In this chapter you will learn how radio waves spread
themselves over the face of the earth, carrying messages
and music from Washington to Saipan and from San
Diego to Suez in less than a second.
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
moves outward in contact with the ground to form the
GROUND WAVE, and the rest of the wave moves upward
and outward to form the SKY WAVE. This is illustrated
in figure 148.
Figure 148.-Formation of the ground wave and sky wave.
The GROUND and SKY portions of the radio wave a.
responsible for two different METHODS of carrying the
messages from transmitter to receiver.
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
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 DIRECT WAVE travels directly from the transmitting antenna to the receiving antenna. For example-two airplanes are several thousand feet in the air and
only a few miles apart. This direct wave is not influenced by the ground, but may be affected by the
atmospheric conditions through which the wave travels.
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.
Shore establishments are able to furnish long-range
ground-wave communication by using frequencies between about 18 and 300 kc. with
EXTREMELY HIGH POWER.
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 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
of ions present in the ionosphere, and these in turn affect
the quality and distance of radio transmission.
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
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
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.
back to the earth. This is of great importance to daylight transmissions for distances up to 1,500 miles.
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
Figure 149 -E-layer and F-layer of the ionosphere.
exceptions of maximum F2 density existing several hours
later. Shortly after sunset, the F1- and F2-layers recombine into a single F-layer.
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
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,
and it REFRACTS or bends the sky wave back to the earth
as illustrated in figure 150.
Figure 150.-Refraction of the sky wave by the ionosphere.
When the wave from an antenna strikes the ionosphere,
the wave begins to bend. If the frequency is correct, and
the ionosphere sufficiently dense, the wave will eventually
emerge from the ionosphere and return to the earth. If
your receiver is located at either of the points B, in
figure 150, you will receive the transmission from point A.
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
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
Ray 3 strikes the ionosphere at the SMALLEST ANGLE
that will be refracted and still return to the earth. Any
smaller angle, like ray 4, will be refracted toward the
earth, but will miss it completely.
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
This variation in the critical frequency is the reason
why you should use issued predictions-FREQUENCY
Figure 151.-Effect of angle of refraction on sky wave.
TABLES or NOMOGRAMS-to determine the MAXIMUM
USABLE FREQUENCY (MUF) for any hour of the day.
Nomograms and frequency tables are prepared from
data obtained experimentally from stations scattered all
over the world. All this information is pooled and you
get the results in the form of a long-range prediction that
removes most of the guess work from radio communication.
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
Figure 152.-Effect of daylight on medium-frequency sky-wave transmission.
And-The presence of the F1- and E-layers with the
F2-layer make long-range, high-frequency communication possible by all three layers, provided the correct
frequencies are used.
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
Figure 153.-Effect of the F2-layer on transmission of high-frequency signals.
completes the refraction and returns the transmissions
of these frequencies to the earth, making possible long-range,
high-frequency communication during the daylight
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
ionosphere and upon the distance between stations. Since
the ionosphere is constantly changing, you must use the
nomograms and tables to pick the correct frequency for
desired distance at a given time of day.
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
In figure 154, the ray strikes the earth at point A
with sufficient force to be reflected back to the ionosphere
Figure 154.-Multiple refraction and reflection of a sky wave.
and then refracted back to the earth a second time. Occasionally a sky wave has sufficient energy to be refracted
and reflected several times, thus greatly increasing the
range of transmission.
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
increased, but if the phases are opposed, they will cancel
each other and weaken the signal.
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
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
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 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
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
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.
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.
Whenever TEMPERATURE INVERSIONS are present, the
AMOUNT OF REFRACTION-called INDEX OF REFRACTION-is
different for the air trapped WITHIN the inversion than it
is for the air outside the 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.
Figure 155.-Duct effect on V.H.F. and U.H.F. transmissions.
Sometime these ducts will be in contact with the water
and may extend a few hundred feet into the air. At
other times the duct will start at an elevation of about
500 to 1,000 feet, and extend an additional 500 to 1,000
feet in the air.
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.
The following rules have exceptions, but you can expect a duct to be formed when-
1. A wind is blowing from land.
2. There is a stratum of quiet air.
3. There are clear skies, little wind, and high
4. A cool breeze is blowing over warm open ocean,
especially in the tropic areas and in the trade-wind
5. Smoke, haze, or dust fails to rise, but spreads out
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
so large that it's not usable with shipboard transmission. So, to be certain a message for a distant point
gets through, RELAY IT-send it to the nearest large shore
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
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
Figure 156.-Recommended frequency chart.
frequencies within this band for communication with various
shore stations. These tables are issued QUARTERLY.
There will be a separate one for EACH major shore station.
They give the recommended frequency for every HOUR
of day for every distance 250 to 5,000 miles for some
stations. The DIRECTION of the receiving station from
your ship is also taken into account.
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.
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
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
Intermediate.) Then line up a ruler through the distance of
transmission (right-hand column) and the local time
(LCT) at the midpoint of the path, which is on the left-hand column. Where the straight edge intersects the frequency scale in the middle of the nomogram, you'll find
the recommended frequency.
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
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,
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
stations and the frequencies they use, the ship-shore
facilities provided by the shore organizations, and the
stations giving DF calibrating service on frequencies
ranging from 150 to 1,500 K.C., as well as those giving
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.