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July 1947 QST
The 1940s and 1950s was an era of much advancement in our knowledge of Earth's upper atmosphere and its affects on radio communications - both good and bad. Industry, government, academic, and amateur groups all played major roles in conducting experiments and publishing findings for the interested community to share and build upon. A year ago I posted an article, along with a bit of editorialization, from the July 1958 edition of Radio-Electronics titled "Communications Via Meteor Burst."
A New Field of Observation
By Oswald G. Villard, Jr.,* W6QYT
An interesting and little-known portion of the rapidly-expanding body of knowledge now being assembled on the ionosphere is the subject of meteors and their effect on radio propagation. In a previous article,1 the author told how Doppler whistles, caused by meteors entering the ionosphere, can be heard on the signals of high-power short-wave broadcasting stations. The purpose of this account is to describe a method of hearing meteor whistles and other effects on the signals of ordinary amateur stations, using straightforward receiving techniques. An amateur station can, in fact, be used for "counting" meteors automatically, with a sensitivity far greater than that of the human eye. That meteors can thus be painlessly "counted," when the sky is overcast or bright with daylight, should be of considerable interest to astronomers as well as to radio engineers concerned with the behavior of the ionosphere, for the new technique of meteor detection by radio promises to yield valuable information in both fields of knowledge. Meteor spotting therefore provides the inquisitively-minded amateur with an interesting opportunity to put his hobby to use in gathering worthwhile scientific information.
Meteors are much in the spotlight of public attention these days, because the V-2 and similar rockets are rapidly encroaching on that domain of the upper air once inhabited exclusively by shooting stars and fireballs. An important question is: what happens when these two different manifestations of matter collide? Will the embryo space ship be completely destroyed, or only punctured like a partridge riddled with buckshot?
These and similar interesting speculations are left to the Jules Vernes of our time. As far as the present discussion is concerned, it suffices that radio has been shown to provide a new tool for the study of meteors. In view of cosmic hiss and solar static, radio equipment may some day be as commonplace a piece of astronomical gear as the telescope.
Some Facts about Meteors
Meteors are, in a very real sense, the driftwood of outer space. They are simply particles of matter - the rubble left over, perhaps, when our solar system was constructed. The particles are graded as to size: the very largest, fortunately, are quite rare; yet the very smallest are so numerous that counting them strains the imagination. Particles large enough to survive the plunge through our atmosphere are called meteorites. There is a crater in Arizona one mile in diameter and 600 feet deep, caused in prehistoric times by the impact of one of these visitors hurtling in from outer space.
The meteors one sees ordinarily are astonishingly small in size - perhaps as big as a pea. On an average night the casual observer will see between two and eight meteors of this size per hour. If a count like this could be maintained over the entire surface of the earth for a period of twenty-four hours, the grand total would be about 24 million meteors. If all the meteors of all sizes which strike the earth every 24 hours could be counted up, the total would come out to be some eight billion meteors!
The most remarkable thing about meteors is their speed. We must think of the brilliant flash of a falling star as being caused by an object hurtling through space at something like 25 miles per second, or about 50 times as fast as the V-2. It is small wonder that when these tiny pellets of cosmic dust collide with particles of our atmosphere, a violent reaction ensues. Those air molecules unfortunate enough to find themselves in the path of a meteor are given a tremendous acceleration by the impact. Glancing off at various angles, they in turn collide with other molecules, and so forth. The resulting agitation is not unlike that produced by the passage of electricity through the rarified upper air, and the result is a visible glow similar to that of the gas in a neon sign. The same ionization that produces the glowing streak, or tail, of the meteor, can also reflect radio waves. Another example of a visible glow produced by ionization is the aurora borealis, which is caused by a mechanism as yet not too clearly understood. Six-meter enthusiasts who have made DX contacts by pointing their beam arrays directly at the aurora, thus bouncing signals back from its sides, have taken advantage of the reflective properties of an ionized region.
Most meteors are distributed more or less uniformly in space and appear at random intervals from random directions. There are certain times of the year, however, when these sporadic meteors are supplemented by clouds of cosmic dust particles all traveling in the same direction, which produce displays called meteor showers. The shower meteors are bits of matter sloughed off by comets, and they follow along the same path as the parent comets even though the comets themselves may have long since burned out or disappeared.
Whistles & Bursts
The effect of meteors on radio propagation has been speculated upon and studied for many years. Ionosphere investigators in 19332 found a change in the over-all level of E-layer ionization during a meteor shower. Later, sudden unexpected rises and dips found in charts of radio field strength were traced to meteors.3 In 1941 reflections from meteor trails were detected on ionosphere echo-sounding records.4 Not long thereafter, the Doppler whistles caused by the motion of the meteor trails were discovered,5 and during the war 100-megacycle radar echoes from meteor ionization were identified and reported.6 Moreover bursts of signal received beyond the normal range of f.m. stations were connected with meteors.7
Recently, however, research in this field has gone ahead rapidly. During the Giacobini-Zinner meteor shower of October 9, 1946, the wartime radar detection of meteors was duplicated with great success,8 while at both Stanford University9 and at Harvard University10 meteors were detected by their Doppler whistles as well. If there had been any doubts up to that time that meteors were the cause of the effects previously noted, they were removed on what meteor investigators will remember as "G-Z day."
Each meteor, entering the ionosphere about 50 or 60 miles up, produces a thin cylinder of very intense ionization until it is burned out or dissipated. Oddly enough, only some 10 per cent of the total energy in a meteor is wasted in friction; the remaining 90 per cent is spent in producing ionization. Moreover, the speed of a meteor changes very little (perhaps 10 per cent) during its brief life, and its course is, for all practical purposes, a straight line.
It is conjectured that the intense ionization contained in the thin cylinder rapidly diffuses outward, thus increasing the diameter of the ionized region, However the intensity of the ionization contained in the cylinder is thereby decreased, and its level soon drops below that required to reflect a radio wave of given frequency. At any frequency, then, the strongest reflection from a meteor trail will be obtained when the dimensions of that trail are such that the largest volume of ionization is present of an intensity sufficient to reflect that frequency. If a very high operating frequency is picked, the cylinder of ionization is capable of reflecting a signal only when it is relatively small in diameter, for only then does it have sufficient intensity. And a small cylinder will return only a weak signal because of its small "echoing area," When the radio frequency is up in the several hundreds of megacycles, the size of a cylinder capable of reflecting those frequencies is so small that only a very feeble reflection can be obtained. Consequently meteor reflections have not so far been noted on frequencies much above 100 megacycles. On the other hand, when lower frequencies are used, the cylinder of ionization can become quite large and still reflect a signal.
The energy reflected from meteor trails produces two different types of effects noticeable at the receiver when c.w. signals are used. The mechanism involved is illustrated in Fig. 1. It is assumed in this sketch that the transmitting and receiving aerials are so located and orientated that direct signal from the transmitter is reduced to a very low value (of the order of a few microvolts) at the receiver. The frequency must be high enough so that no reflection from the ionosphere directly overhead is obtained. Moreover it should be so high that no "long-scatter" signals are returned from points some distance away. In fact, the ideal frequency to use is one just high enough so that no long-distance transmission in any direction is possible, since "long scatter" cannot then exist. Under these conditions it will be readily possible to hear bursts of signal reflected from the sides of the meteor trails, as well as Doppler tones produced by signals scattered from the moving head of each trail. Energy scattered back from the moving head of the ionization columns arrives at the receiver via a path of rapidly-changing length. This path-length change causes an apparent shift in the frequency of the reflected signal, which in turn gives rise to an audible beat note when the reflected signal is combined with energy reaching the receiver via a path of unchanging length.
Once the column of ionization has become fully formed, the signal reflected from it traverses a path of constant length so that no Doppler shift, and hence no beat note, occurs. As far as the receiver can tell, the signal from the transmitter has then suddenly increased in strength; this sudden increase is called a "burst." The burst will be strongest when the position of the column of ionization is such that a line can be drawn from the transmitter and receiver perpendicular to the cylinder at some point. There results a broadside reflection that will be much stronger than the signal scattered back when the column is so positioned that this cannot occur. (It should be remembered that the E region of the ionosphere, where meteor ionization occurs, is a relatively thin layer.) The effect is analogous to the flash of light when sunlight is reflected by a mirror into the eyes of an observer. The mirror can be seen at all times - which is to say it reflects back some light - but only when it is correctly orientated does it produce a flash.
Of the two effects - "Doppler whistles" and "bursts" - the latter are, of course, much more easily detected, because the broadside reflection is so much stronger than the scattered energy returned from the moving head of the average meteor trail. However, the strength of the "whistle" is not so dependent on the orientation of the meteor's path. The head of the column of ionization, being small and of rounded shape, apparently scatters signals back well in many directions. Consequently most meteors that are large enough will produce a whistle. However, only those meteors which travel along exactly the proper path will produce a pronounced burst.
This difference in behavior of "whistles" and "bursts" is readily noticed in practice. Using the set-up described in this article, one often hears the telltale whistle of a meteor boring into the ionosphere, without any perceptible change in received field strength - or "burst" - whatever. These whistles correspond to the meteors that caromed off in such a way that no broadside reflection could occur. Then again, one notices "bursts" without hearing any accompanying whistle. These "bursts," it is reasoned, are produced by meteors following the correct path for broadside reflection, but so small that the energy scattered from their moving heads cannot be heard. The most dramatic-sounding meteors of all are those that begin with a high pianissimo whistle and end with a low, fortissimo grunt, or "burst." Here the "burst" occurs when the meteor trail passes the point at which it can return a broadside reflection. Interestingly enough, this point is also the point at which the Doppler beat note goes to zero, since when the meteor is moving along a path perpendicular to a line drawn between the meteor and the transmitter and receiver, there is no change in path length and hence no Doppler shift of the radio frequency.
It has been found that the ratio of whistles to bursts is rather sharply dependent on the operating wavelength. If one listens to short-wave broadcasting stations in the 25- or 31-meter bands, one hears a relatively large number of whistles as compared to the bursts, although the latter are somewhat obscured by the variable "long-scatter" signal always present. Many of the whistles last as long as one or two seconds. In the 27-Mc. band, however, the whistles are less frequent than the bursts and they are of shorter duration. It is likely that at still higher frequencies (say 100 Mc. or so) whistles would be heard much less frequently, if at all. The bursts also become of shorter duration as the frequency is increased. At 27 megacycles, the average burst is about half a second in duration. At 50 -megacycles, they appear to be still shorter. When the receiver beat oscillator is off, bursts sound like a "thump"; with the beat oscillator on, they sound like a sharp "ping."
The experimental set-up used at Stanford University to detect meteors is extremely simple. The radiated signal is provided by the Stanford Radio Club's transmitter, W6YX. Input to the final stage is 950 watts. Two types of transmitting antennas have been successfully used: the first is a simple half-wave doublet roughly 16 feet long and 8 feet above the ground, giving a radiation pattern consisting of a broad lobe pointed straight up. The doublet was supplanted by the arrangement shown in the photograph; This is nothing more than a three-element beam so arranged that it can be directed vertically upward, or to any intermediate angle, by means of a rope and pulley. A rotatable transmitting antenna is a great help in reducing the signal radiated in the direction of the receiver, since the null off the ends of the elements can be found experimentally and pointed in the direction of the receiving site. This null may or may not be exactly aligned with the direction of the elements, depending on whether the system is exactly balanced to ground or not.
Directivity is not necessarily an advantage in the transmitting antenna, since power gain is obtained by decreasing the width of the beam, which cuts down the area of the sky from which meteor reflections can be obtained. The practical effect is to make the number of echoes heard less frequent. Those which are heard, however, are stronger.
The receiving location at Stanford is an experimental building about a mile away from the amateur transmitter and somewhat below the direct line of sight. The receiving equipment is shown in the second photograph. For purposes of illustration, the NC-200 receiver is shown outside the building - it is normally located inside where there is a heater! A ten-foot post, set in the ground, supports the 16-foot 2-by-4 on which the 11-meter dipole antenna is mounted.
The unique feature of this antenna is that it not only can be turned in any desired direction but can be tilted at will. It is connected to the supporting post by what is in effect a swivel joint. Tilting and turning is accomplished by means of fish lines tied to the ends of the antenna; an awkward method, but one that works! The object is to find that position of the receiving antenna at which the direct signal from the transmitter is almost completely balanced out. Just why this antenna must usually be tilted in order to find the null is not very thoroughly understood; it is conjectured that local distortions of the field by reradiation from adjacent antennas, power lines, etc., gives the incoming wave a polarization that is far from horizontal. It is commonly observed that the apparent direction of arrival of a signal, under similar circumstances, may be far different from the true direction. However, this much can be said for the tilted antenna: in all cases, no matter how bad the unbalance to ground, or how many the obstructions (such as cars) close at hand, it has always been possible to find a sharp null by properly rotating and tilting it. The signal from the transmitter at the receiving site is about 30 db. above S9 on the NC-200 S-meter when a nondirectional antenna is used; using the dipole, this signal can be reduced in strength until it drops into the noise level.
There is no special reason why the receiving and transmitting antennas were located as close together as they were at Stanford, except convenience. As a matter of fact, the closer they are together the more difficult it is to find and maintain a deep null, and the receiving antenna must often be tilted until it is far from horizontal. These difficulties can be avoided by greater separation between transmitter and receiver. If the separation is great enough it may be possible to do away with special receiving and transmitting antennas entirely, provided the aerials available shoot the majority of their power toward the zenith.
It is important that a sensitive receiver be used for meteor detection, and that the antenna be properly matched to it. Receiver sensitivity, in this case, is the equivalent of transmitter power; with the latter set at the 1-kilowatt maximum, and antenna directivity restricted, system performance can only be improved by improving the receiver. The best indication of a receiver's sensitivity is the change in noise level when the first tuned circuit is tuned through resonance with the gain control wide open and with no signal being received. Unless there is a noticeable change when this is done, the set simply isn't sensitive.
The procedure used in making the tests was to radiate an unmodulated signal from W6YX, null out this signal at the receiving site, and then maintain an aural and a visual watch for meteor reflections. Tests of any duration were made in the 11-meter band, where AØ operation is permitted. The transmitter was identified by keying the call letters every ten minutes.
When To Listen
The arrival of a meteor will be announced either by a brief whistle audible over headphones or loudspeaker, or by a sudden "kick" of the receiver's S-meter. Often the whistle and kick will nearly coincide. The pitch of the whistles in most cases descends rapidly to zero, ending in a "grunt." In some instances it may go to zero and then start to rise again, showing that the meteor has approached, passed by at right angles, and then begun to recede. In most cases, however, the meteor will pass through the ionized region or will burn itself out before an "up" whistle can be formed.
An oscillogram of a meteor whistle is shown in Fig. 2. This oscillogram, believed to be one of the first of its kind, was made by transcribing a phonograph recording of the whistle on a 16-mm. motion-picture sound track. At the left of the record (time runs from left to right) will be found random fluctuations caused by the noise output of the receiver in the absence of meteor signal. These fluctuations gradually become regular as the whistle fades in, and the downward change in pitch can readily be seen. As the whistle pitch goes to zero, the strength of the reflected signal increases and presently the receiver is blocked by the "burst" or broadside reflection. The background noise accordingly disappears. The burst then fades away and as the set recovers, the noise again puts in its appearance. During the burst, when the receiver noise is absent, a series of regularly-spaced marks will be found on the record. These are 15-c.p.s. timing pulses added to give an idea of the duration of the burst. /p>
The best hours for hearing meteors, unfortunately, are the wee small ones early in the morning. This is because the earth, while performing its daily rotation, is at the same time moving forward in space along the track of its yearly orbit around the sun. From midnight on, that tiny speck of the earth's surface that we call "home," is moving forward in space at a speed equal to the sum of the motions resulting from the earth's spin and that of its orbital travel. During the afternoon and early evening hours the net forward speed of "home" is the difference of these two motions. The situation is the same as that of a fly clinging to the rim of a moving wagon wheel, considered with respect to the road's surface. The fly is moving forward faster when he is on the top of the wheel than when he is down near the road. When our portion of the earth is moving forward in space most rapidly, the ionosphere directly above us runs into the most meteors, and vice versa.
However, meteors can be "heard" in the late morning and early evening hours too; they are simply less frequent, and it may be necessary to wait a longer time to hear one. It has been found that the fishing is best between the hours of 2 and 4 in the morning. Interestingly enough, these are the best hours for visual observation as well.
On ordinary nights, the number of bursts heard on 11 meters with the set-up described above varied between something like 40 or 50 per hour during the early evening hours, up to something over two hundred per hour during the early morning hours.
The number of whistles heard was roughly one tenth the number of bursts. (A reduction in transmitted power will not greatly affect the number of meteors detected. In the course of some tests with the W6YX buffer-amplifier, whistles and bursts were plainly heard with a. radiated power of roughly 150 watts. And no attempt was ever made to improve the performance of the receiver by adding additional r.f. preamplification!) In the vicinity of the various meteor showers the number will be considerably greater. The Lyrid meteor shower. of April 21, 1947, caused an increase of roughly 3 times in the number of meteors "heard," for example.
There follows a tabulation of the nine principal meteor showers each year, taken from Reference 6:
|Name||Duration in Days||Date of Maximum|
|Eta cquarids||8||May 2-4|
|Delta Acquarids||3||July 28|
The above dates should be taken as only approximate, as there is a variation from year to year. The exact dates of each shower may be obtained in advance by consulting such publications as Sky and Telescope magazine, which may be found in any public library.
The "ticks" in the burst region (C-D) are time signals at intervals of 1/15 second, so the major burst signal lasted approximately 1/3 second in this instance.
A great many other things will be heard as well as meteors. Since the receiver must be wide open, with the direct signal from the transmitter reduced to so low value that it does not operate the receiver's a.v.c. or cause any change in gain, there will be a continuous roar of set noise in the loudspeaker. Needless to say, a receiving location that is electrically quiet is essential. Any cars moving in the vicinity will give rise to reflections which will upset the balance; they usually give rise to a fluttery motion of the S-meter needle and a low-pitched rumble in the loudspeaker.
It is extraordinary to think that meteor trails, occurring as they do some 50 or 60 miles from the observer, should be able to reflect radio signals of about the same strength as airplanes flying overhead roughly one mile away. Yet, echoes from strong meteors often kick the NC-200 S-meter up to S9 or above. The size of the ionized region produced by a meteor must clearly be large. If an airplane, which can be thought of as an irregular object approximately 100 feet in diameter, returns an echo of given strength at a distance of 1 mile, an irregular object such as a meteor trail must be at least 50 times as large in order to return an echo of equal strength at 50 miles. This implies that the meteor trail must be roughly 5000 feet or one mile in diameter. Moreover, since the tests were made at 11 meters, with the transmitter and receiver virtually at the same place, the ionization contained in this trail must be intense enough to reflect an 11-meter wave fired directly at it,. i.e. - at vertical incidence! Presumably if this ionization is allowed to diffuse outward until it is only strong enough to reflect, say, a 7-Mc. wave at vertical incidence - which is a level of ionization commonly encountered in the normal ionosphere - the diameter of the ionization column would then be very much larger.
It is, of course, also possible for radio signals to be reflected from meteor trails at glancing incidence, as might be the case when transmitter and receiver are a hundred or so miles apart and a horizontally-traveling meteor passes over the midpoint of the path. Meteor ionization of given intensity would then reflect radio signals of very much higher frequency. The 6-meter band "opening" during the Giacobini-Zinner meteor shower, reported by E. P. Tilton and others, is an example of this effect. Similarly, the 144-Mc. reflections observed by G. R. Abell, jr.,11 the momentary 28-.Mc. band openings reported by
B. Henke,12 etc., are in all probability caused by the same mechanism. It is interesting in this connection to note that the Federal Communications Commission has recorded at its monitoring station at Grand Island, Nebraska, bursts of signal reflected by meteors from an f.m. station in Boston, Massachusetts, operating in the 42-Mc. band!13 This 1400-mile transmission represents about the maximum possible distance for one-hop E-layer propagation.
Although there is no doubt that meteors can cause the effects described in this article, the evidence at hand is not by any means extensive enough to make it possible to say that they are the only cause. Cosmic-ray bursts, for example, have been seriously proposed as a source of momentary radio reflections. It is furthermore quite possible that whatever mechanism produces sporadic E could also give rise to signal "bursts" of brief duration.
To heighten the mystery still further, although a great many coincidences between visually observed meteors and whistles or bursts have been obtained at Stanford (and it is really impressive to see a big meteor go sailing overhead while listening to its whistle in the loudspeaker!) it has nevertheless been found that a certain percentage of the meteors fully bright enough to be heard by radio, and apparently occurring in the correct portion of the sky, are simply not heard at all.
It is therefore clear that a count of whistles or bursts cannot yet be relied upon to give an indication of the absolute number of meteors colliding with our atmosphere; however, as an indication of the relative number, the method is very sensitive and consequently holds much promise. The behavior during showers, as well as the observed nightly maxima between 2 and 4 A.M. shows that meteors play a very large - if not the sole - part in the formation of whistles and bursts.
But further observation is needed, and in this field the radio amateur is in a position to make a .unique and important contribution to our common knowledge of the ionosphere. Anyone owning a medium-powered transmitter and a sensitive receiver can use them to spot meteors on cloudy nights as well as clear. Reports of meteor ionization effects observed are needed and would be most useful. The author will be glad to correspond with anyone interested in this type of work.
The experiments described in this article have been carried out jointly by W. E. Evans, jr., R. A. Helliwell, W6MQG, L. A. Manning, W6QHJ, and the author. Members of the Stanford Radio Club who have assisted in various ways include: L. A. Roberts, W6YWX, R. O. Beaudette, W7FXI, and J. W. Menne, W0LTW.
* Department of Electrical Engineering, Stanford University, Calif.
1. G. Villard, jr., QST, p. 59, January, 1946.
2 Mitra, Syam and Ghose, Nature, p. 533, Feb., 1934.
3 J. A. Pierce, Proc, I.R.E., p. 892, July, 1938.
4 J. A. Pierce, Physical Review, p. 625, 59, 1941.
5 Chamanlal and Venkatamaran, Electrotechnics, p. 28, 1941.
6. O. P. Ferrell, Physical Review, p. 32, 1946.
7 Electronics, p. 105, Jan., 1945.
8 Bateman, McNish, and Pineo, Science, p. 434, November 8,1946.
9 L. A. Manning, et al., Physical Review, p. 767, Nov., 1946.
10 J. A. Pierce, Physical Review, p. 88, Jan., 1947
11 G. R. Abell. jr., QST, p. 48. Nov., 1946.
12 Bruce Henke, QST, p. 61. Jan., 1947.
13 Federal Communications Commission Docket No. 6651, Sept. 28, 1944.
Posted June 8, 2015