July 1932 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
article was written in 1932, before anyone had in-situ empirical
measurements of the ionosphere, since suitable instrumented sounding
rockets were not yet available. It had only been 30 years since
Oliver Heaviside and Arthur Kennelly first proposed their theory
of the ionized layer that encompasses the Earth. It turned out that
the ionosphere is composed of multiple layers of ionized regions
whose intensities are dependent on solar surface activity, time
of day and night, time of year, and even on terrestrial events like
large volcanoes. Large network communications have been built so
as not to be held hostage by atmospheric conditions by utilizing
land and ocean floor cables, land-based microwave towers, and satellites.
Military and amateur radio users still do a lot of communications
via sky waves that are profoundly affected by the ionosphere. Much
has been learned about the ionosphere in subsequent years, including
an explanation for why the day vs. night absorption and reflection
phenomenon mentioned in this article occurs.
There are many websites, such as
with propagation prediction and historical data.
Summer Short-Wave Reception
A comprehensive discussion of the factors which influence short-wave
reception during both day and night.
Chart showing variation of short-wave reception
during the day.
Radio wave transmission takes place by the propagation of a "ground
wave" along the ground, or a "sky wave" reflected or refracted from
the Kennelly-Heaviside layer, or by both means. The waves are subject
to absorption, both in the ground and in the ionized upper atmosphere.
The ground-wave absorption, in general, increases with frequency
and is reasonably constant, with time, over a given path at a given
frequency; it varies for earth of different conductivities and dielectric
constants. The sky-wave absorption is not a constant with time,
frequency, or path; it appears to be a maximum in the broadcast
band (550-1500 kc.), decreasing with change of frequency in either
direction. In the daytime this absorption of the sky wave is so
great that there is practically no sky wave, from frequencies somewhat
below to somewhat above the broadcast band, the specific limits
varying with the season. Hence sky-wave propagation in the daytime
is only appreciable in the lower and higher frequency ranges. During
the night, however, sky-wave propagation takes place on all except
extremely high frequencies. Sky-wave propagation is subject to material
variations, dependent upon conditions and changes in the ionization
of the Kennelly-Heaviside layer.
Besides daily variation of daylight and darkness, factors such
as latitude, season, magnetic storms; and solar disturbances, have
been found to have effects upon this ionization. These changes in
ionization result in wide variations in the transmission of sky
waves from hour to hour, day to day, and year to year. At the higher
frequencies, received field intensities for a given season and frequency
may vary as much as 1 to 10 from one year to another.
At the higher frequencies, reception at great distances is due
entirely to the sky wave. Above a certain frequency, however, which
may be as low as 4000 kc (see attached graphs), no appreciable portion
of the sky-wave radiation is reflected back to earth from the Kennelly-Heaviside
layer in a certain zone surrounding the transmitter. In the area
bounded by the inner edge of this skipped zone, the received wave
may be composed of both ground wave and sky wave (the sky wave being
appreciable on frequencies up to about 6000 kc. in the summer and
12,000 kc. in the winter); the sky wave intensity in this area is
ordinarily much less at night than in the day,
The outer boundary of the skipped zone is often called the skip
distance. The skip distance increases with frequency, and varies
diurnally and seasonally. Beyond the skip distance, the sky-wave
radiation is received with useful intensity.
Chart showing the variation of short-wave reception
With present knowledge of propagation conditions it is impossible
to postulate any formulas or make any tables or charts which could
be used to determine distance range over any given path accurately.
The attached graphs give average distance ranges as observed by
a number of experimenters to occur most frequently over a number
of transmission paths. Through certain frequency ranges, available
data were so incomplete as to require extrapolation which may be
considerably in error. Wide variations of distance range and skip
distance must be accepted as normal.
The scales of abscissas and ordinates are cubical, (i.e., numbers
shown are proportional to the cube root of the numbers shown). This
scale was chosen because it spaces the data satisfactorily. A linear
scale would crowd the low values too much and a logarithmic scale
would crowd the high values too much.
The graphs show the limits of distance over which practical communication
is possible. They are based on the lowest field intensity which
permits practical reception in the presence of actual background
noise. For the broadcasting frequencies this does not mean satisfactory
program reception. The limiting field intensity is taken to be 10
microvolts per meter for frequencies up to 2000 kc. decreasing from
this value at 2000 kc. to about 1 microvolt per meter at 20,000
kc. When atmospherics or other sources of interference are great,
e.g., in the tropics, much larger received field intensities are
required and the distance ranges are less. The graphs assume the
use of about 5 kilowatts radiated power, and non-directional antennas.
For transmission over a given path, received field intensity is
proportional to the square root of radiated power, but there is
no simple relation between distance range and either radiated power
or received field intensity.
Separate graph sheets are given for day and for night transmission.
Above about 3000 kc. as shown, the distance ranges (and in most
cases also the skip distances) are greater in the winter than in
the summer. The distance ranges in spring and autumn are intermediate
between the limits shown for summer and winter. In general, the
distance ranges for paths which lie partly in day and partly in
night portions of the globe are intermediate between those shown
in the day and the night graphs. For such paths, the distance ranges
are greater than would be expected from inspection of the day graph,
as the waves under these conditions travel over greater distances
in the illuminated portion of the earth's surface; for this reason
it is possible to use a lower frequency for a part day, part night
path than is indicated for the day portion of the path on the day
The distance ranges given in the graphs are the distances for
reliable reception; they are not the limits of distance at which
interference may be caused. A field intensity sufficient to cause
troublesome interference may be produced at a much greater distance
than the maximum distance of reliable reception.
Posted July 8, 2015