Your RF Cafe
|August 1952 Radio & Television News|
These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television News articles I have already posted. All copyrights are hereby acknowledged.
While this article is directed at amateur radio operators who want to explore working in the microwave bands, it is good fodder for anyone who wants a fundamental introduction to waveguides, resonant cavities, distributed elements, and atmospheric propagation. If that describes you, and particularly if you have formulaphobia, then start reading. Even though the article appeared in a 1952 issue of Radio & Television News, the list of frequency band allocations are not much different than today so the information is useful.
Thanks to Terry W. for providing this article.
Fig.1. A 6V6 tube mounted in a wave guide and cavity system for generating microwaves with a conventional receiving tube.
The growing problem of TVI will eventually force hams to "move upstairs" into the microwave bands.
Since May 1945 the Federal Communications Commission has reserved seven bands of microwave frequencies for the exclusive use of hams. Ideally located between 420 and 22,000 mc., this allocation provides over 2,000,000 kilocycles of spectrum as compared to the 16,000 kilocycles now used for approximately 99 percent of all ham transmissions.
It is now seven years since these bands were assigned to the hams yet, with the exception of the 420-450 mc. band, activity in these bands is negligible. This state of affairs is inconsistent with the ham's traditional role as a "radio pioneer."
The fact that 15,000,000 television receivers are now in the hands of the public with more to come as new stations bring larger areas of the country within the fold will provide a powerful impetus in "kicking the amateurs upstairs." If there is to be any lasting peace and harmony between the millions of televiewers and the thousands of amateurs, the only real solution lies in a voluntary, or perhaps involuntary, exodus of radio hams to the high frequency, or microwave, side of the television frequency bands.
Ham transmissions on the present popular amateur bands will continue to cause interference in nearby receivers with the attendant protests from irate TV fans. Although it is technically and theoretically possible to eliminate such interference, it is sometimes financially unfeasible to do so. The mere fact that one amateur station can operate in a certain area without causing interference in nearby television receivers is no guarantee that the equipment will continue free from TVI. All the ham has to do is change frequencies or equipment or for a neighbor to buy a different type of TV receiver and the problem of interference becomes critical.
The public's investment in television receivers now aggregates three billion dollars as compared to the approximately thirty million dollars which hams have tied up in their equipment. Thus the hams' stake in radio gear has been exceeded a hundredfold by the public's investment in home receivers. In terms of the number of households affected (or in votes at election time), the relationship is even more top-heavy. At the present time there are over 200 times more television households than there are ham radio homes. Even the ham often finds himself in the unenviable position of having his ham equipment interfere with his own television reception! A showdown is in the making at the present time and eventually the ham will wind up in the microwave region and, in the opinion of the author, when this happens it will be the greatest "blessing in disguise" ever vouchsafed the radio art.
The situation is even more critical than it was in 1922 when radio broadcasting developed virtually overnight into a hydra-headed monster of gigantic proportions. At that time, amateur radio had to move up in the radio spectrum and operate on the high frequency side of radio broadcasting. This enforced move made the ham's equipment and techniques obsolete and unsuitable. Specifically he had to discontinue operations on frequencies of 1500 kc. and lower where he had employed a spark coil or rotary spark transmitter, crystal receivers, monstrous antennas, and wireless telegraphy. He was forced to operate in the band from 1500 kc. upward with equipment requiring the use of vacuum tubes and careful attention to impedance matching.
In a matter of months this enforced move resulted in several important developments. The move led to the discovery of the Kennelly-Heaviside layer, reliable determination of sky wave skip phenomena, and the establishment of the advantages of short-wave operation.
The ham was delighted to discover that with a fraction of the power needed on the lower frequencies he could communicate world-wide - even to the antipodes. Even a small receiving tube, such as the now-obsolete UV201A, was sufficient to serve as a transmitter to reach all the way to Australia. The range of communication jumped from the usual hundred miles or less to distances circling the globe. Furthermore the ham could make his contacts by voice or radio-telephony instead of code or radio-telegraphy.
Today a similar situation exists except that now the amateur will move to a much higher and more spacious spectrum. He can achieve efficiencies in circuitry never before possible since he can dispense with lumped or specially-provided inductances, condensers, and even resistors with their losses. He can develop and control the electric and magnetic fields through distributed inductance, capacitance, and impedance by means of the physical arrangement of simple metallic shapes.
On microwaves, the amateur will again be recognized as one of the nation's most valuable sources of original research and experimentation instead of a mere nuisance as he has now become in the opinion of millions of televiewers. On microwaves, where amateur radio now more properly belongs, hams by their very number and geographical distribution will open a new era in amateur radio. They will quickly overtake the billions of dollars' worth of professional microwave development that has thus far taken place without his participation. He can ultimately save the taxpayers untold sums which are now going into microwave research and development. In the past decade it has been demonstrated that professional microwave activities have been unable to keep microwaves simple and inexpensive enough to encourage their widespread usage. Only the radio amateur is in a position to substitute empirical (cut-and-try) methods for the calculated complex and planned procedures of government and industry. There are relevant discoveries yet to be made which can best be made by a free exchange of information and experiences by amateurs operating largely "without rhyme or reason" techniques.
The radio amateur today has a large number of frequencies in which he is free to operate. These frequency bands are listed in Table 1.
It is estimated that most of the licensed amateur radio activity in the United States is concentrated in the high frequency band, representing a total of only 3570 kc. out of the total 2,256,670 kc. assigned to hams. They, plus the bulk of the rest of the hams operating on the 50-54 mc. and 144-148 mc. bands, are the source of the TVI. The hams have congregated into 11,670 kc. of the amateur spectrum subject to TVI while doing little to equip themselves and engage in operations on the balance of the 2,245,000 kc. assigned to them. They are jammed into less than one-half of one percent of the spectrum and are ignoring the more than 99% percent of the spectrum in which TVI would be virtually nonexistent.
Microwaves have been generally recognized to be the frequencies between 300 and 3000 mc. (known as ultra-high frequencies) and between 3000 and 30,000 mc. (known as super-high frequencies). The FCC has allocated 9.8 percent of all ultra-high frequencies and over 7.3 percent of all super-high frequencies for the exclusive use of hams. On a non-exclusive basis, the amateurs may also use all frequencies above 30,000,000 kc. on to infinity or cosmic rays, including the bands known as "infrared," "light," "ultra-violet," "x-rays," "gamma rays," and beyond.
Basically, the difference between microwave operation and transmissions at the lower frequencies is a matter of equipment. In the case of microwave operation there is no need for specially-provided transformers, coils, condensers, or resistors. As shown in Fig. 3, all of these components are replaced by positions taken along a closed or shorted pipe (called a wave guide). In practice. this pipe is usually rectangular with its wide dimension exceeding a half wavelength. Fig. 4 is a photograph of a commercially-available model and an improvised unit made of screen wire. The home-built unit can be made out of foil or any other conductive or non-conductive material as long as the inner surface is a good conductor. If a simple can is used instead of a pipe, the unit is called a "cavity." Only the frequencies which have electric and magnetic field distributions that fit inside of such a can or cavity will exist in same. Thus, it is a frequency controlling element hat replaces quartz crystals on the lower frequencies. For the amateur frequencies, the cavities can have a "Q" on the order of 10,000 or more.
On the frequencies that the radio amateur best understands (frequencies below 450 mc.) , he has been conveying energy by means of conductors such as circuit wiring. On the microwave frequencies, he conveys the current by means of electric and magnetic field displacements within the wave guide. In other words, microwaves are characterized by the displacement technique while conventional frequencies use conduction or the cumbersome electric power line technique.
On microwaves, the phenomenon of space radio propagation is extended to the passage of energy within the equipment itself and the transmission line system. The method by which this takes place is shown in Fig. 5. One side of the wave guide pipe (Fig. 4) simulates the ionosphere while the opposite side simulates the earth. Fig. 5 shows a several-hundred-foot medium frequency broadcasting tower used for sky wave transmissions by reflections between the ionosphere and the earth. Fig. 5A shows a rectangular pipe (artificial or fabricated wave guide) which replaces "Nature's wave guide" and performs the same function. In Fig. 5A the pipe is less than a half wavelength or at cut-off. Energy will not proceed down the pipe and attenuation is maximum. Fig. 5B shows what happens if the wave guide is wider than a half wavelength. Energy will propagate down the wave guide. Fig. 5C shows what happens if the wave guide is made even wider. Energy will be propagated even better with less attenuation or losses. In order to keep this explanation simple it is desirable that the dimension of the guide not approach or exceed a full wavelength. If the guide is wider than a full wavelength, the energy divides itself and becomes similar to two wave guide pipes. Two energy patterns or modes would then exist side by side. In addition, the narrow side of the rectangular wave guide would accommodate a pattern. The narrow side walls function to keep the other two walls properly spaced. They also determine how much power can be handled by the wave guide. The wave guide of Fig. 4 can handle up to 3,200,000 watts of power without breakdown or flashover. Fig. 2 is a graph of the performance of a wave guide suitable for the 10,000 to 10,500 mc. amateur microwave band. The rectangular pipe has an inside dimension of .9" x .4". Part A in Fig. 5 corresponds to 6562 mc. on the graph of Fig. 2. Part B in Fig. 15 might correspond to 7500 mc. on the graph of Fig. 2. Part C might correspond to 10,500 mc. on the graph of Fig. 2. At 13,123 mc., two modes of energy will form, changing this particular energy designation from "cut-off frequency of transverse electric mode 1,0" to "cut-off frequency of transverse electric mode 2,0."
It is preferable to operate in the dominant or first mode for reasons of simplicity. It is feasible, and research has been conducted along these lines, to use several modes, each a separate channel of communication. If the wave guide is two wavelengths in width, there would be four modes of energy. If it is three and one-half wavelengths in width, there would be seven modes of energy, etc. Fig. 2 also shows the attenuation in decibels-per-foot for a particular size wave guide. In this case, in the 10,000 mc. amateur microwave band, it is less than .035 db-per-foot. The wave guide could be over 28 feet long before the energy would be attenuated 50 percent. By selecting an appropriate size wave guide, minimum attenuation can be obtained for any frequency. This same concept holds true even on low frequencies except that at 4000 kc., for example, the wave guide pipe would have to be substantially greater than a half wavelength, or 123 feet in width. It is only because of the shorter wavelengths which make possible convenient physical dimensions that it is possible to take advantage of microwave techniques that would be impossible or unfeasible to employ on lower frequencies. The technique would otherwise function on any wavelength as long as physical dimensions and associated costs are not prohibitive. The amount of power which such a wave guide can handle depends upon the height of the guide. For the wave guide of Fig. 2, the power handling capacity is 235,000 watts. The smallest size wave guide, such as the one required for 21,000 to 22,000 mc., will still exceed 60,000 watts power handling capacity. Since communication at microwave frequencies can be carried on with a fraction of a watt power (even microwatts) there need be no concern that the user might, in any way, exceed the power handling capacity of a wave guide.
To further appreciate wave guide phenomena, one need but recall what happens to an auto radio receiver when the car is driven through an underpass. The underpass is, in reality, a wave guide. Since the broadcast might be 1000 kc. (300 meter wavelength), such an aperture or wave guide would have to exceed 500 feet in diameter in order for the signals to go through. Police radios and two-way vehicular systems have no difficulty in communicating in such a wave guide since their operating wavelength is substantially shorter and will fit inside such boundaries. The wavelengths involved for the amateur microwave bands range from as little as a half inch on 22,000 mc. to as much as 14 inches on 1200 mc.
There are many other methods of handling microwave energy of which the "G string" and the "helical coil" are particularly interesting examples. In the case of the helical coil, the coaxial inner conductor connection is extended into a coil which serves as a wave guide. With the "G string," the coaxial connector inner connection extends as a straight wire while the coaxial connector outer connection flares out into a horn which focuses the energy onto this straight wire.
Tubes for Producing Microwaves
There are several tube types or tube techniques for generating microwaves. Where one can be purchased at surplus, a reflex klystron is a useful means of generating microwave frequencies. Fig. 6 is a cross-section view of a type which is approximately correct for the 3300-3500 mc. amateur band. It has a cathode, a pair of grids, and a repeller. A repeller is equivalent to a plate but is biased negatively instead of positively. The grids operate at a high positive potential. A cavity connects to the grid extremities to form a tuned circuit. If modulation or audio is impressed on the repeller voltage, the tube will serve as an FM transmitter.
Fig. 7 shows how a reflex klystron tube is coupled to the wave guide. The output electrode extends into the wave guide as if it were a quarter-wave grounded antenna, in low frequency applications. Energy then propagates down the wave guide. An adjusting screw tunes the cavity contained within the tube itself. Such a tube and wave guide is nearly correct for the 10,000 to 10,500 mc. amateur microwave band.
Fig. 1 shows a conventional tube enclosed within a wave guide cavity. In this application only the tube frequencies which can exist for that size microwave plumbing are available and utilizable. The grid and plate leads can be adjusted external to the guide. The photograph also shows an elaborate wave guide attenuator consisting of a carbon-coated resistor that can be inserted into or withdrawn from the wave guide. A gauge is used to indicate how much attenuation is being inserted.
Other means of providing microwave energy include:
1. Tubes having very close inter-electrode spacings while maintaining low orders of interelectrode capacitance by their geometrical design.
2. By using conventional tubes with the transit time between cathode and plate equal to more than a period of oscillation in order to maintain proper phase relations even though the transit time is too long with respect to the same period or cycle of oscillation. It can be corrected for a subsequent period. The electron transit time may take two or more periods of time to reach the plate from the cathode but it must arrive at the plate during the correct part of the period. This is accomplished by means of suitable voltages.
3. By use of a spark gap within a shielded wave guide. A spark gap generates the frequency spectrum while the wave guide plumbing enclosing or connected to it permits only the microwaves to propagate.
In its simplest form, a microwave transmitter is merely a signal source which may be a tube or a spark gap and a wave guide pipe. The outer end of the pipe will squirt energy into space from the end of such a wave guide. If a horn extends from that end, the energy may be concentrated or directed as desired. The beam may be sharpened or broadened by changing the length and angle of the flared horn.
The simplest microwave receiver is a silicon or other type of receiving crystal connected to a pair of headphones. A more elaborate receiver may consist of a crystal detector followed by several stages of audio or video amplification. Still more elaborate is a crystal mixer stage in which the crystal output is mixed with a local oscillator (which may be the transmitting tube) to yield an i.f. frequency which is then handled by a superheterodyne circuit similar to the one used on the lower frequencies.
Fig. 8 is the schematic of a one-tube microwave receiver used by the author in his laboratory experiments. The same tube serves as a combination r.f. amplifier, detector, first audio amplifier, as well as providing for possible a.v.c. connections. Fig. 9 shows how this receiver appears, complete with its antenna system made of brass foil. The horn and wave guide section slips over the tube with a coaxial tuning plunger connecting to the grid of the first half of the Type 6F8G tube. The antenna system comprises an electromagnetic horn, tapering to a round wave guide. The coaxial plunger, consisting of a movable short, permits adjustment of the wave guide system.
The statement or belief that microwaves can only be used within the unobstructed horizon is completely erroneous. Such ideas were also prevalent before the amateurs opened up the short-wave band in 1922 and when "five-meter" radio opened up in 1932. Skepticism was rampant when police two-way radio began expanding on v.h.f. around 1935 and when radar on microwaves moved up into the 200 mc. region and above in 1940-41. In every case, equipment has operated beyond the horizon, with many instances having been recorded showing transmissions of several thousands of miles. Once this fact was established, our research experts were able to layout study programs for yielding an explanation as to how this could occur. New and relevant factors became known. On short-waves, it was the Kennelly-Heaviside layer, first believed to be a single layer and later found to consist of several layers - each of which was responsible for a new set of radio communications ranges. On very-high frequencies, it was the dispersion effect at the horizon plus natural wave guide paths resulting from walls of buildings, sides of hills or mountains, walls of a canyon, or boundaries set by wayside wires and fences, etc.
On microwaves, the possible range of operations is unlimited if the following conditions affecting propagation through space are recognized.
1. Direct path communication within the unobstructed horizon. This is approximately equal (in miles) to 1.41 times the square root of the antenna height (in feet) above the intervening terrain plus the same conditions for the second station. For example:
Station 1 has a radio horizon of 1.41 * √2500 = 1.41 * 50 or 70.5 miles while Station 2 has a radio horizon of 1.41 * √9 = 1.41 * 3 or 4.23 miles. The two stations can thus intercommunicate over a distance of 74.73 miles by direct path.
2. Indirect path or reflected communication. This type of transmission may exist either within the horizon, beyond the horizon, or by a reflection within the horizon passing the energy on to another reflecting or pickup point beyond the horizon of the originating station. To understand how "this happens, one should consider the source signal as a beam of light and every solid object encountered enroute as a reflecting mirror. Whatever a mirror of such shape would do to light, a similar thing will happen with respect to microwaves. Reflections will be more effective when obstructions enroute are substantially larger than the wavelength. This is quite likely to happen since microwaves are normally less than one foot long. Even dense cloud formations have reflective possibilities. At their greatest height, they can develop great ranges, even for stations operating at sea level with very small unobstructed horizon.
3. Wave guide paths. Microwave energy may recognize the space between two wires as the equivalent of the two walls of a wave guide pipe. It will treat one wire as if it were the ionosphere, and the other the earth, and try to propagate skywave fashion between such boundaries. The limit of such a communications range is the limit of the availability and existence of suitable wire arrangements around the country. Even the space between railroad tracks can serve as a wave guide, as can tunnels, underpasses, canyons, gorges, etc.
4. Atmospheric ducts. These are a function of weather and can make microwaves a tremendously valuable tool in weather forecasting. Microwave propagation at great distances can be tied in with weather conditions. Microwave energy recognizes the boundaries of a stratification of temperature or pressure aloft as a wave guide. It also considers the adjacent boundaries of two strata a wave guide. If signal energy from a transmitter can enter one of these atmospheric ducts or wave guides, the range of communication can become very great - often up to thousands of miles. This fact has been verified on many occasions and is undergoing continuing research by governmental and subsidized institutions. In investigations of this type the hams will be invaluable because of their large groups, geographical distribution, and the number of hours they spend on the air. The phenomenon is often missed by the professional groups working the modern 40·hour week.
Although thousands of persons are currently employed in the microwave industry involving the expenditure of billions of dollars, very few of these persons and only a small portion of the total funds are actually used for propagation studies. Instead, most of the time and money has gone into the design and construction of complex and expensive radar and microwave relay systems.
The author feels confident that when the hams really get into microwaves in sufficient number, the "CQ" call will yield just as interesting responses as those enjoyed now. Microwaves also offer infinite possibilities for a ham organization like the ARRL to live up to its name. Microwaves are an excellent medium for radio relaying and for working out communications networks with a wide selection of echelons to communicate during emergencies and civil defense operations.
To utilize the microwave frequencies, the radio ham has to become more of a mechanic than an electrician. He must get used to pipes called wave guides and metallic structures having certain shapes, configurations, and dimensions. He will use "cut-and-try" methods and simple arithmetic in his computations. He will be required to perform simple sheet metal and machining operations in building his apparatus but will probably purchase certain of his gear such as the wave guide probes and coaxial connectors, if they are readily available, otherwise he will build or improvise them from whatever is at hand. There is not one single thing connected with microwave operation on the ham bands that cannot be built or improvised very cheaply if the ham is willing to experiment. Microwaves offer a real challenge to the alert ham-a challenge very few hams will be able to resist!
Posted May 27, 2013