Your RF Cafe
July 1958 Radio-Electronics[Table of Contents]
These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio-Electronics articles I have already posted. All copyrights are 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.
See also Meteor Detection by Amateur Radio, July 1947 QST
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 bending.
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
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.
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 per minute.
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 ratio.
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.
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 of Standards
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 day.
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 communication performance.
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 scatter.
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
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; December, 1957.
3 "ARRL Merit Award for 1955 Goes to W4HHK and W2UK," QST, Vol. 40, No. 10, page 62; October, 1956.
4 Scatter Propagation Issue, Proceedings of the IRE, Vol. 43, No. 10, pages 1173-1526; October, 1955.
Posted May 6, 2014