CHAPTER 21
WAVE PROPAGATION
RADIO WAVES COVER THE EARTH

With your own broadcast receiver, you have probably made a few observations on the behavior of radio waves. The 1,000-watt station 700 miles away may come in clearer and stronger than a 5,000-watt station only 400 miles away. You know also that your receiver does not pick up stations as well in daylight as it does at night, and that you get more distance and better reception in the winter than you do in the summer. Again you may have observed that some places in the United States are good radio locations, while others are naturally poor places for receiving radio programs.

THE RADIO WAVE

When it leaves a vertical antenna, the radio wave resembles a huge doughnut lying on the ground, with the antenna in the hole at the center. Part of the wave

The GROUND WAVE is used for SHORT-RANGE COMMUNICATION at high frequencies with low power, and for LONG-RANGE COMMUNICATION at low frequencies and very high power. Day-time reception from most commercial stations is carried by the ground wave.

The SKY WAVE is used for long-range, high-frequency daylight communication. At night, the sky wave provides a means for long-range contacts at LOWER FREQUENCIES.

THE GROUND WAVE

The ground wave is made up of four parts-DIRECT, GROUND-REFLECTED, TROPOSPHERIC, and SURFACE waves. The relative importance and use made of each part is dependent on several factors. The chief factors are-frequency, distance between the transmitting and receiving antennas, height of the antenna, the nature of the ground over which the wave travels, and the condition of the atmosphere at the lower levels.

The GROUND-REFLECTED WAVE permits two airplanes several miles distant and at low altitudes to communicate with each other. The wave arrives at the receiving antenna after being reflected from the earth's surface. When the airplanes are close enough and at the correct altitude to receive BOTH direct waves and ground-reflected waves, the signals may be either reinforced or weakened, depending upon the relative phases of the two waves.

The TROPOSPHERIC WAVE is the part of the wave that is subject to the influences of the atmosphere at the low altitudes. The effects of the atmosphere on this type of wave propagation are most pronounced at frequencies above the high end of the H-F band. Communication by the use of the tropospheric wave is gaining in importance, both from the standpoint of its usefulness, and its frequent unpredictable ranges. This type of communication is discussed in more detail later in this chapter.

The SURFACE WAVE brings most of the low and medium frequency broadcasts to your receiver. These frequencies are low enough to permit this wave to follow the surface of the earth. The intensity of the surface wave decreases as it moves outward from the antenna. This ATTENUATION-rate of decrease-is influenced chiefly by the conductivity of the ground or water and the frequency of the wave.

As it passes over the ground, the surface wave induces a voltage in the earth, setting up eddy currents. The ENERGY to create these currents is PIRATED or taken away from the surface wave. In this way, the surface wave is weakened as it moves away from the antenna. increasing the frequency rapidly increases the rate of attenuation. Hence surface wave communication is limited to the lower frequency.

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Since the electrical properties of the earth over which the surface waves travel are relatively constant, the signal strength from a given station at a given point is nearly constant. This holds true in practically all localities, except those that have distinct rainy and dry seasons. There, the difference in the amount of moisture will cause the soil's conductivity to change.

It is interesting to note that the conductivity of salt water is 5,000 times as great as that of dry soil. The superiority of surface wave conductivity by salts water explains why high-power, low-frequency transmitters are located as close to the edge of the ocean as practicable.

Do not think that the surface wave is confined to the earth's surface only. It also extends a considerable distance up into the air, but it drops in intensity as it rises.

THE SKY WAVE

In behavior, the SKY WAVE is quite different from the ground wave. The part of the expanding lobe that moves toward the sky "bumps" into an IONIZED layer of atmosphere, called the IONOSPHERE, and is bounced or bent back toward the earth. If your receiver is located in the area where the returning wave strikes, you will receive the program clearly even though you are several hundred miles beyond the range of the ground wave.

THE IONOSPHERE

The ionosphere is found in the rarified atmosphere, approximately 30-350 miles above the earth. It differs from the other atmosphere in that it contains a higher percentage of positive and negative ions.

The ions are produced by the ultra violet and particle radiations from the sun. The rotation of the earth on its axis, the annual course of the earth around the sun, and the development of SUN-SPOTS all affect the number

You must understand that the ionosphere is constantly changing. Some of the ions are re-combining to form atoms, while other atoms are being split to form ions. The rate of formation of ions and recombination depends upon the amount of air present, and the strength of the sun's radiations.

At altitudes above 350 miles, the particles of air are too sparse to permit large-scale ion formation. At about 30 miles altitude, few ions are present because the rate of recombination is too high. Also few ions are formed, because the sun's radiations have been materially weakened by their passage through the upper layers of the ionosphere with the result that below 30 miles, too few ions exist to affect materially sky wave communication.

LAYERS OF THE IONOSPHERE

Different densities of ionization make the ionosphere appear to have layers. Actually there is no sharp dividing line between layers. But for the purpose of discussion a sharp demarkation is indicated.

The ionized atmosphere at an altitude of between 30 and 55 miles is designated as the D-LAYER. Its ionization is low and has little effect on the propagation of radio waves except for the ABSORPTION of energy from the radio waves as they pass through it. The D-layer is present only during the day. This greatly reduces the field intensities of transmissions that must pass through daylight zones.

The band of atmosphere at altitudes between 55 and 90 miles contains the E-LAYER. It is a well-defined band with greatest density at an altitude of about 70 miles. This layer is present during the daylight hours, and is also present in PATCHES, called "SPORADIC E," both day and night. The maximum density of the regular E-layer appears at about noon, local time.

The ionization of the E-layer at the middle of the day is sufficiently intense to refract frequencies up to 20 mcs.

The F-layer extends from the 90-mile level to the upper limits of the ionosphere. At night only one F-layer is present. But during the day, especially when the sun is high, this layer separates into two parts, F₁ and F₂, as illustrated in figure 149.

As a rule, the F₂-layer is at its greatest density during early afternoon hours. But there are many notable

SPORADIC E LAYER

In addition to the layers of ionized atmosphere that appear regularly, erratic patches occur at E-layer heights much as clouds appear in the sky. These clouds are referred to as SPORADIC-E IONIZATIONS. These patches often are present in sufficient number and intensity to enable good radio transmission over distances where it is not normally possible.

Sometimes sporadic ionizations appear in considerable strength at varying altitudes, and actually prove harmful to radio transmissions.

EFFECT OF IONOSPHERE ON THE SKY WAVE

The ionosphere has three effects on the sky wave. It acts as a CONDUCTOR, it absorbs energy from the wave,

Don't think that the antenna reaches as near the ionosphere as is indicated in figure 150. Remember the tallest antenna is only about 1,000 feet high.

The ability of the ionosphere to return a radio wave to the earth depends upon the ANGLE at which the sky wave strikes the ionosphere and upon the FREQUENCY of the transmission.

For discussion, the sky wave in figure 151 is assumed to be composed of four rays. The angle at which ray 1. strikes the ionosphere is too nearly vertical for the ray to be returned to the earth. The ray is bent out of line, but it passes through the ionosphere and is lost.

The angle made by ray 2 is called the CRITICAL ANGLE for that frequency. Any ray that leaves the antenna at in angle GREATER than theta (θ) will penetrate the ionosphere

As the FREQUENCY INCREASES, the size of the CRITICAL ANGLE DECREASES. Low frequency fields can be projected straight upward and will be returned to the earth. The HIGHEST FREQUENCY that can be sent directly upward and still be returned to the earth is called the CRITICAL Frequency. At sufficiently high frequencies, the wave will not be returned to the earth, regardless of the angle at which the ray strikes the ionosphere.

The critical frequency is not constant. It varies from one locality to another, with differences in time of day, with the season of the year, and according to sunspot cycle.

This variation in the critical frequency is the reason why you should use issued predictions-FREQUENCY

Nomograms and frequency tables are prepared from data obtained experimentally from stations scattered all

Refer again to figure 151. The area between points B and C will receive the transmission via the REFRACTED SKY WAVE. The area between points A and E will receive its signals by GROUND WAVE. All receivers located in the SKIP ZONE between points E and B will receive NO transmissions from point A, since neither the sky wave nor the ground wave reaches this area.

EFFECT OF DAYLIGHT ON WAVE PROPAGATION

The INCREASED IONIZATION during the day is responsible for several important changes in sky-wave transmission-

First-It causes the sky-wave to be returned to the earth NEARER to the point of transmission.

Next-The EXTRA ionization increases the ABSORPTION of energy from the sky-wave. If the wave travels a sufficient distance into the ionosphere, it will lose all its energy.

In figure 152, you see the results of daylight in increasing refraction and absorption. These two factors usually combine to reduce the effective daylight communication range of low-frequency and medium-frequency transmitters to surface wave ranges.

HIGH-FREQUENCY, LONG-RANGE COMMUNICATION

The high ionization of the F₂-layer during the day, enabling refraction of high frequencies which are not greatly absorbed, has an important effect on transmissions of the HF band. Figure 153 shows how the F₂-layer

The waves are partially bent in going through the E-layer and F₁-layer, but are not returned to the earth until the F₂-layer completes the refraction. At night, when only one layer is present, very-high-frequency waves may pass right through the ionosphere.

The EXACT FREQUENCY to be used to communicate with another station depends upon the condition of the

MULTIPLE REFRACTION

Many times the REFRACTED WAVE will return to the earth with enough energy to be bounced back up to the ionosphere, and then be refracted back to the earth a second time.

In figure 154, the ray strikes the earth at point A with sufficient force to be reflected back to the ionosphere

FADING

FADING is the result of variations in signal strength at the receiver. There are several causes. Some are easily understood, others are more complicated. One cause is probably the direct result of interference between single-hop and double-hop transmissions. If the two waves arrive IN PHASE, the signal strength will be

Interference fading is also severe in regions where the ground and skywave are in contact with each other. This is especially true if the two are approximately of equal strength. Fluctuations of the sky wave with a steady ground wave can cause worse fading than sky-wave transmission alone.

The way the waves strike the antenna and the variations in absorption in the ionosphere are also responsible for fading. Occasionally, sudden ionospheric disturbances will cause complete absorption of all sky-wave radiations.

Receivers that are located near the outer edge of the skip zone are subjected to fading as the sky wave alternately strikes and skips over the area. This type of fading is sometimes so complete that the signal strength may fall to near zero level.

FREQUENCY BLACKOUTS

FREQUENCY BLACKOUTS are closely related to some types of fading, but this fading is complete enough to blot-out the transmission completely.

Changing conditions in the ionosphere shortly before sunrise and after sunset may cause complete BLACKOUTS at certain frequencies. The HIGHER frequencies pass through the ionosphere, while the LOWER ones are absorbed by it.

IONOSPHERIC STORMS-turbulent conditions in the ionosphere-often cause communication to be erratic. Some frequencies will be completely blotted out, while others may be reinforced. Sometimes these storms develop in a few minutes, and at other times they require as much as several hours. A storm may last several days. You can expect these storms to recur at about every 27 days.

When frequency blackouts occur, you will have to be on the ball to prevent complete loss of contact with other ships or stations. When the storms are severe, the critical frequencies are much lower, and the absorption in the lower layers of the ionosphere is much higher.

V. H. F. AND U. H. F. COMMUNICATION

In the recent years, there has been a trend toward the use of frequencies above 30 mc., for short-range, ship-to-ship, and ship-to-airplane communications.

Early concepts suggested that these transmissions traveled in straight lines. This naturally leads to the assumption that the V.H.F. transmitter and receiver must be within sight of each other to supply radio contact.

Extensive use and additional research show the early "line-of-sight" theory to be frequently in error because radio waves of these frequencies are refracted. The transmitter does not always need to be in sight of the receiver.

This type of communication still is called by its popular name, "Line of sight transmission." But it is better to call it V.H.F. and U.H.F. transmission. It is true that U.H.F. and V.H.F. waves follow approximately straight lines, and large hills or mountains cast a radio shadow over areas in much the same way as light creates a shadow. A receiver located in shadow will receive a weakened signal, and in some cases, no signal at all.

In theory, the range of contact is the distance to the horizon, and this distance is determined by the heights of the two antennas. But communication is often possible many miles beyond the assumed horizon range. Be sure to remember this point when your ship is in waters where radio security is essential.

EFFECT OF ATMOSPHERE ON V. H. F. AND U. H. F. TRANSMISSIONS

The abnormal ranges of V.H.F. and U.H.F. contacts are caused by abnormal atmospheric conditions within a few miles of the earth. Normally, you will find the warmest air near the surface of the water. The air gradually becomes cooler as you gain altitude. However, unnatural situations often develop where WARM bands of air are above the COOLER layers. This unusual situation is called a TEMPERATURE INVERSION.

The differences in the index of refraction form CHANNELS or DUCTS that will pipe V.H.F. and U.H.F. signals many miles beyond the assumed normal range.

If an antenna extends into the duct or if. wave motion lets the wave enter a duct after leaving an antenna, the transmission may be conducted long distances to another ship whose antenna extends into the duct. This is illustrated in figure 155.

WHEN WILL DUCTS BE FORMED?

When operating this high-frequency equipment, you must be able to recognize the weather conditions that lead to DUCT FORMATIONS. Since the duct is not visible to the eye and since complete aerological information is not always available, you must rely on a few simple visible evidences and a lot of common sense.

1. A wind is blowing from land.
2. There is a stratum of quiet air.
3. There are clear skies, little wind, and high barometric conditions.
4. A cool breeze is blowing over warm open ocean, especially in the tropic areas and in the trade-wind belt.
5. Smoke, haze, or dust fails to rise, but spreads out horizontally.
6. Your receiver is fading rapidly.
7. The moisture content of the air at the bridge is considerably less than at the sea's surface.
8. The temperature at the bridge is 1 or 2 degrees F. HIGHER than at the sea's surface.

GENERAL USE OF FREQUENCIES

Each frequency band has its own special uses. The uses depend upon the nature of the waves-surface, sky, or space-and the effect that the sun, the earth, the ionosphere, and the atmosphere have upon them.

It is almost impossible to lay down fixed rules for the use of what frequency for what purpose. Some general statements can be made, however, on what FREQUENCY BANDS are best used for what purposes. COMINST, in Article 6520, lists each frequency band and what its best use is.

Most rules for the use of frequencies deal with VARIATIONS that are beyond human control. This is particularly true of medium- and high-frequency transmissions using the SKY wave.

Make intelligent use of nomograms and tables.

One SURE rule-if you want to be reasonably certain that a LONG-RANGE COMMUNICATION gets through, use HIGH POWER and LOW FREQUENCY. That's what the international communication systems and most of your big FOX stations use. However, this takes an antenna array

Note in figure 156 how the SKY WAVE builds up to a peak of daytime usefulness in the H.F. band. At night the peak is in the top third of the M.F. band. Note also how the usefulness of the GROUND, or surface, WAVE declines steadily as the higher frequencies are reached, until it is altogether useless in H.F. But as the SPACE WAVE, it becomes the only means of communication in V.L.F. and for a certain range above V.L.F.

And be sure you remember that all SKY WAVE transmission-and that means almost all from 1,600 to 30,000 kc.-is associated with SKIP DISTANCES. In other words you can get great range, but in the process you'll skip a lot of receiving stations in between-possibly the one you most want to receive your message.

THE NAVY FREQUENCY BAND

Most important to you in the chart is the shaded area in the M.F. and H.F. bands-from 2,000 to 18,100 kc. (2 to 18.1 mc.) . That, as you should already know, is the standard band for NAVAL COMMUNICATIONS from SHIP-TO-SHIP and SHIP-TO-SHORE. It's the band you'll use most frequently for TRANSMITTING messages, the one which your standard transmitters, such as the TBK, TBL, and TBM, cover.

It's right in the SHORT-WAVE area. Thus, it's SKY-WAVE TRANSMISSION and is affected by SKIP DISTANCES. As the chart shows, when you want range in DAYTIME, use the UPPER PORTION of the band-roughly from 3 mc. to 18 mc. But for NIGHT communication, drop down below 3.5 mc. The three frequencies most commonly used in this band are 2,716 kc. (2.716 mc.), 2,844 kc. (2.844 mc.), and 4,235 kc. (4.235 mc.)-the good old NERK series.

To help you in the use of this band, and to utilize properly knowledge of SKIP DISTANCE, the Navy publishes NRPM's containing tables which show the best

Look at the table in figure 156. It's a sample, but it's for communication with Balboa during February 1945. Your ship is 750 miles off the Pacific coast of Central America during that February, the time is 1200 GCT, and you wish to get a message to NBA, Balboa. Look at your table for the proper time, then move over to the third column-500 to 1,000 miles-in the second vertical row of figures, since Balboa is east of you. The recommended frequency is 4 mc. Send your message.

These tables will be supplied to your ship in the form of NRPM's and will cover three months, with a separate table for each month and for each shore station.

NOMOGRAMS

As a further aid, you'll also be supplied with NOMOGRAMS-again in NRPM's. They cover three-month periods, and each nomogram covers a range of 10° in latitude. A nomogram may be used for any path where the midpoint lies within the range of latitude of the particular nomogram. It will give you the proper frequency for any time of day in Local Civil Time-LCT-for any transmission of from 1 to 2,200 miles. With proper use of nomograms and the frequency tables you should be all set for communicating in the Navy H-F band.

Figure 157 shows a nomogram that is typical of the series you'll use most frequently. To use one, first locate approximately the midpoint of the transmission path on the map shown on the last page of each published nomogram series. Determine the latitude, local time, and the "zone" at this midpoint. (The zones are labeled E, W, and I on the map to represent East, West and

For instance, you want to make a transmission of 800 nautical miles at 0400. You have consulted the map and found that the midpoint of the path is at roughly 30° north latitude and lies in the I (Intermediate) zone. Local time at this point is approximately the same (0400). Line up your straight edge on the nomogram in figure 157 between the Intermediate Zone MILES Line on the right side and the Intermediate Zone TIME Line on the left. Then look at your recommended frequency column in the middle. You'll note that it is intersected right at 8 mc. by the diagonal line made by the ruler. That's your frequency-8 mc.

BUT THERE ARE OTHERS

The so-called Navy band is not the only one used. It's the standard ship long-distance communications frequency-your chief TRANSMITTING frequency. But the major FOX skeds are more generally broadcast way down the line in the V.L.F. and L.F. bands. NGP's major sked, for instance, is broadcast on 19.8 kc., and NSS's is on 18 kc. True, the big stations also broadcast FOX in the M.F. and H.F. bands and some of the secondary skeds are broadcast only in the higher frequencies. But if you want to be sure to get that FOX, flip the receiver dial way, way down.

Scooting up again into V.H.F. and U.H.F., you enter your TACTICAL bands. When it's radio phone communication over the TBS or TDQ/RCK, go on up and start playing around with the SPACE WAVE. And remember your range limitations.

As a final tip on the proper use of frequencies, be sure you know the proper PUBLICATIONS to use. Appendix I of COMINST gives the big shore station circuits, the FOX

Also, of course, there's the CONFIDENTIAL publication-The U. S. Naval Radio Frequency Usage Plan-which lists them all and what the Navy's currently using them for. And there are the IRPL Radio Propagation Handbook (DNC13-1), USF-70, current NRPM's and circular letters to turn to for further up-to-date frequency data when needed. DNC-22 gives the dope on V.H.F. propagation.