US2798207A - Magnetic microwave attenuators - Google Patents

Description

July 2, 95 F. REGGIA 2,798,207

MAGNETIC MICROWAVE ATTBNUATORS Filed Aug. 17, 1951 2 Sheets-Sheet 1 30 30 TUNED 25 MICROWAVE MAGNET/C 5 015 50 OSCILLATOR ATTENNATOR C SE EE E5 LOAD LOWCURRENIMZT REGULAIE I F. POWER sup/ 1. JIMPLIHER gg- REE vanes IN V EN TOR.

Fran/c figggz'a v ATTORNEY July 2, 1957 F. REGGIA 2,798,207

MAGNETIC MICROWAVE ATTENUATORS Filed Aug. 17, 1951 I 2 Sheets-Sheet 2 25- AIIENUAIUON 50- 22- 10-. IIENUAT/QN FIELD CURRHVT 10- I o I I I l I 0 a5 1.0 1.5 20 5.0 3.5

FIELD CURRENT N F g.4

IN VEN TOR iFrank Bggza United States Patent Q States of America as represented by -the Secretary of Commerce Apeneanen August 1-31951, serial $142,397 3 Claims. (Cl. sas -s1 (Grantee under "Title as, us coaedssayse'c. 266) i The invention described herein may be manufactured and used *by or for the'Government of the United States for governmental purposes without the payme'ntto me of any royalty thereon in accordance with the provisions of the Act of March 3, 1883, as amended(45" Stat. 467; 35 U. S. C. 45).

The present invention relates to the development of variable attenuators for ultrahigh-frequency electterna'gnetic waves in transmission lines, which attenuators do not involve the use of slots 'in' the' c'oaxiallines orwa've' guides, mechanical controls, or movable components in the "transmission line;

At'present the most common type or variable'at tenuator fdr thesehigh frequencies is the piston type 'inhanical attenuator. These attenuatorshav'e several inherent difficulties such ashigh insertion losses, inaccuracies due to backlash in the gears, the n'ece'ssity for locating attenuator controls at the sam'e place in lin'e as the attenuator is located and the limitation that the mechanical displacement of the control perdecibel attenuation '-is-s'niall and is fixed by the diameter of the tube. Also the me chanical attenuator is restricted to-thoseuses that require only slow and rather infrequent changes in attenuation. It is obvious, for example, that this type of attenuator could not be used to amplitude modulate an electromagnetic wave even at audio frequencies.

It is'a primary object of this invention to'provide a variable attenuator for the ultrahigh-frequency ranges that has very low insertion losses.

It is another object of the invention to provide a variable attenuator that has no moving parts.

It is another object of the invention to provide a variable attenuator that can be used as an amplitude modulator.

Another object is to provide a variable attenuator which can have its controls located at any'convenientlo'cation.

Another object of the invention is to provide a variable attenuator whose control can have any desired mechanical displacement (of the control) per decibel.

In accordance with the present invention there is provided an attenuating device that possesses the specified characteristics as outlined above. The device consists of a section of transmission line which contains a special dielectric material inside the line. The two poles of an electromagnet are placed on opposite sides of this section of the cable containing the special attenuating material so that this material will be subjected to whateverfield exists between the poles. Provisions are made for varying this field from zero to very high intensities. It has been found that by increasing the intensity of this field the attenuation of the radio frequency wave caused by the special attenuating material will be greatly reduced.

Other uses and advantages of the invention will become apparent upon reference to the specificationahd drawings.

In the drawings Figure 1 is a view partially in crossse'ction and partially in plan of one embodiment of the invention;

2 ,798,207 Fatented July 2, 1957 Figure 2 is the'cross-sectional View of the attenuator taken along line '2 2 of Figure l.

Figure 3 is a graphshowing'the relationship between the "electromagnetic field current and the attenuation in decibels.

'Figur tis a'showing of a modification of the special section of the coaxial conductor employed when the invention is used to 'amplitudeunodul'ate the electromagnetic Wave. p, I

' Figure 5 is another attenuation versus-fie'ld-current s fp Figure 6 is a block diagram era system in which the present invention is used to "stabilize "the power output from the power source.

The losses due to the insertion of an attenuator with magnetic properties in a system in which high frequency power is "being transmitted over acoaitial line or a wave guide are or thrjeiet'y'p'ejsz r'efiectidn, hysteresis, and eddy current losses. Thereflection losses can be madealmost negligible by impedance matching, and'the 'eddy current losses'canbe'reduc'ed to a lowv'alue by using an attenuator which has a high resistivity. Some of the magnetic material'sdevelopedliii'recerit'years have such'very high resistivities that almost all losses ih these materials are due to hysteresis losses rather than a combination of hysteresis and eddy current losses. It the total eddy cur- 'reiit and reflection losses are reduced to a negligible amount, only thehysteresis losses in the magnetic mate rialare of any consequency. By developing a means for controlling the hysteresis losses 'in a magnetic material the inventor'has made possible a high'frequen'cy attenuator which eliminates movin pait sfrom'the transmission system and also does away with all of the difficulties that are inherent in mechanical attenuators.

Whenhighfrequencypower is being transmitted down a coaxial line and magnetic material is inserted in the line, thechange in flux density in that material is "equal to "the change in magnetoinotive force times the permeability and, as'is Well knownfth'e permeability is 'a function of the flux density. It a second magnetomotive force is applied across the'magnetic material the change influ'ii density due to the change in the first magnetorn'otive force is still equal to the, change in the first magntomotive 'forc'e times the permeability. The sec ond ma'g'netotnotive force does not directly affect this relationship. However, the permeability is no longer a simple function of the flux density caused. by the first m gnetomotive force alone, but is a complex function or the total flux density; that caused by the total of the first and second magnetomotive forces. As the second maghetomotive force is increased the change in flux density caused by a change in the first magnetomotive force is' 'deci e ase'd and "since, 'as is well'known, the power lost ina magnetic material is proportional to some power of the change influx density, the power lost in the attenuator is decreased.

.In 'the present invention the radio frequency power lbstinthe' attenuator is "varied by varying the second magnetomotive toice which is applied across the attenuatoi' from an outside sourceofinaghetomotive force.

ln'Figure l there is shown one embodiment of a practicaldevice'for e'm'pld'yingthe above principles; An ultrahigh-frequency generator '1 supplies radio-frequency ower to'a coaxial line 2 in which is'inserted a special section of cable at 3. The coaxial line is connected to some power-absorbing load 4. Inserted in the section'of cable3 is a special magnetic material 6. (See also Figure 2. This material is subjected to a biasing fieldby means of the U-shaped electromagnet 7 whose two poles sand 9 are fitted tightly agaiiis'ttheopposite sides of the coaxial section 3. Thccurfei i't'to the field coils il and 12 a supplied by the battery 1's. It'is obvious, of course,

that a single coil Wound around the base of the U might also be used. A variable resistor 14 is inserted in the line to provide a control for the field current. The means used to controlthe attenuation is this variable resistor 14. The use of the resistor to control attenuation makes it possible to locate the control at any convenient point in the system and also makes it possibleto have any desired mechanical displacement per decibel that is expedient.

The magnetic material 6, as stated before, must have a high permeability and a very high resistivity. There are commercially available today several high permeability, high resistivity, ferromagnetic materials. Of these Polyiron and the magnetic ferrites are well known. these materials have permeabilities higher than 4000 and resistivities as high as It will be seen that since the resistivity is so high the eddy current losses can be kept extremely small and therefore almost all of the loss in the material is due to the hysteresis-effect. The magnetic ferrites are considered to be very low loss material at frequencies up to about 200 megacycles per second, but at frequencies above this point they become lossy and therefore are suitable for application to this invention; In the range in which these materials become lossy the decibel losses vary approximately linearly with respect to frequency. I

Figure 3 is a graph of the deeibelattenuation plotted against the current supplied to the field coils 11 and 12. The initial attenuation, that is, the attenuation in the absence of a field across the poles 9 and 10 and with av constant frequency applied to the transmission line depends entirely upon the properties of the attenuator material and its physical dimensions. In this particular case the initial attenuation was 30 decibels. As direct current was applied to the field windings, the attenuation decreased almost linearly to decibels. At this point the curve began to level off and the lowest attenuation ob tained was 12 decibels or a total reduction of 18 decibels.

Some of t The percentage change in decibels was 60 percent and the ratio of the initial power absorbed in the material to the final power. absorbed was 60 to 1. This graph is typical of the type of results obtained. However, far greater ranges ,of attenuation have been obtained as shown by Figure 5. In this modification an attenuator made of a /2-inch length of Croloy No. 20, amagnetic ferrite, and subjected to a frequency of 3700 megacycles per second, was made to vary the attenuation from a high of approximately 40 decibels to a low of 1 decibel. It will be noted that one of the objects of the invention was to develop a variable attenuator with low insertion losses.

The variable attenuators used in the prior art had insertion losses of approximately 20 decibels; that is, with the attenuator set to the least possible amount of attenuation the generator had to supply 100 units of power for every one unit of power reaching the load. However, with the attenuators according to the present invention the attenuation that cannot be eliminated is less than 1 decibel.

As can readily be seen, this represents a tremendous saving of power.

As pointed out above with reference to Figure 3, over a range of approximately 15 decibels the decrease in attenuation varied linearly with the increase of the field current. It is apparent then that the radio-frequency wave can be amplitude modulated without distortion if an alternating field current is applied that will cause the attenuation to varyover a range that is within the linear portion of the curve. It is a property of some of these special materials that the same reduction in attenuation is caused by a field applied in one direction as by a field applied in the opposite direction. Therefore, if an alternating current field were applied across the attenuator and this field were allowed to oscillate about zero field as a center, the'modulating frequency would be doubled. This is true because each half cycle of the modulating field would have exactly the same effect on the radio-frequency wave. Therefore unless it is desired to obtain a modulating frequency that is equal to twice the applied audio frequency, it is necessary to superimpose on the alternating field current a direct field current sufiicient in magnitude to prevent the total field current from crossing the zero axis. There is a limitation on the size of both the direct and alternating field currents. The curve as shown in Figure 3 indicates that the attenuation curve is linear over'only a portion of the curve, and it is necessary to stay within these limits it serious distortion is to be avoided. Therefore the direct-current bias is chosen so that in the absence of an alternating field current the material 6 has a sufiicient field across it to locate the initial decibel losses in the center of the linear portion of the current. Referring to the curve it is seen that in this instance the value is'0.625 ampere. Then the alternating field current can have a peak-to-peak value of 1.25 amperes. By choosing these values, both objectives are accomplished; i. e., the total field never goes through zero and never extends past the linear sector of operation.

In Figure 4 there is shown a system according to the present invention that can be used to amplitude modulate a radio-frequency wave.

The attenuating material 6, as shown in Figure 2, is completely inclosed along its axial length by the outer conductor 15 of the coaxial line. This outer conductor is made of brass, and if an alternating-current field were applied across this section most of the audio-frequency energy would be dissipated in the brass. Therefore, section 3 of Figure 1 cannot be used satisfactorily in a modulating unit, and it was found necessary to use a section of the type shown at 16, in Figure 4. In this modification the pole pieces 8 and 9 are made of the same material as the attenuator. As pointed out before, these materials have high permeabilities and have very small losses'at low frequencies. Therefore the magnetic ferrites make ideal materials for use in magnetic circuits at frequencies under megacycles per second.

The pole pieces 8 and 9 in this form of the invention extend through the sides of the coaxial section 5 and are in firm contact with the attenuator '7. The brass outer conductor 15 has been removed at the places where the poles extend through the outer conductor, but the conductor extends part way up the sides of the poles 3 and 9. These sides are shown at 17 and 18. The sides 17 and 18 form Wave guides below cutoff; that is, wave guides below their cutoff frequency which therefore have a very high impedance. As a result the leakage of radiofrequeney power, which normally occurs when there are openings in the outer conductor of a coaxial line, is kept to a minimum.

The pole pieces 8 and 9 are excited by means of the U-shaped electromagnet 19, which receives audiofrequency power from the audio-frequency generator 21.

This means of amplitude modulation of the radiofrequency wave'has made it possible to accomplish amplitude modulation without introducing frequency modulation. Heretofore it has not been possible to accomplish this without a great deal of difiiculty. It is possible to accomplish this without much difiiculty by means of the present invention, since the wave is modulated at a point in the system which can be effectively decoupled from the radio-frequency oscillator. in accordance with standard practice decoupling can be accomplished by inserting a buffer (some of which are shown at number 30 in Figure 6) in the transmission line between the power source and the attenuator.

In regard to using the attenuators of the present device as amplitude modulators it is of interest to note that Croloy No. 20 will produce a sharp peak of attenuation with the field poles located in one position but not in all positions about the circumference of the cable. Therefore if the Croloy No. 20 were subjected to a rotating magnetic field the radio-frequency wave would again be amplitude modulated. v

Figure 6 is a block diagram showing another use of the present invention. In this particular circuit the variable attenuator is used to keep the power input to the load constant. A microwave oscillator 22 is used to supply power to the load 23. The magnetic attenuator 24 is employed to keep the power to the load constant. The tuned probe 25 samples the power going to the load and produces a voltage that is proportional to the power. This voltage is fed into the direct-current amplifier 26 which in turn controls the power supply 27 for the electromagnet of the attenuator.

The operation is as follows: The power output of the oscillator 22 is adjusted to the desired power level. If for any reason the power reaching the tuned probe 25 changes, the voltage output to the direct-current amplifier 26 will be changed. This in turn varies the output of the amplifier to the power supply 27, which in turn varies the control current to the electromagnet of the attenuator. As an example, it will be presumed that the power level to the load drops. The output of the tuned probe will be reduced, and this will decrease the output of the direct-current amplifier. This in turn will decrease the voltage to the power supply 27 and thereby increase the control current to the attenuator. This will reduce the attenuation and raise the power level. The buffers 30 are used to isolate the different stages along the transmission line.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of my invention as defined in the appended claims. Of these the use of the present invention with wave guides is the most obvious.

I claim:

1. A variable attenuator for traveling electromagnetic waves covering a frequency range of 200-3700 rnc. comprising: a coaxial TEM mode transmission line, a body of magnetic ferrite material inserted in and substantially filling the cross-sectional area of said coaxial transmission line over the entire length of said body, electromagnetic means for establishing a magnetic field below magnetic saturation across said body and in a direction 6 perpendicular to the axis of said transmission line and means for varying the magnetic flux in the field established by said electromagnetic means, said body having a response at a particular frequency within said frequency range such that the attenuation versus magnetic flux curve decreases from a relatively high initial value at substantially no flux to a second value, then rises sharply to a maximum value greater than said initial value and decreases substantially linearly to a minimum value which is considerably lower than the second value of attenuation.

2. The invention according to claim 1 in which said electromagnetic means produces a magnetic field which rotates about said body in a plane transverse to the axis of said hollow transmission lines.

3. The invention according to claim 1 in which the flux density is held constant at a value to cause maximum attenuation and 'said electromagnetic means produces a rotating magnetic field.

References Cited in the file of this patent UNITED STATES PATENTS 2,197,123 King Apr. 6, 1940 2,286,428 Mehler June 16, 1942 2,392,434 Trucksess Jan. 8, 1946 2,402,948 Carlson July 2, 1946 2,562,744 Schultz July 31, 1951 2,629,079 Miller Feb. 17, 1953 2,645,758 Van de Lindt July 14, 1953 2,745,069 Hewitt May 8, 1956 FOREIGN PATENTS 806,150 Germany June 11, 1951 OTHER REFERENCES Belgers: Measurements on Gyromagnetic Resonance, Physica XIV, No. 10, February 1949, pages 629-641. (Copy in 333-246.)

Hewitt: Microwave Resonance in Ferromagnetic Semiconductors, Physical Review, vol. 73, No. 9, pages 1118-9. (Copy in Patent Ofiice Library.)

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