March 1940 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
Reading through this article reminds me
of studying for the amateur radio exams. In fact, the information presented in this
1940 QST piece does not seem to be lacking anything that contemporary discussions
include. My point is that a great amount of knowledge had already been amassed about
earth's upper atmosphere a mere four decades after the first transatlantic radio
communications were accomplished by
on December 12, 1901 from Poldhu in Cornwall, England, to Newfoundland, Canada.
Considering that at the time no instrumented sounding rockets had been launched
into the extreme upper layers (F1 & F2, beginning at around 120 mi | 200 km),
a lot had been discerned about characteristics as they pertain to radio communications.
Balloons were used for direct measurements at lower altitudes, but inferences based
on observed behavior of long distance communications provided most of the related
Normal and Irregular Characteristics Which Affect Wave Propagation
Fig. 1 - Simplified scale drawing showing possible paths
of wave travel. (1) A low-frequency-wave reflected by the E layer; (2) a wave of
higher frequency reflected from the F2 layer; (3) wave which passes completely through
the ionosphere because its frequency is too high to permit reflection.
As time goes on, the unremitting efforts of physicists in probing the upper atmosphere
bear the fruit of continual additions to our knowledge of the things that happen
to the signals We launch so hopefully into space. It's helpful, now and then, to
take stock of that knowledge. The accompanying article, extracted from the Bureau
of Standards' Letter Circular LC-575, is an up-to-date summary of known ionosphere
effects. "Must" reading for the chap who wants to keep well informed.
In the high atmosphere, above about 50 kilometers (30 miles), the air particles
are separated so far that collisions between them are far less frequent than in
the lower atmosphere, and when an air particle is ionized by ultraviolet radiation
from the sun it remains ionized for a considerable time. Therefore at any given
time a large proportion of the air particles are in an ionized condition. This does
not occur much below about 50 kilometers (30 miles), because the ionizing radiations
from the sun are largely absorbed in the higher regions of the atmosphere. Likewise
there is not very great ionization density above about 400 kilometers (250 miles),
because the air is so rare at such heights that there are not enough atoms to provide
for great ionization density. The region in which the ionization is great, great
enough in fact to affect radio wave transmission, is thus between 50 and 400 kilometers
(30 and 250 miles) above the earth's surface, and this region is called the "ionosphere".
The ionization in the ionosphere is not uniformly distributed with altitude but
is stratified, and there are certain definite layers in which the ionization density
is such as to reflect radio waves. These layers do not remain always at the same
height but vary diurnally, seasonally, and otherwise, in both height and ionization
density. There may be a considerable number of such layers at a given time. There
are two principal ones, called the "E" and "F" layers. The E layer is at a height
of 90 to 140 kilometers at different times, usually about no kilometers. The term
F-layer is ordinarily reserved for the other layer as it exists at night; in the
daytime during most of the year it divides into two layers which are called the
"F1" and "F2". The night F layer is at a height of about 180 to 400 kilometers.
The F1 layer exists in the daytime, at a height of about 140 to 250 kilometers.
The F2 layer also exists in the daytime, at a height of about 250 to 350 or more
kilometers in the summer, and about 150 kilometers in the winter day. (The "virtual"
heights, defined later, are somewhat higher than these values.) The fourth layer,
which is semi-permanent, is the "D" layer; it exists only in the daytime, and its
height is of the order of 50 to 90 kilometers. Little has been done on the determination
of the quantitative characteristics of the D layer, its effects being largely inferred
rather than directly observed. Existing knowledge covers mainly the E, F, F1, and
The structure of the ionosphere may be visualized in an elementary way from Fig. 1,
which is for a typical summer daytime condition, the E, F1, and F2 layers all being
present. This diagram is drawn to scale, so the angles of reflection of radio waves
from the layers may be estimated correctly. The three layers are shown as mere thin
lines, for simplicity. The layers have in fact a certain thickness, and the density
of ionization varies somewhat in this thickness. At the right of the diagram is
a rough illustration of a possible distribution of ionization density with height.
Dotted lines indicate two of many possible paths of radio waves from a transmitter
to a receiver as transmitted by reflection from the ionosphere layers. This picture,
simple as it is, does in fact represent the basic mechanism of radio wave transmission
over long distances. When we consider the variations of ionization and height of
the layers with time, and the effects of the ionization upon the received intensity
and the limits of transmissible frequency at any particular time, the picture loses
its simplicity. However, most of the phenomena of long-distance radio transmission
are completely explainable in terms of the ionosphere.
Fig. 2 - Showing the relation of virtual height (h) to the
height actually reached by the wave in reflection from the ionosphere.
The principal ionosphere characteristics which control or determine long-distance
radio transmission are the height and the ionization density of each of the ionosphere
layers. Since each layer has a certain thickness it is necessary to define the sense
in which the term "height" is used. When a ray or train of waves is reflected by
a layer, it is slowed down as soon as it starts to penetrate into the layer. The
process of reflection thus goes on from the point at which the waves enter the layer
until they have been fully turned down and leave the layer. This is true whether
the waves travel vertically or obliquely to the ionosphere. It is illustrated for
the oblique case in Fig. 2. The waves follow a curved path in the layer until
they emerge at the same vertical angle at which they entered. The time of transmission
along the actual path BCD in the ionized layer is the same as would be required
for transmission along the path BED if there were no ionized particles present.
The height h from the ground to E, the intersection of the two projected straight
parts of the path, is called the "virtual height" of the layer. This is the important
quantity in all measurements and applications.
The virtual height of a layer is measured by transmitting a radio signal from
A, and receiving at F both the signal transmitted along the ground and the echo,
or signal reflected by the ionosphere, and measuring the difference in time of arrival
of the two. The signal is a special, very short pulse in order that the two may
be separated in an oscillograph, as the time differences are mere thousandths of
a second. The difference between the distance (AE + EF) and AF is found by multiplying
the measured time difference by the velocity of light. From this and the known distance
AF, the virtual height h is calculated. It is usual to make AF zero by transmitting
the signal vertically upward and receiving it at the same place (and it is for this
case that the term "virtual height" rigorously applies). The virtual height varies
somewhat with frequency.
The effectiveness of the ions in reflecting the waves back to earth depends on
the number of ions present in a unit of volume, i.e., the ionization density The
higher the frequency, the greater is the density of ionization required to reflect
the waves back to earth. It has been shown that, for electron ionization, the relation
(for the ordinary ray, explained below) is
N = 0.0124f2
where N is the number of electrons per cubic centimeter and 1 is the highest
frequency, in kilocycles per second, at which waves sent vertically upward are reflected
back to earth. Waves of all frequencies higher than this pass on through the ionized
layer and are not reflected back to earth, while waves of all lower frequencies
are reflected. This frequency is called the "critical frequency," and measurement
of it is, with the equation just given, a means of measuring the maximum ionization
density in an ionized layer. (Waves of higher frequencies than the critical are
sometimes reflected by another mechanism - see discussion of "Sporadic E" later.)
Measurements of critical frequency are usually made by means of vertical or nearly
vertical transmission (that is, with the transmitter and receiver not far apart).
The process is to measure the virtual height, by the method described above, repeating
the determination at successively increasing frequencies until the waves are no
longer received back from the layer. The highest frequency at which waves sent vertically
upward are received back from the layer is the critical frequency of that layer.
Typical results of such measurements are illustrated in Figs. 3, 4 and 5, for different
times of year, day and night. They show critical frequencies as sharp increases
in virtual height.
Fig. 3 - Virtual height plotted against frequency for a
typical winter day, with critical frequencies indicated.
In Fig 3, starting at a frequency below 2000 kc. the virtual height is found
(in this example) to be about 110 kilometers, and remains at about this height until
about 3300 kc. The critical frequency of the E layer at the time of this measurement
is thus 3300 kc.; i.e., this is the highest frequency at which vertically-incident
waves are reflected back to earth. All such waves of higher frequency penetrate
through the E layer and go on up to a higher layer, the F2. The F2 layer has a greater
ionization density and so it reflects back waves of frequency greater than 3300
kc. It is not until frequencies greater than 11,500 kc. are used that the F2 layer
fails to reflect them, in the case illustrated.
Near the critical frequency the waves are excessively retarded in the ionized
layer, which accounts for the rise of the curve at the critical frequency. At the
right of the curve appear two critical frequencies for the F2 layer. This is an
indication of double refraction of the waves due to the earth's magnetic field,
two components of different polarization being produced. One is called the "ordinary"
wave and the other the "extraordinary" wave. The symbols o and x, respectively,
are used for these components. The critical frequency of a layer n is represented
by the symbol fn, and to such symbol the o or x is added as a superscript. Thus
the critical frequencies of the F2 layer for the ordinary and extraordinary waves
are indicated by the respective symbols, SYMBOL HERE and SYMBOL HERE.
In the case of the E layer, the ordinary wave usually predominates and the extraordinary
wave is so weak it does not affect radio reception. At Washington the critical frequency
for the extraordinary wave is about 750 kc. higher than for the ordinary wave (for
frequencies of 4000 kc. or higher). The difference in frequency is proportional
to the intensity of the earth's magnetic field at the place of reflection, and is
therefore different at different places on the earth. In reporting results of measurements
of critical frequency it is now customary to give the values for the ordinary wave;
practice varied in the past.
Fig. 4 - Representative summer day conditions. The extraordinary
components are clearly indicated.
Besides the virtual heights and critical frequencies, the absorption of the energy
of radio waves by the ionosphere is an important factor in limiting radio transmission.
This absorption exists because the ions set in motion by the radio waves collide
with air molecules and dissipate as heat the energy they have taken from the radio
waves. Consequently the energy thus absorbed from the radio waves is greater, the
greater the distance of penetration of waves into the ionized layer and the greater
the density of ions and air molecules in the layer, and hence the greater the number
of collisions between ions and air molecules. Absorption is especially great in
the daytime, and it occurs chiefly in the low ionosphere, in the D or E layers.
It also occurs in the high ionosphere, near critical frequencies. Much of the low-layer
absorption disappears with the decrease of low-layer ionization at night. Higher
frequencies are less affected by absorption than are lower frequencies, for waves
passing through the same ionized layers.
Regular Variations of Ionosphere Characteristics
There are three principal types of variation of critical frequencies which are
fairly regular with time. These are diurnal variations, seasonal variations, and
year-to-year variations with the sunspot cycle.
The diurnal and seasonal variations of the critical frequencies of the normal
E layer are particularly regular. The critical frequencies vary with the altitude
of the sun, being highest when the sun is most nearly overhead. Thus the diurnal
maximum of the E critical frequency (fE) is at local noon, and the seasonal maximum
is in mid-summer. At night this layer usually does not reflect waves of frequencies
higher than about 1000 kc.
The diurnal and seasonal variations of the critical frequencies of the F2 layer
at Washington are quite different from those of the E layer. The winter F2 critical
frequencies exceed any regular critical frequency found during the summer. In the
winter a broad diurnal maximum occurs in the daytime, centered around 1:00 P.M.
local time. In the summer a broader diurnal maximum centers about sunset. During
the night the winter critical frequencies are usually lower than the corresponding
summer values. Thus, the highest F2-layer critical frequencies occur during the
winter day, and the lowest F-layer critical frequencies occur during the winter
night; the summer day and night values are between.
The F2 virtual heights are much lower during a winter day than during a summer
day. The F virtual heights at night are about the same in winter as in summer.
The seasonal effects in the ionosphere synchronize with the sun's seasonal position,
not lagging a month or two as do the seasons of weather. Winter conditions in the
F2 layer obtain during a period of several months from about October to March, and
summer conditions for a period of several months from about April to September.
Around the equinoxes, there is a transition period of a month or two in which the
change occurs between winter and summer conditions; in these transition periods
the ionosphere characteristics (except for the effects of sporadic E) fluctuate
between winter and summer conditions and are thus more erratic than during the rest
of the year.
There are important changes in ionosphere characteristics in the 11-year sunspot
cycle. From the sunspot minimum in 1933 to the sunspot maximum in 1937 the F- and
F2-1ayer critical frequencies doubled (for most hours of the day), and the E-layer
critical frequencies became 1.25 times as great. A consequence is that the best
radio frequencies for long-distance transmission were approximately twice as great
in 1937 as in 1933 (except for summer daytime, when they were about 1.5 times as
great). In about 1944 they will return to minimum values.
Fig. 5 - At night, either summer or winter, the E layer
is relatively weak, while the F1 and F2 layers combine to form a single layer, the
F. Critical frequencies are considerably lower at night than during the daytime.
Table 1 - Typical Average Ratios of Maximum Usable Frequency
to Critical Frequency (For One-Hop Transmission)
* Sporadic E transmission has no critical frequency. The values
given are ratios of maximum usable frequency to the approximate upper limit of frequency
of the stronger sporadic-E reflections at vertical incidence.
The condition of the ionosphere varies somewhat with latitude. For all latitudes
of continental United States the differences from the Washington values appear to
be negligible, but the values in Alaska and in the Canal Zone are somewhat different.
Applications to Radio Transmission
From the vertical-incidence critical frequencies and virtual heights of the ionosphere
layers, at any given time, it is possible to calculate the upper limit of radio
frequency that can be transmitted over any distance. The calculated values of maximum
usable frequency are found to agree with direct observation of radio transmission
over such distances.
When radio waves are transmitted along the earth over any distance, they strike
the ionosphere obliquely (Fig. 1). Such obliquely incident waves can be reflected
back down with lower ionization densities than can vertically incident waves. It
results that the larger the angle of incidence (angle of wave path with the vertical),
i.e., the greater the transmission distance, the higher is the upper limit of frequency
of waves that can be reflected from a layer of given ionization density or critical
frequency. This upper limit of frequency, for transmission via an ionosphere layer,
for a particular time and transmission distance, is called the "maximum usable frequency."
It may be calculated roughly, to a first approximation, by multiplying the critical
frequency of the layer by the secant of the angle of incidence.
The accurate calculation of maximum usable frequencies from vertical-incidence
critical frequencies is quite complicated. For convenience, typical values for the
conversion are given in Table 1.
To obtain the maximum usable frequency for transmission over a given distance
by way of a given layer, multiply the critical frequency by the ratio given in the
table. Where blanks appear in the table, and for distances over 3500 kilometers,
the distance is too great for one-hop transmission, i.e., transmission over such
distances requires multiple reflection from the ionosphere with intervening reflection
from the ground.
The distance at which a given frequency is the maximum usable frequency is also
the minimum distance over which that frequency is receivable. This minimum distance
for any frequency is called the skip distance; at any less distance it is impossible
to receive on that or higher frequencies, except for sporadic or scattered reflections.
The highest maximum usable frequencies (in north temperate latitudes) occur during
the winter day and the lowest during the winter night. The summer values for both
night and day lie between these two extremes except as modified by sporadic-E or
Maximum Usable Frequencies over Long Paths
Since the local time of day, and hence the ionosphere characteristics, may vary
a large amount throughout a long transmission path, it is necessary to consider
what part of the path determines the conditions of transmission. For single-hop
transmission (transmission by a single reflection from the ionosphere), it is the
region half-way between the transmitter and receiver whose conditions determine
the transmission, because it is there that the reflection from the ionosphere takes
place. In multi-hop transmission, when the radio waves are reflected from the ionosphere,
then from the ground, then back to the ionosphere, etc., the determining conditions
are in the middle of each hop.
The maximum possible distance of transmission between the transmitter and receiver
whose conditions determine the transmission, because it is there that the reflection
from the ionosphere takes place. In multi-hop transmission, when the radio waves
are reflected from the ionosphere, then from the ground, then back to the ionosphere,
etc., the determining conditions are in the middle of each hop.
The maximum possible distance of transmission by a single hop is limited by the
geometry of the earth's surface and the layer, and also by absorption or other limitation
at the ground of those waves which are nearly tangential to the earth's surface.
It is found in practice that the minimum angle with the ground of the radio waves
transmitted or received (over land) averages about 3 1/2 degrees. From these considerations
the geometry indicates that the maximum distance along the earth by a single hop
is ordinarily about 3500 kilometers for the F2 layer, and about 1700 kilometers
for the E layer. Single-hop transmission may sometimes be possible at greater distances
than these while at the same time multi-hop transmission over the same path may
be more efficient.
Because of the variation of ionosphere characteristics with longitude, different
frequencies may be necessary for transmission in different directions from a given
place. For example, around sunset in winter lower frequencies are used in transmitting
eastward than in transmitting westward from the same location. This does not mean,
however, that different frequencies would be necessary or desirable in opposite
directions over the same path.
For very long paths in which widely different longitudes (i.e., times of day)
are involved, it sometimes happens that the waves travel different parts of the
way by different layers. For such cases, it is necessary to take account of the
heights of the different layers to determine the lengths of the several hops.
Effects of Ionosphere Irregularities
The primary effects of the ionosphere on radio wave propagation are those already
described, which are due to the normal or regular characteristics of the ionosphere.
The modes of variation of those characteristics have been shown to be of a regular
and fairly predictable nature. There are some other ionosphere phenomena which are
irregular in their times of occurrence, and make radio phenomena in general much
less predictable. Five types of such phenomena have been identified: sporadic-E-layer
reflections, scattered reflections, sudden ionosphere disturbances, prolonged periods
of low-layer absorption, and ionosphere storms. While all five are irregular in
time, the first two are primarily due to irregularities in space.
It sometimes happens that waves are reflected by the E layer on frequencies higher
than that at which the E-layer waves normally disappear and the reflection of waves
by higher layers begins. Thus, in the example shown in Fig. 3 waves may sometimes
be reflected at the E-layer height of 110 kilometers on frequencies of 4 or 5 or
more megacycles. These reflections are due to a process different from the normal
reflection in the ionized layer; the process is probably one of reflection from
a sharp boundary of stratified ionization. The existence of these "sporadic E" reflections
necessitates a redefinition of the term "critical frequency," previously defined
as the highest frequency at which waves sent vertically upward are received back
from the layer. When sporadic E reflections occur they may be received simultaneously
with reflections from higher layers; for example, vertical-incidence reflections
might be received at 8 Mc. from both the E and the F2 layers. The E-layer critical
frequency, more precisely defined, is the value (3300 kc. in the example shown in
Fig. 3) at which the observed virtual height shows a sudden rise to large values
as the frequency is increased. Except for the occasional occurrence of sporadic
E reflections, all waves of higher frequency pass through the E layer and are not
reflected by it.
Sporadic E leads to interesting results in long-distance radio transmission.
It accounts for long-distance transmission up to higher frequencies than by any
other means. Strong vertical-incidence reflections by sporadic E sometimes occur
at frequencies up to about 12 Mc. By reason of the large angles of incidence possible
with the E layer, this makes occasional long-distance communication possible on
frequencies as high as 60 Mc. Such communication is generally for only a short time
and for restricted localities. Sporadic E is thus patchy or sporadic in both geographical
distribution and time.
Sporadic E occurs most commonly in the summer, May to August, particularly in
the morning and evening, but may occur any time of day or night. It occurs occasionally
at all seasons, particularly in the evening. It occurs more at high latitudes than
in equatorial regions.
An irregular type of reflection from the ionosphere occurs at all seasons and
is prevalent both day and night. These reflections are most noticeable within the
skip zone, or at frequencies higher than those nominally receivable from the regular
layers. Like sporadic E, they occur at frequencies which may exceed the F2 critical
frequencies, but are unlike sporadic E in that they are complex and jumpy thus causing
signal distortion; they occur and disappear fitfully, and are almost useless for
communication purposes. Some types are of very weak intensity. The scattered reflections
are characterized by very great virtual heights, usually somewhere from 400 to 1500
kilometers. Their occurrence was for a time thought to indicate the existence of
another layer above the F2 layer which might be called the G layer. It is now, however,
thought that they are of several types, and that some of them are due to complex
reflections from small, ephemeral, scattered patches or "clouds" of ionization in
or between the normal ionosphere layers, and thence to one or more layers and ground
by single or multiple reflection.
Sudden Ionosphere Disturbances
The most startling of all the irregularities of the ionosphere and of radio wave
transmission is the sudden type of disturbance manifested by a radio fadeout. This
phenomenon is the result of a burst of ionizing radiation from a bright chromospheric
eruption on the sun, causing a sudden abnormal increase in the ionization in the
D layer (below the E layer), frequently with resultant disturbances in terrestrial
magnetism and earth currents as well as radio transmission. The radio effect is
the sudden cessation of radio transmission on frequencies above about 1500 kc.
The drop of the radio signals to zero usually occurs within a minute. The effects
occur simultaneously throughout the hemisphere illuminated by the sun, and do not
occur at night. The effects last from about ten minutes to an hour or more, the
occurrences of greater intensity in general producing effects of longer duration.
The effects are more intense, and last longer, the lower the frequency in the high-frequency
range (i.e., from about 1500 kc. up). It is consequently sometimes possible to continue
communication during a radio fadeout by raising the working frequency.
The radio, magnetic, and other effects are markedly different from other types
of changes in these quantities. The effects are most intense in that region of the
earth where the sun's radiation is perpendicular, i.e., greater at noon than at
other times of day and greater in equatorial than in higher latitudes.
Taking due account of the variation of the effects with frequency and distance,
varying effects in differing directions can be explained. Reception in the United
States from stations in the southern hemisphere usually exhibits greater effects
than reception from other directions (because of passing the equatorial regions).
Similarly, when the disturbance occurs at a time when it is morning at the receiving
point the effects are usually greater in reception from the east than from the west,
and vice versa for the afternoon (because of passing the region where it is noon).
A radio fade-out sometimes occurs when it is night at the receiving point, but only
when the path of the waves is somewhere in daylight.
Prolonged Periods of Low-layer Absorption
This phenomenon is similar to the sudden ionosphere disturbance in its effects
and characteristics except that its beginning as well as recovery is gradual and
it has a longer time duration, commonly several hours. The intensity diminution
is in general not as severe as in the more intense fadeouts, but sometimes the intensities
fall to zero.
The low-layer absorption effect appears to be due to increased ionization in
the D layer (below the E layer), exactly as for the sudden ionosphere disturbances.
The increased ionization is caused by an abnormally great outpouring of ultraviolet
light from the sun, but in this case it is not so sudden as in the eruptions which
cause the sudden ionosphere disturbances. The variation of the efects with frequency,
and other characteristics, are .the same as for the sudden ionosphere disturbances.
Both phenomena occur at all seasons, but the prolonged periods of low-layer absorption
have been found to occur in a group of several weeks' duration at periods of high
sunspot activity, the groups being separated by more or less quiet periods of several
months. They frequently but not always occur during periods when sudden ionosphere
disturbances are numerous.
An ionosphere storm is a period of disturbance in the ionosphere in which there
are great anomalies of critical frequencies, virtual heights, and absorption. Radio
transmission is poor (except for the low frequencies, below 500 kc., which are sometimes
improved). An ionosphere storm usually lasts one or two days, and occurs both day
and night. It is usually accompanied by a magnetic storm (i.e., a period of unusual
fluctuation of terrestrial magnetic intensity). During the first few hours of very
severe ionosphere storms the ionosphere is turbulent, stratification is destroyed,
and radio wave propagation erratic. During the later stages of very severe storms
and during the whole of more moderate storms, the upper part of the ionosphere,
principally the F2 layer, is expanded and diffused. The critical frequencies are
much lower than normal and the virtual heights much greater, and therefore the maximum
usable frequencies are much lower than normal. It is often necessary to lower the
working frequency in order to maintain communication during one of these storms.
There is also increased absorption of radio waves during an ionosphere storm. Ionosphere
storms are most severe in auroral latitudes .and decrease in intensity as the equator
is approached. Ionosphere storms occur approximately simultaneously over wide geographical
areas. The condition of the ionosphere is much less uniform from point to point
than on undisturbed days.
Posted October 16, 2020(original 12/9/2015)