric gases (O2 and N2) can be ionized by stripping away electrons under the influence of solar radiation and certain other sources of energy (see FIG. 1). In the ionosphere the air density is so low that positive ions can travel relatively long distances before recombining with electrons to form electrically neutral atoms. As a result, the ionosphere remains ionized for long periods of the day - even after sunset. At lower altitudes, however, air density is greater, and recombination thus occurs rapidly. At those altitudes, solar ionization diminishes to nearly zero immediately after sunset or never achieves any significant levels even at local noon.

Ionization and recombination phenomena in the ionosphere add to the noise level experienced at VHF, UHF, and microwave frequencies. The properties of the ionosphere are therefore important at these frequencies because of the noise contribution. In addition, in satellite communications there are some trans-ionospheric effects.

GROUND WAVE PROPAGATION The ground wave, naturally enough, travels along the ground, or at least in close proximity to it ( FIG. 3).

There are two basic forms of ground wave: space wave and surface wave.

The space wave does not actually touch the ground. As a result, space wave attenuation with distance in clear weather is about the same as in free space (except above about 10 GHz, where absorption by H2O and O2 increases dramatically). Of course, above the VHF region, weather conditions add attenuation not found in outer space.

The surface wave is subject to the same attenuation factors as the space wave, but in addition it also suffers ground losses. These losses are due to ohmic resistive losses in the conductive earth. Surface wave attenuation is a function of frequency, and increases rapidly as frequency increases. For both of these forms of ground wave, communications is affected by the following factors: wavelength, height of both receive and transmit antennas, distance between antennas, and terrain and weather along the transmission path.

FIG. 3

Ground wave communications also suffer another difficulty, especially at VHF, UHF, and microwave frequencies. The space wave is like a surface wave, but is radiated many wavelengths above the surface. It’s made up of two components (see FIG. 2): direct and reflected waves. If both of these components arrive at the receive antenna they will add algebraically to either increase or decrease signal strength. There is nearly always a phase shift between the two components because the two signal paths have different lengths. In addition, there may be a 180 (pi radians) phase reversal at the point of reflection (especially if the incident signal is horizontally polarized).

Multipath phenomena exist because of interference between the direct and reflected components of the space wave. The form of multipath phenomenon that is, perhaps, most familiar to many readers (at least those old enough to be 'pre-cable') is ghosting in television reception. Some multipath events are transitory in nature (as when an aircraft flies through the transmission path), while others are permanent (as when a large building or hill reflects the signal). In mobile communications, multipath phenomena are responsible for reception dead zones and 'picket fencing.' A dead zone exists when destructive interference between direct and reflected (or multiple reflected) waves drastically reduces signal strengths. This problem is most often noticed at VHF and above when the vehicle is stopped; and the solution is to move the antenna a quarter wavelength. Picket fencing occurs as a mobile unit moves through successive dead zones and signal enhancement (or normal) zones, and sounds like a series of short noise bursts.

At VHF, UHF, and microwave frequencies the space wave is limited to so-called 'line of sight' distances. The horizon is theoretically the limit of communications distance, but the radio horizon is actually about 15% further than the optical horizon. This phenomenon is due to refractive bending in the atmosphere around the curvature of the Earth, and makes the geometry of the situation look as if the Earth's radius is four-thirds the actual radius.

The surface wave travels in direct contact with the Earth's surface, and it suffers a severe frequency-dependent attenuation due to absorption by the ground ( FIG. 3). The zone between the end of the ground wave and where the sky wave touches down is called the skip zone, and is a region of little or no signal. Because of this phenomenon, I have seen situations on the 15m band (21.390 MHz) where two stations 65 km apart ( Baltimore, Maryland, and Fairfax, Virginia) could not hear each other, and their communications have to be relayed via a ham station in Lima, Peru! The surface wave extends to considerable heights above the ground level, although its intensity drops off rapidly at the upper end. The surface wave is subject to the same attenuation factors as the space wave, but in addition it also suffers ground losses. These losses are due to ohmic resistive losses in the conductive earth, and to the dielectric properties of the Earth.

Horizontally polarized waves are not often used for surface wave communications because the Earth tends to short circuit the electrical (E) field component. On vertically polarized waves, however, the Earth offers electrical resistance to the E-field and returns currents to following waves. The conductivity of the soil determines how much energy is returned.

IONOSPHERIC PROPAGATION

Now let us turn our attention to the phenomena of skip communications as seen in the short-wave bands, plus portions of the medium-wave and lower VHF regions. Ionospheric propagation is responsible for intercontinental broadcasting and communications.

Long-distance radio transmission is carried out on the HF bands (3-30MHz), also called the 'short-wave' bands. These frequencies are used because of the phenomenon called skip. Under this type of propagation the Earth's ionosphere acts as if it’s a 'radio mirror,' to reflect the signal back to Earth. This signal is called the sky wave. Although the actual phenomenon is based on refraction (not reflection, as is frequently believed) the appearance to the casual ground observer is that short-wave and low-VHF radio signals are reflected from the ionosphere as if it were a kind of radio mirror. The actual situation is a little different, but we will deal with that issue in a moment.

The key lies in the fact that a seeming radio mirror is produced by ionization of the upper atmosphere. The upper portion of the atmosphere is called the 'ionosphere' because it tends to be easily ionized by solar and cosmic radiation phenomena. The reason for the ease with which that region (50-500 km above the surface) ionizes is that the air density is very low.

Energy from the Sun strips away electrons from the outer shells of oxygen and nitrogen molecules, forming free electrons and positive ions. Because the air is so rarified at those altitudes, these charged particles can travel great distances before recombining to form electrically neutral atoms again.

As a result, the average ionization level remains high in that region.

Several sources of energy will cause ionization of the upper atmosphere.

Cosmic radiation from outer space causes some degree of ionization, but the majority of ionization is caused by solar energy. The role of cosmic radiation was first noticed during World War II when military radar operators discovered that the distance at which their equipment could detect enemy aircraft was dependent upon whether or not the Milky Way was above the horizon (although it was theorized 10 years earlier). Intergalactic radiation raised the background microwave noise level, thereby adversely affecting the signal-to-noise ratio.

The ionosphere is divided for purposes of radio propagation studies into various layers that have different properties. These layers are only well defined in textbooks, however, and even there we find a variation in the height above the Earth's surface where these layers are found. In addition, the real physical situation is such that layers don’t have sharply defined boundaries, but rather fade one into another. The division into layers is therefore somewhat arbitrary. These layers (shown earlier in FIG. 1) are designated D, E, and F (with F being further subdivided into the F1 and F2 sub-layers).

D-layer

The D-layer is the lowest layer in the ionosphere, and exists from approximately 50 to 90 km above the Earth's surface. This layer is not ionized as much as higher layers because all forms of solar energy that cause ionization are severely attenuated by the higher layers above the D-layer. Another reason is that the D-layer is much denser than the E- and F-layers, and that density of air molecules allows ions and electrons to recombine to form electroneutral atoms very quickly.

The extent of D-layer ionization is roughly proportional to the height of the Sun above the horizon, so will achieve maximum intensity at midday.

The D-layer exists mostly during the warmer months of the year because of both greater height of the sun above the horizon and the longer hours of daylight. The D-layer almost completely disappears after local sunset, although some observers have reported sporadic incidents of D-layer activity for a considerable time past sunset. The D-layer exhibits a large amount of absorption of medium-wave and short-wave signals, to such an extent that signals below 4-6 MHz are completely absorbed by the D-layer.

E-layer

The E-layer exists at altitudes between approximately 100 and 125 km.

Instead of acting as an attenuator it acts primarily as a reflector although signals do undergo a degree of attenuation.

Like the D-layer, ionization in this region only exists during daylight hours, peaking around midday and falling rapidly after sunset. After night fall the layer virtually disappears although there is some residual ionization there during the night-time hours.

The distance that is generally accepted to be maximum that can be achieved using E-layer propagation is 2500 km, although it’s generally much less than this and can be as little as 200 km.

One interesting and exciting aspect of this region is a phenomenon called Es or sporadic E. When this occurs a layer or cloud of very intense ionization forms. This can reflect signals well into the VHH region of the radio spectrum. Although generally short lived, there can be openings on bands as high as 2 meters (144 MHz). These may last as little as a few minutes, whilst long openings may last up to a couple of hours. The phenomenon also affects lower frequencies like the 10 meter and 6 meter amateur bands as well as the VHF FM band. Sporadic E is most common in the summer months, peaking in June (in the northern hemisphere). Distances of between 1000 and 2500 km can be reached using this mode of propagation.

F-layer The F-layer of the ionosphere is the region that is the principal cause of long-distance short-wave communications. This layer is located from about 150-500 km above the Earth's surface. Unlike the lower layers, the air density in the F-layer is low enough that ionization levels remain high all day, and decay slowly after local sunset. Minimum levels are reached just prior to local sunrise. Propagation in the F-layer is capable of skip distances up to 4000 km in a single hop. During the day there are actually two identifiable, distinct sublayers in the F-layer region, and these are designated the 'Fl' and 'F2' layers. The F1 layer is found approximately 150-250 km above the Earth's surface, while the F2 layer is above the F1 to the 450-500 km limit. Beginning at local sundown, however, the lower regions of the F1 layer begin to de-ionize due to recombination of positive ions and free electrons. At some time after local sunset the F1 and F2 layers have effectively merged to become a single reduced layer beginning at about 300 km.

The height and degree of ionization of the F2 layer varies over the course of the day, with the season of the year, and with the 27 day cycle of the sun.

The F2 layer begins to form shortly after local sunrise, and reaches a maximum shortly before noon. During the afternoon the F2 layer ionization begins to decay in an exponential manner until, for purposes of radio propagation, it disappears sometime after local sunset. There is some evidence that ionization in the F-layer does not completely disappear, but its importance to HF radio communication does disappear.

IONOSPHERIC VARIATION AND DISTURBANCES

The ionosphere is an extremely dynamic region of the atmosphere, especially from a radio operator's point of view, for it significantly alters radio propagation. The dynamics of the ionosphere are conveniently divided into two general classes: regular variation and disturbances. We will now look at both types of ionospheric change.

Ionospheric variation

There are several different forms of variation seen on a regular basis in the ionosphere: diurnal, 27 day (monthly), seasonal, and 11 year cycle.

Diurnal (daily) variation

The Sun rises and falls in a 24 hour cycle, and because it’s a principal source of ionization of the upper atmosphere, one can expect diurnal variation.

During daylight hours the E- and D-levels exist, but these disappear at night. The height of the F2 layer increases until midday, and then decreases until evening, when it disappears or merges with other layers. As a result of higher absorption in the E- and D-layers, lower frequencies are not useful during daylight hours. On the other hand, the F-layers reflect higher frequencies during the day. In the 1-30 MHz region, higher frequencies (>11 MHz) are used during daylight hours and lower frequencies (<11 MHz) at night. FIG. 4B shows the number of sunspots per year since 1700.

27 day cycle

Approximately monthly in duration, this variation is due to the rotational period of the Sun. Sunspots ( FIG. 4A) are localized on the surface of the Sun, so will face the Earth only during a portion of the month. As new sunspots are formed, they don’t show up on the earthside face until their region of the Sun rotates earthside.

FIG. 4A

Seasonal cycle

The Earth's tilt varies the exposure of the planet to the Sun on a seasonal basis. In addition, the Earth's yearly orbit is not circular, but elliptical. As a result, the intensity of the Sun's energy that ionizes the upper atmosphere varies with the seasons of the year. In general, the E-, D-, and F-layers are affected, although the F2 layer is only minimally affected. Ion density in the F2 layer tends to be highest in winter, and less in summer. During the summer, the distinction between F1 and F2 layers is less obvious.

11 year cycle

The number of sunspots, statistically averaged, varies on an approximately 11 year cycle ( FIG. 4B). As a result, the ionospheric effects that affect radio propagation also vary on an 11 year cycle. Radio propagation in the short wave bands is best when the average number of sunspots is highest. Peaks occurred in 1957, 1968, 1979, and 1990.

Events on the surface of the Sun sometimes cause the radio mirror to seem almost perfect, and make spectacular propagation possible. At other times, however, solar disturbances disrupt radio communications for days at a time.

There are two principal forms of solar energy that affect short-wave communications: electromagnetic radiation and charged solar particles.

Most of the radiation is beyond the visible spectrum, in the ultraviolet and X-ray/_-ray region of the spectrum. Because electromagnetic radiation travels at the speed of light, solar events that release radiation cause changes to the ionosphere about 8 minutes later. Charged particles, on the other hand, have a finite mass and so travel at a considerably slower velocity.

They require 2 or 3 days to reach the Earth.

Various sources of both radiation and particles exist on the Sun. Solar flares may release huge amounts of both radiation and particles. These events are unpredictable and sporadic. Solar radiation also varies over an approximately 27 day period, which is the rotational period of the Sun. The same source of radiation will face the Earth once every 27 days, so events tend to be somewhat repetitive.

FIG. 4B

Solar and galactic noise affect the reception of weak signals, while solar noise will also either affect radio propagation or act as a harbinger of changes in propagation patterns. Solar noise can be demonstrated by using an ordinary radio receiver and a directional antenna, preferably operating in the VHF/UHF regions of the spectrum. If the antenna is aimed at the Sun on the horizon at either sunset or sunrise a dramatic change in background noise will be noted as the Sun slides across the horizon.

Sunspots

A principal source of solar radiation, especially the periodic forms, is sun spots ( FIG. 4A). Sunspots can be as large as 100 000-150 000 km in diameter, and generally occur in clusters. The number of sunspots varies over a period of approximately 11 years, although the actual periods since 1750 (when records were first kept) have varied from 9 to 14 years ( FIG. 4B). The sunspot number is reported daily as the statistically massaged Zurich smoothed sunspot number, or Wolf number. The number of sunspots greatly affects radio propagation via the ionosphere. The low was in the range of 60 (in 1907), while the high was about 200 (1958).

Another indicator of ionospheric propagation potential is the solar flux index (SFI). This measure is taken in the microwave region (wavelength of 10.2 cm, or 2.8 GHz), at 1700 U.T. Greenwich Mean Time in Ottawa, Canada. The SFI is reported by the National Institutes of Standards and Technology (NIST) radio stations WWV ( Fort Collins, Colorado) and WWVH ( Maui, Hawaii).

The ionosphere offers different properties that affect radio propagation at different times. Variations occur not only over the 11 year sunspot cycle but also diurnally and seasonally. Obviously, if the Sun affects propagation in a significant way, then differences between night-time and daytime, and between summer and winter, must cause variations in the propagation phenomena observed.

Ionospheric disturbances

Disturbances in the ionosphere can have a profound effect on radio communications - and most of them (but not all) are bad. In this section we will briefly examine some of the more common forms.

Sporadic E-layer

A reflective cloud of ionization sometimes appears in the E-layer of the ionosphere; this layer is sometimes called the Es layer. It’s believed that the Es layer forms from the effects of wind shear between masses of air moving in opposite directions. This action appears to redistribute ions into a thin a layer that is radio-reflective.

Sporadic-E propagation is normally thought of as a VHF phenomenon, with most activity between 30 and 100 MHz, and decreasing activity up to about 100 MHz. However, about 25-50% of the time, sporadic-E propagation is possible on frequencies down to 10-15 MHz. Reception over paths of 2300-4200 km is possible in the 50 MHz region when sporadic-E propagation is present. In the northern hemisphere, the months of June and July are the most prevalent sporadic-E months. On most days when the sporadic-E phenomenon is present it lasts only a few hours.

FIG. 5

Sudden ionospheric disturbances (SIDs)

The SID, or 'Dellinger fade,' mechanism occurs suddenly, and rarely gives any warning. Solar flares ( FIG. 5) are implicated in SIDs. The SID may last from a few minutes to many hours. It’s believed that SIDs occur in correlation with solar flares or 'bright solar eruptions' that produce immense amounts of ultraviolet radiation that impinge the upper atmosphere. The SID causes a tremendous increase in D-layer ionization, which accounts for the radio propagation effects. The ionization is so intense that all receiver operators on the sunny side of the Earth experience profound loss of signal strength above about 3 MHz. It’s not uncommon for receiver owners to think that their receivers are malfunctioning when this occurs. The sudden loss of signal by sunny-side receivers is called Dellinger fade. The SID is often accompanied by variations in terrestrial electrical currents and magnetism levels.

An interesting anomaly is seen when SIDs occur. Although short-wave reception is disrupted, and may stay that way for awhile, distant very low frequency (VLF) signals, especially in the 15-40 kHz region, experience a sudden increase in intensity. This is due to the fact that the SID event results in deep ionization way into the D-layer. This ionization increases absorption of HF signals. But the wavelength of VLF signals is close to the distance from the Earth's surface to the bottom of the D-layer, so that space acts like a gigantic 'waveguide' (as used in the transmission of microwaves) when the SID is present - thus propagating the VLF signal very efficiently.

Ionospheric storms

The ionospheric storm appears to be produced by an abnormally large rain of atomic particles in the upper atmosphere, and is often preceded by a SID 18-24 hours earlier. These storms tend to last from several hours to a week or more, and are often preceded by 2 days or so by an abnormally large collection of sunspots crossing the solar disk. They occur most frequently, and with greatest severity, in the higher latitudes, decreasing toward the Equator. When the ionospheric storm commences, short-wave radio signals may begin to flutter rapidly and then drop out altogether. The upper ionosphere becomes chaotic, turbulence increases, and the normal stratification into 'layers' or zones diminishes.

Radio propagation may come and go over the course of the storm, but it’s mostly absent. The ionospheric storm, unlike the SID which affects the sunny side of the Earth, is worldwide. It’s noted that the maximum usable frequency (MUF) and critical frequency tend to reduce rapidly as the storm commences.

An ionospheric disturbance observed over November 12-14, 1960 was preceded by about 30 minutes of extremely good, but abnormal propagation. At 15.00 hours EST, European stations were noted in North America with S9+ signal strengths in the 7000-7300 kHz region of the spectrum, which is an extremely rare occurrence. After about 30 minutes, the bottom dropped out, and even AM broadcast band skip (later that evening) was non-existent. At the time, WWV was broadcasting a 'W2' propagation prediction at 19 and 49 minutes after each hour. It was difficult to hear even the 5 MHz WWV frequency in the early hours of the disturbance, and it disappeared altogether for the next 48 hours. Of course, as luck would have it, that event occurred during the first weekend of the ARRL Sweepstakes ham radio operating contest that year.

GREAT CIRCLE PATHS

A great circle is a line between two points on the surface of a sphere that lies on a plane through the sphere's center. When translated to 'radio speak,' a great circle is the shortest path on the surface of the Earth between two points. Navigators and radio operators use the great circle for similar, but different reasons: the navigator in order to get from here to there, and the radio operator to get a transmission path from here to there.

The heading of a directional antenna is normally aimed at the receiving station along its great circle path. Unfortunately, many people don’t understand the concept well enough, for they typically aim the antenna in the wrong direction. Radio waves don’t travel along what appears to be the best route on a flat map. Instead they travel along the shortest distance on a real globe.

Long path versus short path The Earth is a sphere (or more precisely, an 'oblique spheroid'), so from any given point to any other point there are two great circle paths: the long path (major arc) and short path (minor arc). In general, the best reception occurs along the short path. In addition, short-path propagation is more nearly 'textbook' compared with long-path reception. However, there are times when the long path is better, or is the only path that will deliver a signal to a specific location from the geographic location in question.

USING THE IONOSPHERE

The refraction of HF and some medium-wave radio signals back to Earth via the ionosphere gives rise to intercontinental HF radio communications.

This phenomenon becomes possible during daylight hours, and for a while after sunset when the ionosphere is ionized. FIG. 6 reiterates the mechanism of long-distance skip communications. The transmitter is located at point T, while receiving stations are located at sites R1and R2.

Signals 1and 2 are not refracted sufficiently to be returned to Earth, so they are lost in space. Signal 3, however, is refracted enough to return to Earth, so it’s heard at station R1. The skip distance for signal 3 is the distance from T to R1. At points between T and R1, signal 3 is inaudible, except within ground wave distance of the transmitter site (T). This is the reason why two stations 50 km apart hear each other only weakly, or not at all, while both stations can communicate with a third station 3000 km away. In American amateur radio circles it’s common for South American stations to relay between two US stations only a few kilometers apart.

Multi-hop skip is responsible for the reception of signal 3 at site R2. The signal reflects (not refracts) from the surface at R1, and is retransmitted into the ionosphere, where it’s again refracted back to Earth.

The location where skip signals are received (at different distances) depends partially upon the angle of radiation of the transmitting antenna.

A high angle of radiation causes a shorter skip zone, while a lower angle of radiation results in a longer skip zone. Communication between any particular locations on any given frequency requires adjustment of the antenna radiation angle. Some international short-wave stations have multiple antennas with different radiation angles to ensure that the correct skip distances are available.

FIG. 6

SUPER-REFRACTION AND SUBREFRACTION

At VHF frequencies and well up into the microwave bands there are special propagation modes called super-refraction and sub-refraction. Depending upon the temperature gradient and humidity, the propagation may not be straight line. At issue is something called the K-factor of the wave. FIG. 7 shows these modalities. The straight line case has a K-factor of one, and is the reference. If super-refraction occurs, then the value of K is greater than one, and if sub-refraction occurs it’s less than one.

FIG. 8 shows a case where super-refraction occurs. The value of K can be substantially above one in cases where a hot body of land occurs next to a relatively cool body of water. This occurs off Baja, California, the Persian Gulf and Indian Ocean, parts of Australia and the North African coast. In those areas, there may be substantial amounts of super-refraction occurring, making directional antennas point in the wrong direction.

FIG. 9 shows a case of sub-refraction. In this case, there is a relatively cold land mass next to relatively warm seas. In the Arctic and Antarctic this situation exists. The K factor will be substantially less than one in these cases. In fact, the signal may be lost to terrestrial communications after only a relatively short distance.

FIG. 7 TRANSMITTER RECEIVER

FIG. 8 LAND; WATER

FIG. 9 LAND WATER

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