July 1958 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
Long distance communications (DX) thrives on ionized atmosphere.
It causes boundary layers at various altitudes that function
as reflecting surfaces and thereby facilitate the 'bouncing'
of radio waves off the sky so that they bend around the curvature
of the Earth. Early sounding rocket experiments verified that
indeed ionized layers of atmosphere did exist as suspected based
on theories of aurora activity during periods that correlated
with observed solar flare activity. Intense solar flare periods
are associated with large numbers of sun spots, since coronal
mass ejections (CMEs) are produced which supply enormous amounts
of electrons traveling at high speed earthward. CME matter of
course shoots in all directions, but only those moving in the
direction of the Earth matter to us. Amateur and professional,
as well as military and academic communications practitioners
learned to exploit not only the fairly regular diurnal ionization
of the upper atmosphere for DX operation, but also learned to
use the ionized trails left by meteors passing through the sky
for temporary long distance paths not possible with predictable
scenarios. In fact, many contests have been won based on a lucky
meteor 'burst' that opened a communications channel just long
enough to make official contact with another operator somewhere
in a distant land. It might not sound like such a big deal today
where you can talk to someone anywhere in the world on a standard
telephone or via the Internet using VoIP, but there was a time
when it was magic. To many Hams, it still is magic.
RF Cafe visitor Denny Condron (Tracking
Down a Mystery Signal) is an avid meteor detector and
posted a link to this article on the
RadioMeteors Google group.
Meteor Detection by Amateur Radio, July 1947 QST
Communications via Meteor Bursts
Meteors and modern techniques combine to give longer-range
two-way) communications in the 30-100-mc band
By G. Franklin Montgomery
During the early Forties, when the FM broadcast service was
operating in its old band at 42 to 50 mc, Federal Communications
Commission monitoring engineers discovered a strange effect.
Short bursts of signal were received occasionally from FM stations
as far as 1,370 miles away. This distance was much greater than
the 100 miles or so considered maximum line-of-sight. The signal
bursts were brief, usually lasting for less than a second. They
showed none of the characteristics associated with known abnormal
propagation such as ionospheric sporadic-E transmission or atmospheric
Fig. 1 - Sketch of typical meteor-burst signals
shows amplitude and duration of received information.
After a careful study, E. W. Allen, Jr.1 identified
the bursts with individual meteors, the small outer-space particles
of matter that plunge into the atmosphere by the billions each
day. This intermittent vhf propagation over intermediate distances
has led to some unusual developments.
Meteor bursts are signal reflections from the atmospheric
ionization that these high-speed particles produce. Ranging
in size from almost microscopic grains to an occasional mass
of rock or iron weighing several tons, meteors enter the atmosphere
at velocities as great as 40 miles per second. At an altitude
of about 60 miles, their friction with the thin atmosphere produces
temperatures high enough to melt vaporize the meteor anti to
split molecules of the air into ions and free electrons.
In the meteor's wake, a miniature ionosphere is formed in
the shape of a slender, slowly expanding cylinder, perhaps several
miles in length. The ions and electrons produced by this process
eventually recombine to form neutral gas molecules, but it may
take many seconds for them to do so.
After the war, to study meteor behavior, a number of experimenters
began measuring radio reflections from meteor trails with vhf
radar equipment. About 1950, the idea occurred almost simultaneously
to several engineers that meteors might provide useful radio
communication from one point to another. Shortly thereafter,
groups at the Defence Research Telecommunications Establishment
(Canada), the National Bureau of Standards, the Stanford Research
Institute and RCA Laboratories began work to develop regular
communication by this means.2 Amateurs in the US
also began work with similar techniques.3
Fig. 2 - Message capacity plotted against
bandwidth or transmission speed.
Suppose we have two stations, A and B, about 1,000 miles
apart. Each station has a transmitter and receiver with antennas
beamed toward a patch of sky (about 60 miles up) that can be
seen from both stations. Each transmitter radiates a carrier
wave continuously. Receiver A is tuned to transmitter B, receiver
B to transmitter A. (Transmitter frequencies must differ slightly
so transmitter A will not interfere with receiver A, nor transmitter
B with receiver B.) Ordinarily, each station receives only a
very weak signal or none at all. But from time to time, a meteor
will pass through the part of the atmosphere included in the
antenna beams of both stations. As it does so, it produces a
trail of ionization. If this trail is oriented properly with
respect to the stations, both will receive a signal burst that
may last from less than 1/10 second to many seconds before it
fades into the background noise.
Fig. 1 shows the kind of variation that is typical of separate
meteor bursts. In addition to the main outlines of an abrupt
rise and slow decay in signal amplitude, subsidiary fading is
often observed on some bursts. The fading is attributed to breakup
of the trail by high-altitude atmospheric winds.
Stations A and B can communicate with each other during these
brief intervals. With ordinary equipment, however, communication
is difficult or impossible with the signal lasting for so short
a time. Modern information-handling techniques now come into
play. If A is to send a message to B, the message can be stored
at A in record form - on magnetic drums or tape, for example.
When a signal burst occurs at A, the recorded message is played
back at high speed and modulates A's transmitter. The same high-speed
message at the output of B's receiver is recorded at B. Subsequently,
when the signal burst has passed, B's record is played back
at normal speed so that the message can be understood.
Interior of trailer. Two racks to the rear
contain receiving and control equipment. Two central racks are
magnetic tape storage, record and play. back units for transmitting
and receiving. Tape is stored in vertical perforated metal tanks.
Foremost rack contains monitoring equipment.
This general procedure is the basis for burst communication.
Storing information at normal speed, playback and transmission
at high speed, reception and recording at high speed, and playback
at normal speed are common to all systems now in development.
Teletypewriter, voice and even facsimile modulation have been
used successfully in meteor-burst experiments. Usually the goal
is two-way communication, with both stations transmitting and
receiving different messages simultaneously, using the same
signal burst. This end has already been achieved with teletypewriter
messages sent at up to 40 times the normal speed of 60 words
In addition to the recording equipment, control circuits
are required at each station to perform certain functions automatically.
When a burst is received, there must be a threshold device to
judge whether the signal amplitude of the burst is large enough
to warrant starting transmission. If it is, the high-speed record
and playback units must be signaled to start. When the received
burst amplitude falls below a preset threshold, these units
must be stopped, and so on. In a two-way system, it is almost
certain that the received signals at both stations will not
behave in exactly the same manner. A code of check signals is
often sent around the loop from one station to the other and
back before starting transmission from either. This procedure
may sound like a waste of precious transmitting time, but it
is done within a few milliseconds and greatly reduces the possibility
of transmission errors.
Fundamental to any communication system is the signal-to-noise
ratio at the receiver while the message is in progress. If the
message is sent more rapidly than normal, the receiver bandwidth
must be increased to avoid message distortion. An increased
receiver bandwidth admits more noise, which in itself may distort
the message. In conventional systems where communication is
continuous, a compromise must therefore be made between speed
of transmission and degradation of the received signal-to-noise
Long or Short; Large or Small?
In a meteor-burst system, where communication is intermittent,
this compromise has unique consequences. The signal bursts in
such a system vary widely in amplitude and duration. A few bursts
have very large peak amplitudes; many more have small amplitudes.
A particular burst of any amplitude may be short, or it may
last for a relatively long time. The bursts can be predicted
only statistically. Whether the next burst will be large or
small and exactly when it will occur are both matters of chance.
Experimental meteor-burst communication station.
Four Yagi antennas are arranged in double arrays for transmitting
and receiving. Radio and terminal equipment are housed in trailer
to the rear.
Now, in setting up a system, we may choose to operate only
with the few bursts of large amplitude and transmit at high
speed. Alternatively, we may use the many bursts of smaller
amplitude, as well as the large ones, and transmit slowly.
The striking feature of meteor-burst statistics is that it
pays to use only the large bursts. The price that must be paid
for using them effectively is transmission speed and bandwidth.
In addition, a longer wait is required between transmissions,
although this waiting period may be unimportant in some applications.
The faster we are prepared to transmit and record during
a burst, and the fewer bursts we use, the greater the message
capacity of the system. (A convenient measure of message capacity
is the total length of message that can be sent over the system
in a period of time long enough to include a large number of
bursts.) Fig. 2 illustrates this dependence. Relative message
capacity and relative bandwidth or transmission speed are plotted
using logarithmic scales. The capacity is not directly proportional
to the bandwidth but increases more slowly. In a typical case,
it is proportional to the bandwidth or speed raised to about
the 0.4 power.
Courtesy National Bureau of Standards Courtesy National Bureau
The choice of operating parameters for a meteor-burst system
is made more difficult by the erratic nature of the bursts.
Their random occurrence and amplitudes would not be particularly
troublesome if we could depend upon known average occurrence
rates and average amplitudes. But we cannot.
We do know that a typical system of moderate power may operate
with four or five 1-second bursts per minute on the average.
Throughout the day, however, the actual number of bursts per
minute change from a high rate in the pre-dawn hours to a low
rate at sunset. The total number of bursts varies from day to
At any given time, there is a best section of sky toward
which the antenna beams should be pointed to intercept the greatest
number of useful trails. The position of this best section also
changes throughout the day and probably with the seasons. All
these meteor-burst characteristics have been measured extensively,
but they must be known even more exactly to achieve the best
In one sense, meteor-burst systems4 are competitors
with ionospheric forward-scatter systems. Both provide communication
over the same distances in the same part of the vhf spectrum,
roughly 30-100 mc. However, meteor-burst systems may be able
to use higher frequencies than are profitable for ionospheric
One distinct advantage of meteor bursts is the lower power
require. Most experimental work has been done with transmitter
powers from 100 watts to a few kilowatts. Operational scatter
systems usually require tens of kilowatts and high-gain antennas.
While high-gain antennas are certainly useful for meteor-burst
work, satisfactory results have been obtained with simple Yagis
of moderate gain. The principal disadvantage, of course, is
the complex message-handling equipment needed at the terminals.
Perhaps this disadvantage will seem less severe after more experience
with these devices.
Voice of the Satellites
Believed to be the only commercially available
recording of the radio signals from Sputnik I and II and Explorer
I, II and III, Voices of the Satellites is the work of Professor
Thomas A. Benham of Haverford College, Haverford, Pennsylvania.
The signals were recorded with a specially built convertor-amplifier.
Professor Benham, who is totally blind, also produces a nonprofit
science magazine on tape, Science for the Blind, which issues
400 recordings per month. His satellite recording, available
on a 5-inch tape reel or 10-inch LP record, is distributed by
Taben Recordings, Box G-224, Ardmore, Pa, ($3.95)
The high frequencies (3 to 30 mc), which we depend on for
both intermediate- and long-distance communication, have been
badly crowded for many years. During ionospheric storms, high-frequency
communication is often unreliable. Even so, there is little
hope that future technical improvements will provide space for
all of the services that would like to use this part of the
spectrum. The situation is rather like an overcrowded bus with
more riders than seats. Those standing must either ride uncomfortably
until someone abandons a seat, or give up the idea of riding
altogether. Meteor-burst communication, it is hoped, will provide
a larger bus. The hope is bright enough that much effort is
being spent on its development.
1 E. W. Allen, Jr., "Reflections of
Very-High-Frequency Radio Waves from Meteoric Ionization," Proceedings
of the IRE, Vol. 36, No.3, pages 346-352; March, 1948.
2 Meteor-burst Communication Papers,
Proceedings of the pages IRE, Vol. 45, No. 12 pages 1642-1736;
3 "ARRL Merit Award for 1955 Goes
to W4HHK and W2UK," QST, Vol. 40, No. 10, page 62; October,
4 Scatter Propagation Issue, Proceedings
of the IRE, Vol. 43, No. 10, pages 1173-1526; October, 1955.
Posted May 6, 2014