December 1947 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.
Short wave radio was a boon to both professional and amateur radio
operators because of its ability to be received over longer distances
using significantly lower transmitter power. The problem was (and
still is) that short wave bands typically suffer from atmospheric
ionization effects that vary depending on time of day, local weather,
solar activity, pollution, and other phenomena. Long wave's advantage
was that although it required higher power and longer antennas,
it was (and is) extremely reliable. For other than the most critical
applications, idiosyncrasies of short wave communications were accepted
as the price of more convenient and lower cost operation. Widespread
adoption of short wave communications brought extensive studies
and characterization of atmospheric influences in particular frequency
bands. Discovery of distinct 'F' layers (regions) in the ionosphere
and their effects on radio transmission has allowed radio operations
to predict and accommodate the affected propagation paths.
There are a few really nice propagation prediction website that
give up-to-the-minute data:
What does the 'F' in F-Layer mean? According to my sources the
'F' refers not to frequency, but to 'free' electrons in the ionosphere.
How to Use Radio Propagation Predictions
By Fred Shunaman
The various layers of the atmosphere. Though all affect
radio transmission at certain frequencies, F-layer reflections
are most important for long-distance communication. Scale
at left in miles.
Old-timers remember when the best radio wave was the longest
one. The long wave was reliable. It maintained the same strength
day and night at all times of the year, and the strength dropped
off steadily with increasing distance from the transmitter. Short
waves - roughly those under 1000 meters - were "unreliable." They
varied in strength with the time of day and the season and showed
other "unpredictable" vagaries.
The reliability of the long waves was obtained at the cost of
high power. With the coming of broadcasting, it was found that a
station of a few hundred watts could be heard (when conditions were
good, such as on a cold winter night) farther than a long-wave station
of many kilowatts. Then the amateurs started to work at even higher
frequencies, first on 150 meters (2 mc), then 80 (3.5 mc), later
40 and 20 (7 and 14 mc). At each increase of frequency they were
able to transmit farther with less power.
But other and more disquieting discoveries were made. A station
which pounded in with ear-splitting volume one night might simply
not be there at all the next. Amateurs found they were not able
to communicate with old friends in the next state, but were being
received solid several thousand miles away. Thus was "skip distance"
discovered. One point was clear: if some way could be found to figure
out these high frequencies, a new era in low-power, long-distance
communication would open up. Scientists amateurs, and communications
companies set out to learn more about these mysterious waves.
First, the scientists Kenelly and Heaviside suggested that the
upper parts of the atmosphere were ionized - that the ultraviolet
rays of the sun break up the atoms in these upper regions where
the air is so thin as to resemble a poorly exhausted vacuum tube
- into negative electrons and positive ions. These upper regions
of the atmosphere, they said, were filled with a partly conductive
material, something like the interior of a gas-filled vacuum tube.
This ionized layer reflected radio waves of high frequencies back
to earth, but lower-frequency waves were merely absorbed by it.
The higher the frequency, the farther the waves penetrated the layer;
thus the greater the distance at which the reflected wave reached
the earth (Fig. 1). Above a certain critical frequency, the waves
keep right on through the layer and are lost to earth entirely.
In 1923 the English scientist Appleton proved the existence of
the Kenelly-Heaviside layer by beaming radio waves straight up and
measuring the time the reflected waves took to return. He found
that the higher-frequency waves took longer to come back, proving
that higher frequencies penetrate farther into the layer.
Height of the layer as measured by this earliest radar was about
70 miles. But as frequencies were raised toward 3 mc, the echoes
suddenly disappeared - to return from a distance of more than 150
miles. Obviously there were two reflecting layers. Above 8 mc a
double echo was noted, as if the higher layer were split in two.
Fig. 1 - Long waves follow path 1, shorter ones, 2 or
3, and very short waves, path 4.
The lower layer is now called the E-layer, and the higher one
- or two - the F1- and F2-layers. The F2-layer is the most important
one to long-distance, high-frequency communication. The E-layer
is more important in daylight and at frequencies below 4 mc, though
at times its effects may be felt up to 20 mc. Below these layers
are the C- and D-layers, whose effects on radio waves have been
little studied, but which are beginning to be considered important
at certain frequencies.
The Bureau of Standards at Washington has been one of the leading
explorers of the ionosphere, and began in the late '30's to issue
rough predictions of the probable ranges at various frequencies.
The predictions were very approximate, and covered only four periods-summer
and winter noon and midnight.
The demand for reliable radio communication during World War II
made much more detailed and accurate knowledge of ionosphere conditions
necessary. Numerous stations were established at widely scattered
points, and continuous records were made and transmitted to Washington
for interpretation and correlation with those from other stations.
These stations now number 58, and gather ionospheric data from all
parts of the earth. The Central Radio Propagation Laboratory (CRPL)
of the Bureau of Standards uses this information to issue monthly
predictions of usable radio frequencies. These are put out in the
form of monthly booklets, 3 months in advance. They contain maps
and charts showing the maximum usable frequency for communication
between any 2 points on the earth's surface at any time of day.
The booklets are entitled Basic Radio Propagation Predictions and
are used in connection with another publication, Instructions for
the Use of Basic Radio Propagation Predictions (Bureau of Standards
Circular 465), which contains further maps, charts, nomographs,
and instructions for use with the Predictions.
The Predictions consist of a series of charts of the ionosphere.
Fig. 2 is one of them. The maps show the maximum usable frequencies
(muf) for points at 4000 kilometers away from any point on the earth
at any given time of day. Points on the same latitude do not have
the same ionospheric conditions at the same time of day throughout
the world. Therefore it is necessary to have 3 sets of charts. The
western (W) chart includes South America, all the United States
but the northwestern tip, most of Canada, the Atlantic Ocean and
a bit of Africa. The eastern (E) chart is applicable to Asia and
Australia, and the intermediate (I) to Europe, Africa, northwest
Canada, Alaska, and a belt of the Pacific.
Fig. 2 - One of the ionosphere charts. There are six
of these F2 and one E-layer chart.
Fig. 3 - Part of the world map, showing the two great-circle
paths mentioned in the text.
Predictions not easy to apply
The Predictions are not particularly easy to use. It is often
necessary to use 2 charts in conjunction, checking them against
a third when the E-layer may affect results. A further difficulty
is that radio waves follow great-circle paths, while the charts
are square Mercator projections. This difficulty is solved with
a great-circle chart in the Instructions. To use the Predictions,
put a piece of tracing paper over the map of the world in the Instructions
(Fig. 3). Mark the points between which communication is to be established.
Also trace the equator line on the tracing paper. Then place the
tracing paper on the great-circle chart (Fig. 4) with the equator
lines coinciding, and slide it back and forth until one of the curved
lines on this chart connects the 2 points. Draw the great-circle
path between them along that line. Put the tracing paper on the
correct ionosphere chart with the local station on the vertical
line marking the local time of desired communication.
Finding the correct frequency now depends on whether the distance
is more or less than 4000 km (2500 miles).
If the distance is exactly 4000 miles" the problem may be relatively
The mid-point of the great-circle path is located and the maximum
usable frequency (muf) read direct from the F2 4000 chart for the
given zone. For example, suppose contact is to be made between New
York and Georgetown, British. Guiana, a distance of approximately
4000 km. The tracing paper is placed over the world map and the
2 points, as well as the equator, are marked on it. Then the paper
is placed over the great-circle chart and moved back and forth till
a great circle joins the 2 points. The path between the two is drawn,
as well as the mid-point of that path.
The tracing paper is then placed over the F2, 4000 W map with
the equator lines again coinciding and the New York point on the
hour meridian of the time desired. The muf of the mid-point is read.
For example, in December 1947, the muf of the mid-point of the great-circle
path between New York and Georgetown is 21 mc for 6 am, 34 mc for
12 noon, and 21 mc for 6 p.m.
Since unpredictable conditions may cause the predictions to be
in error, a frequency 15% lower than the. muf is taken as the optimum
working frequency (owf). Thus all muf's are multiplied by 0.85 for
actual working frequency.
If the distance is greater than 400 km, 2 points 2000 km from
each end of the path are taken instead of the mid point. The tracing
paper is placed over the map as before and both points, as well
as the equator, marked. The meridian of Greenwich may be drawn also,
as a time reference. It is usually easier to use it for great distances
than the local time of either of the 2 stations at the ends of the
path. The great-circle chart is again used to find the actual radio
path between the 2 points. Instead of marking the mid-point of the
path control points 2000 km from the ends of the path are marked.
(Experience has shown that muf's at great distances are little different
than at 4000 km.) The maximum usable frequency for each of these
points is found (on the appropriate chart) and the lower of the
two taken as the muf for the entire path.
For example, communication between New York and Shanghai is desired.
Drawing the path and control point the muf's for the New York control
point are found to be 28, 12, and 12 mc for 0600, 1200, and 1800
GMT, respectively. At the Shanghai end. using the F2 4000 E chart
(not shown) all 3 muf's are found to be 12 mc. The muf for the whole
distance is then 12 mc for the given times, and the owf 10.2 mc.
For distances less than 4000 km a little more work must be done.
The muf is first found exactly as for the 4000 km. Then the muf
of the mid-point found on the F2 0 chart, which shows the critical
frequencies for waves projected directly up from the sending station.
Since the distance is between 0 and 4000 km, it might be expected
that its muf would be between these two. And so it is, but in a
way that does not vary directly with distance. A nomograph provided
in the Instructions for distances less than 4000 km. A straight
edge is placed between the muf at 0 and that at 4000 km, and the
correct frequency is read where the straight edge intersects the
line representing the distance of the station with which communication
is to be established.
Fig. 4 - This great circle chart is used to find the
radio transmission paths on Fig. 3.
Radio Propagation Chart - December 1947
(Maximum usable frequencies are given
At lower frequencies the E-layer may enter into the picture.
An E-layer chart in the Predictions helps the radioist to use reflections
from that layer. After selecting the best frequency from the F charts,
consult the E chart. If the E-layer muf is higher than that reflected
by the F-layer, use the E-layer frequency.
Adapting the predictions
All these methods are excellent for transmission between 2 fixed
points, such as 2 commercial or government stations. They are not
so good for the amateur or short-wave listener. The amateur wants
to know what frequencies to use to cover a large area - possibly
a continent - or in what direction to send a CQ to get results and
dx on his transmitter's frequency. The short-wave listener would
like to find out when to listen for elusive short-wave broadcasters.
Both are on the alert for times when higher frequencies than those
predicted are useful, for these are the periods of dx.
A few attempts have been made to broaden the scope of the Predictions
to cover amateur and general requirements. The most successful of
these is based on the observation that a given muf "cloud" drifts
along the earth from east to west, maintaining a constant angle
with the sun. The amateur who works on 10 meters, for example, can
note the area over which 30 mc or higher is the muf. If this area.
covers the control points between him and any desired station, he
can work that station on 10 meters. Another system calculates owf's
for narrow strips along the American coast and that of other countries.
The table presented here represents a new approach to the problem.
Based on latitude 40 N in the western zone, a large number of calculations
have been made, giving the optimum frequency for any part of the
day for any distance and any direction. As conditions drift west
with the sun, this chart will be correct for any part of the United
States on the 40th parallel, and approximately correct for considerable
distances north and south of that line.
The table shows conditions at intervals of 3 hours and at ranges
from 2500 miles (4000 km) to 12,500 miles (20,000 km) at intervals
of 2500 miles. These intervals are close enough to permit interpolation
for times and distances between those given. The same is true of
Use of the table is simple. The user merely consults it for the
current hour and notes the optimum working frequencies in each direction
for the various ranges. A combination of high working frequencies
and great distance spells dx. Low frequencies in a given direction
indicate the limits within which a receiver or transmitter should
be held to work or hear from stations in those great-circle directions.
A great-circle chart based on a point near the user's location
is exceedingly useful for ascertaining the bearing of distant points.
Less convenient is the chart published in the Instructions, but
distances as well as directions can be obtained from it. A globe
and piece of string is possibly the simplest means of finding direction
For example, to find the muf for working between New York and
San Francisco at 9 p.m. Eastern Standard Time during December, 1947,
first find the bearing and distance by any of the means above. The
distance is between 2500 and 3000 miles, and thus fits most closely
the 2500-mile table. Bearing is a little north of west (in spite
of the fact that San Francisco is south of New York). At 21 hours
the maximum usable frequency west is 24 mc. For reliable work this
should be converted to the optimum working frequency (owf) by multiplying
by 85%. The owf is then about 20.5 mc. The nearest amateur band
is 14 mc, though short-wave listeners may look for Treasure Island
right up to its 21-mc frequency.
Again, suppose a short-wave listener is interested in getting
Shanghai or Nanking. Distance is ascertained to be approximately
12,000 km and the bearing slightly west of north. Looking at the
chart, it appears that 13 mc at 9 p.m. offers the best opportunity,
as read from the 7500-mile chart in the direction N. Checking with
NW, the best times appear to be between 3 and 9 p.m., with a frequency
as high as 33 mc at 6 p.m. However, the bearing is much more nearly
north than northwest, leaving 9 p.m. the best hour.
Only one direction is given for the 12,500-mile range, as it
is the same distance in all directions. There is very little difference
in muf over a wide angle from NE to NW. The operator with a rotatable
antenna may find it worth his while to try various paths.
Posted July 2014