April 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Much has been learned and
published about the various layers of the
since its first being measured with sounding rockets in the 1950s. Both professional
and amateur communications operators are very interested in its properties. Long
term (seasonal) and short term (daily) patterns have been discerned that help make
some of the ionosphere's effect on the propagation of electromagnetic (radio) signals
somewhat predictable, but there are still many unpredictable events that hamper
- and often enhance - communications. Sun spots and coronal mass ejections, volcanic
eruptions, hurricanes, and other natural events cause sporadic changes in the ionosphere.
Real-time atmospheric propagation conditions are monitored and reported on many
websites worldwide (see dr2w.de,
The quality of a long-distance communications system depends on
the ability of the ionosphere to bend and reflect radio waves back to earth.
By H. Charles Wood
Fig. 1 - Groups of well-formed sunspots (arrows) photographed
in February 1956 near the maximum of an 11-year cycle. Ultraviolet radiation during
height of solar cycle is sufficient to produce highly ionized F2 layer
capable of supporting radio signals of frequencies up to 50-55 MHz.
Some radio waves penetrate the ionosphere and fly outward to the stars; others
are stopped, bent, and reflected to touch the earth miles away from their origin.
Propagation - the movement of electromagnetic (radio) waves through the air,
or "ether" as it used to be called - is the means by which wireless communications
systems communicate. The distance over which these waves travel is a function of
frequency; angle of radiation; and atmospheric noise such as sunspots, aurora borealis,
and other solar activity.
This article discusses the various phenomena which must be considered when predicting
Ground Wave and Sky Wave
Engineers agree that radio waves are electromagnetic waves consisting of traveling
electrostatic and electromagnetic fields of energy whose lines of force are at right
angles to each other in a plane perpendicular to its path. Except for frequency,
the electromagnetic field has the same characteristics as light waves - both have
a speed of 300-million meters per second, and both are capable of being refracted
or reflected. Two types of electromagnetic waves are emitted by a transmitting antenna,
one travels along the ground and is called the ground wave. The other, referred
to as the sky wave, travels through the atmosphere and has little contact with the
earth along most of its path.
At low frequencies (below 500 kHz), the ground wave is the most important, frequently
traveling a thousand miles or more. In the broadcast band, ground waves are received
200 miles or more away over land and perhaps twice that distance over sea water.
At higher frequencies, however, the range drops off rapidly. Above 4 MHz, the ground
wave is only useful for very short distance communications.
The sky wave leaves the transmitting antenna at various angles of elevation to
the earth's surface, ranging from 1 or 2 degrees to 90 degrees, and would travel
out into empty space were it not that, under certain conditions, it can be reflected
or refracted high in the atmosphere to return to the earth at distances varying
from zero to about 2500 miles (4000 kilometers) from the transmitter. By a series
of alternate reflections by the upper atmosphere and the earth's surface, electromagnetic
waves can be transmitted or propagated around the world.
Fig. 2 - Block diagram of a typical ionosonde. This radar-type
device explores the ionosphere and records the MUF for a specific time of day. There
are 150 observatories around the world.
The medium which causes the bending or reflection of the sky wave is called the
"ionosphere", a region in the upper atmosphere where free ions and electrons exist
in sufficient quantity to cause a change in the reflection index. Ultraviolet radiation
by the sun is considered to be responsible for producing most of the ions. In the
ionosphere, ionization does not change at a uniform rate, that is, proportionate
to height, rather the ions form in layers with each layer consisting of a highly
concentrated central region, tapering off in intensity above and below this area.
Three major layers of ionization exist in the upper reaches of the atmosphere: the
"D" region, "E" region, and "F" region, with the F layer being further subdivided
into F1 and F2.
The D-region, lowest of the ionospheric layers, ranges in height from 30 to 50
miles above the earth. At this altitude the atmosphere is still relatively dense
and atoms broken into ions by solar radiation quickly recombine. Here the amount
of ionization is directly dependent upon the amount of sunlight. Therefore, it is
maximum at noon and almost zero at dusk. In the D-layer, most electromagnetic wave
energy (sky waves under 5 MHz) is expended in the form of heat. Thus, the D-layer
acts as an absorber with little effect on bending radio waves back to earth.
The E-layer, the second significant ionized layer in the atmosphere, has a mean
height of about 6.5 miles and because it, too, is in an area of comparatively high
atmospheric density, ionization varies with the elevation of the sun, much the same
as the D-layer, although not to as great an extent. And, also for reasons mentioned
previously, a radio wave below 5-7 MHz passing through the E-layer loses a large
portion of its power in the form of heat.
The F-layer, the third and most important ionized area for long-distance short-wave
communications purposes, is approximately 150 to 250 miles above the earth. At this
altitude, the atmosphere is so thin that electrons and ions are slow in recombining.
Ionization reaches a maximum shortly after noon and decreases very slowly to a minimum
shortly before dawn. At sunrise, intensity increases rapidly to a maximum within
an hour or two. During daylight hours, the F-layer sometimes splits into two distinct
areas, called the F1 and F2-layers. The F1-layer
is of little importance to radio communications except that it acts as an absorber
much like the D- and E-layers and disappears shortly after sunset. The F2
region of the ionosphere is normally the most important parameter in estimating
the performance of a high-frequency transmission but, unfortunately, it is also
the most unpredictable of the three ionospheric layers.
The Critical Angle
The amount by which a radio wave is bent in the F2 region is dependent
upon two factors: the density of the ionized layer, and the frequency of the wavelength.
The greater the ionization, the more it bends at a specific frequency; or for a
specific degree of ionization intensity, the refraction will be greater as the frequency
is lowered. It thus becomes apparent that if the ionization is intense enough and
the frequency is low enough, a radio wave entering the F2-layer at a
90-degree angle will be reflected back to earth; conversely, if the frequency is
raised or the amount of ionization is decreased, a point will be reached where the
refraction will not be sufficient to return the radio wave. Fig. 3 shows a
specific radio wave whose frequency is of such a value as to penetrate the ionosphere
at a 90-degree angle but is reflected back to earth when the angle is decreased.
The angle at which the radio wave begins to bend back toward earth is called the
critical angle. Fig. 3 also shows low-angled waves being bent back to earth
in a single-hop propagation. As mentioned earlier, multi-hop transmissions are accomplished
by alternate reflections of the electromagnetic wave between the earth and the F2-layer.
Because long-distance high-frequency radio communications depends upon the ability
of the ionosphere to return the radio signals back to earth, it is apparent that
predictions of ionization intensities in the various regions of the F2
and other layers are essential to the calculation of sky-wave circuit (the communications
path over which intelligence is transmitted) performance. Predictions of the F2
characteristics are computed about three months in advance by the Environmental
Science Service Administration and published in a booklet called "Ionosphere Propagation
Predictions." These predictions, as well as the accompanying isograms which show
the maximum usable transmitter frequency for a specific hour of a specific month,
are essential tools for the communications engineer. A discussion of the methodology
used by ESSA in the compilation of the charts appears later in this article. The
mechanics involved in using the charts are beyond the scope of this article; however,
they are described in ESSA Handbook #90.
The Optimum Frequency
Fig. 3 - Typical daytime ionosphere layer diagram showing
high-angle radiation penetrating the F2-layer, but reflected back toward
earth as the wave-angle is reduced. All multi-hop transmissions are at wave-angles
below the critical angle.
The complexity of the propagation phenomenon, the diversity of radio programming
services, and the fluctuations in traffic density within the high-frequency bands
preclude any clear and simple criteria for the selection of optimum frequencies.
And because these are elusive factors, a high-frequency communications engineer's
job is not an easy one. Since the density of the F2-layer and its height
above the surface of the earth vary with the time of day and the season of the year,
and because of the 11-year sunspot cycle (a factor we shall discuss later), it is
common practice for communications engineers to select a new set of operating frequencies
and time schedules every three months.
In general (within the high-frequency spectrum), radio noise tends to decrease
as frequency increases; also, propagation losses become less severe as frequency
increases. Therefore, the higher the operating frequency the better the signal-to-noise
ratio. However, the frequency can be increased to a point where the critical frequency
is exceeded, or where the ionosphere reflection becomes improbable. This point is
called the Maximum Usable Frequency (MUF). Through the use of the prediction charts
and a working knowledge of the geographic variation in the electron density of the
F2-layer, it is possible to predict this upper limit. It would therefore
seem to the engineer's advantage to simply operate very close to the MUF. Unfortunately,
the high-frequency circuit calculation is not that simple. Because the F2-layer
density changes constantly, it would be necessary to continuously change the operating
frequency throughout a 24-hour day - a laborious task to say the least.
If enough is known about the ionosphere to determine the critical angle, etc.,
at specific frequencies 90 percent of the time, these frequencies are considered
adequate as estimates of upper limits for system planning. Selected operating frequencies
are approximately 10 percent below the MUF. This minimizes circuit interruptions
from irregularities in ionospheric conditions and reduces to a minimum the number
of frequency changes required to maintain acceptable continuity throughout the transmission
period. This operating frequency is called the Optimum Traffic Freq. (OTF).
Because energy absorption in the ionosphere's D- and E-layers increases as frequency
decreases (assuming that the transmitter's output power remains constant), the power
available at the receiver's input decreases. The atmospheric noise level also increases
with a decrease in frequency. These phenomena combine to lower the signal-to-noise
ratio and reduce circuit reliability.
When the required signal-to-noise ratio (the minimum acceptable level for communications)
equals the available signal-to-noise ratio, the circuit may be expected to have
acceptable quality on half the days within the month. In other words, the probability
of satisfactory performance on any given day will be 0.5. The probability of satisfactory
signal-to-noise ratio at any given hour is defined as circuit reliability. As the
available signal-to-noise ratio exceeds the required signal-to-noise ratio, circuit
Since the available signal-to-noise ratio decreases as the transmitter frequency
is decreased, it becomes apparent that a point can be reached where a further reduction
in frequency will result in unacceptable circuit reliability. This point is defined
as Lowest Useful Frequency (LUF). The LUF depends on transmitter power, the factors
that determine the path-loss (frequency, season, geographic location), and noise
level. Of course, one of the principal factors is D- and E-level absorption and,
since their intensity is maximum at noon, LUF is highest at noon. When calculating
an operating frequency, the engineer must select one that is above the LUF but not
greater than the OTF.
Selection of Frequency Complement
Absolute continuity of any high-frequency radio service is impossible to achieve
even if an unlimited choice of operating frequencies is available. For a 24-hour
day, frequency complements are based on the concept of Maximum Feasible Continuity
or that a theoretical increase in circuit continuity will be negligible if additional
frequencies are used, but a significant decrease in continuity is possible if fewer
frequencies are used. The required frequency complement depends upon the type of
circuit used. Communications engineers classify all circuits in two groups: circuits
requiring Maximum Feasible Continuity and circuits requiring Moderate Continuity.
Circuits requiring maximum continuity are heavily loaded telegraph and telephone
circuits which maintain high traffic at all times. Telegraph circuits in this category
are usually operated by high-speed machines which transmit 100 or more words per
minute, while telephone circuits generally employ several multiplex channels of
a single-sideband system with multi-line terminations at each end. Such circuits
usually have large directional antennas and employ diversity reception and high-power
transmitters. The standard frequency complement for these circuits assures at least
one frequency between OTF and LUF at all times plus two additional frequencies to
permit flexible operation in the event of atmospheric disturbance.
When choosing frequencies for this service, the communications engineer selects
one high frequency, one low frequency, and one middle frequency. The high frequency
is strictly a daytime frequency (computed to be well below the OTF for at least
four hours every day during the period the circuit is operated).
The low frequency of the three-frequency complement is exclusively a night channel,
and is chosen as the highest frequency for which less than two hours of skip is
indicated on the lowest of the OTF curves for the required operating period.
The middle frequency selection is made to maximize the number of hours during
which at least one frequency is between LUF and OFT for the required circuit operation.
Circuits requiring only moderate continuity are usually those that provide communications
where the needs are sufficiently critical to warrant extension of telephone, cable,
or v.h.f. facilities. Many such circuits are used to provide occasional service
to remote areas or are operated in situations where occasional delays or traffic
slow-downs can be tolerated. The standard frequency complement for these circuits
is usually one day frequency and one night frequency.
Therefore, the logical selection for the daytime frequency must be one that will
be above the mid-day LUF but far enough below the OTF to give skip-free service
to the intended areas. The night-time frequency is chosen as the highest frequency
for which less than four hours of skip is indicated on the lowest OTF curve for
the operation period.
As mentioned earlier, solar bombardment of the earth's upper atmosphere by ultraviolet
rays is the major influence in the production of ionized layers. Because the earth
is constantly bathed by these rays, the F2 layer is always present; however
its density and height above the earth changes constantly.
Although a completely satisfactory measure of solar activity is not available,
the average number of sunspots over a 12-month period (as observed at Zurich, Switzerland)
has formed the basis for much of the radio propagation analysis, and is presently
being used as an index of the solar activity. Other celestial occurrences are under
study at this time in an effort to improve predictions.
A linear relationship between the F2-layer and the sunspot number
was first established in the early 1930's. Observations made during the succeeding
11-year solar cycle continued to correlate well with the sunspot number. This led
to the conclusion that, from a practical point of view, it seemed advisable to use
this phenomenon as a basic reference point. Generally speaking, as the sunspot number
increases during the given 11-year solar cycle, the F2-layer increases
in density and altitude, permitting the higher frequencies to "open up" to various
parts of the world. Fig. 1 shows a group of well-formed sunspots observed in
Three other sunspot variations must be taken into account when predicting solar
activity: (1) solar rotation (which lasts about 27 days; this causes the longer-lived
sunspots to reappear several times on the earth-side of the sun's surface); (2)
seasonal changes brought about by the difference in solar bombardment reaching the
two hemispheres during the winter and summer months; (3) detectable waxing and waning
of sunspot numbers over long periods of time (one hundred years or more).
Occasionally, during times of maximum 11-year cycles, tremendous bursts of solar
energy, called "flares", eject highly charged electrified particles millions of
miles into space around the sun. Energy bursts such as these often cause the F2
layer to temporarily disappear, bringing about a complete radio circuit breakdown
above about 4 MHz.
It is apparent that for purposes of radio communications, it is necessary to
have as much information as possible about ionospheric characteristics. Therefore,
sounding stations have been installed in more than 150 locations around the world
to provide a steady flow of solar data to a central location where it is analyzed,
computed, and relayed to users.
Height and density of the F2 layers are obtained at the sounding stations
by the use of an ionosonde, a pulse radar device in which the exploring frequency
can be varied over a wide range of frequencies from about 1 MHz to 25 MHz. The ionosonde
is equipped with an antenna system to direct radio energy at a 90-degree angle and
is designed to measure the elapsed time for a pulse to travel to the F2
layer and return. As the frequency of the ionosonde is increased, the point at which
the radio signal is no longer returned is recorded. This is the Critical Frequency.
Fig. 2 is a diagram of a typical ionosonde.
Posted June 6, 2023
(updated from original post