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July 1932 Radio-Craft
 [Table
of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
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 This
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
DX Coffee (now defunct),
DX.QSL.net, and the
ARRL, with propagation
prediction and historical data.
Summer Short-Wave Reception
Chart showing variation of short-wave reception during the day.
A comprehensive discussion of the factors which influence short-wave reception
during both day and night.
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,
Chart showing the variation of short-wave reception at night.
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.
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 graph.
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 August 4, 2023
(updated from original post
on 7/8/2015)
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