US2784382A - Magnetic high frequency attenuator - Google Patents

Description

MarchS, 1957 E.F.HARR|S 2,784,382

MAGNETIC HIGH FREQUENCY ATTENUATOR Filed April 5, 1952 2 Sheets-Sheet 1 A; "Lair-4" l. fw

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March 5, 1957 E. F. HARRIS 2,784,332

MAGNETIC HIGH FREQUENCY ATTENUATOR Filed April 5, 1952 2 Sheets-Sheet 2 F -A q. 4 24' 62 MT Z5 Z4 a 4 L I 2 $7115 TJZLLIT Edward F ffizrn's MAGNETIC HIGH FREQUENCY ATTENUATOR Edward F. Harris, Columbus, Ohio, assignor to Thompson Products, Inc., Cleveland, Ohio, a corporation of Ohio Application April 5, 1952, Serial No. 280,833

2 Claims. (Cl. 333-451) This invention relates to a magnetic high frequency attenuator and more particularly to a structure for attenuating high frequency electro-magnetic energy in accordance with the strength of a magnetic field.

According to this invention, an insulator matrix having finely divided magnetizable particles embedded therein is placed in a high frequency electro-magnetic energy transmission structure and subjected to a variable magnetic field. It has been found that such a matrix imposes a high degree of attenuation on the high frequency energy when the magnetic field is small or non-existent but the degree of attenuation decreases rapidly as the magnetic field increases and, with a magnetic field of sufl'lcient strength, the degree of attenuation of the high frequency energy will be extremely small. This structure is highly advantageous as a switching means and also as a means for modulating the high frequency energy in accordance with a control signal. In particular, no moving parts are used and the structure is, in practice, readily and economically manufacturable. A further advantage is that this structure can operate efiiciently over a wide range of frequencies.

In a preferred embodiment of this invention, the matrix is of annular form and is disposed around the inner conductor of a co-axial transmission line and may provide a support means therefor. Opposite poles of an electromagnet are disposed on opposite sides of the line to induce a transverse field through the matrix.

As an alternative, a coil may be disposed around the matrix to provide an axial field. Difi'iculty may be experienced in obtaining a field of sufficient strength with this method, but it is entirely satisfactory (and desirable due to the simplicity with which it can be made and assembled) in instances where a strong field is not necessary or desirable.

In order to reduce any eddy current losses, a section of the outer conductor surrounding the matrix is preferably removed and replaced by an annular member of dielectric material together with a shorting wire and capacitor combination. By proper selection of components, the input and output impedance of the attenuator section may be made substantially constant over a wide range of frequencies.

The magnetic field may be changed between predetermined constant values to provide a switch or may be continuously variable. If desired, the field may be controlled by a low frequency signal source and a modulator arrangement. ,In such a modulator arrangement, an A.-C. current alone should not be applied to the coil of the electro-magnet since the attenuation will decrease with increased magnetic field in either direction and a frequency twice the frequency of the A.-C. current will appear in the output. This effect is eliminated, by this invention, by providing a constant biasing voltage in series with the low-frequency signal voltage.

An object of this invention, accordingly, is to provide improved structure to effect variable attenuation of high frequency electro-magnetic energy.

ited States Patent cording. to this invention.

Another object of this invention is to provide a variable coaxial line attenuator operable over a wide range of input frequencies.

A further object of this invention is to provide high frequency attenuator structure controlled by a low frequency control signal with a substantially linear relation between the magnitude of the high frequency energy output and the magnitude of the control voltage.

This invention contemplates other objects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate preferred embodiments and in which:

Figure l is an elevational view of a preferred type of attenuator structure used in conjunction with a coaxial line;

Figure 2 is a cross-sectional view taken substantially along lines II-II of Figure 1;

Figure 3 is a sectional View through another preferred form of attenuator;

Figure 4 is a schematic diagram of a circuit equivalent to the preferred forms of attenuators of Figures 1, 2 and 3;

Figure 5 is a schematic diagram of a preferred arrangement for controlling attenuation in accordance with a control signal; and

Figure 6 is a schematic diagram of another preferred arrangement of using the attenuators of this invention.

In Figures 1 and 2, reference numeral 10 generally designates a preferred type of variable attenuator ac- In the attenuator 10, an insulator matrix 11 of annular form and having finely divided magnetizable particles embedded therein is disposed around the central conductor 12 of a co-axial line section and an electro-magnet 14. having an energizing coil 15 has opposite poles 16 and 17 disposed on opposite sides of the matrix 11. It has been found that, with no magnetizing flux through the matrix 11, high frequency electro-magnetic energy transmitted into the coaxial line section will be highly attenuated by the matrix 11. When the electro-rnagnet 14 is energized by cur.-

rent flow through the energizing coil 15, a transverse magnetic flux will be set up in the matrix 11 and the attenuation of high frequency energy by the matrix 11 will be greatly reduced. With sufiicient flux density in the matrix 11, the attenuation of high frequency energy by the matrix may be reduced to a negligible quantity.

To insure maximum flux in the matrix 11, and to reduce any eddy current losses, a portion of the outer conductor of the co-axial line section is removed to define a pair of axially separated outer conductors 18 and 19 with an annular member 20 of insulating dielectric material disposed between the conductors 18 and 19, around the matrix 11 and between the poles 16 and 17 of the electro-magnet 14. It may be here noted that the matrix 11, in addition to serving its attenuating function, serves to support the outer conductors 18 and 19 and the member 20 in spaced relation to the inner conductor 12. If desired, additional spacers 21 and 22 of insulating material may be provided between the inner conductor 12 and the outer conductors 18 and 19, respectively.

To provide a current flow path between the outer conductors 18 and 19 and to improve operation over a wide range of high frequency operating frequencies, the outer conductors 18 and 19 are preferably connected at one side by a shorting wire 23 and on the opposite side by a capacitor 24 having leads 25 and 26 soldered or otherwise electrically connected to the outer conductors 18 and 19, respectively.

Figure 4 is a schematic illustration of a circuit equivalent to the attenuator 10 in which lines 18 and 19 represent the outer conductors 18 and 19, respectively. Line 12' represents the inner conductor 12, resistances 11a, 11b and lie represent the matrix 11, capacitances 11d and 11:: represent the capacitance of the matrix 11, inductances 25 and 26' represent the leads 25 and 26, respectively, inductance 23' represents the inductance of the shorting wire 23, capacitance 20' represents the capacitance of the member 20, and capacitance 24 represents capacitor 24.

It will be noted that the attenuator is thus equivalent to a resistive pi attenuator section in parallel with a reactive pi filter section.

With variations in the magnetic flux in the matrix 11, the resistance thereof as represented by the resistances 11a, 11b and 110 in Figure 4 will vary in such a manner as to maintain a substantially constant characteristic impedance of the section over a wide range of attenuation ratios and over a wide range of high frequencies. In practice, the capacitance of the member 20 as represented by the capacitance 20', Figure 41, is negligible and the equivalent reactive pi filter section of the attenuator 10 is substantially a low pass filter with a series resonant circuit 25', 24 and 26' across the inductive arm 23, Figure 4. This filter section will have a substantially constant characteristic impedance in the pass band and the series resonant circuit 25, 24- and 26 preferably has a resonant frequency at or about the high frequency end of the pass band so as to extend the high frequency range of operation with substantially constant characteristic impedance.

In Figure 3, reference numeral 30 generally designates another preferred form of attenuator according to this invention, reference numerals 11, 12, 18, 1?, 20, 21, 22, 23, 24, 25 and 26 in this figure designating parts identical to those described above in connection with Figures 1 and 2. In this form, however, the electro-magnet 14 is replaced by a coil 31 disposed around the member 20 which may be excited to produce axially extending lines of magnetic flux in the matrix 11. It has been found that the attenuator will operate in the same manner whether the lines of flux are transverse or axial. The arrangement of Figure 3, however, is not capable of inducing as high a flux, or capable of reducing the attenuation to as high an extent, as the arrangement of Figures 1 and 2. It is highly satisfactory and desirable, one to the simplicity with which it can be made and assembled, where a high flux or high reduction in attenuation is not necessary or desirable.

In either of the preferred embodiments of attenuators described above, the magnetic field may be changed between predetermined constant values so that the attenuator acts as a switch. In the alternative, the magnetic field may be continuously variable. If desired, the field may be controlled by a low frequency signal source in a modulator arrangement. Such a modulator arrangement is highly advantageous in certain applications such as, for example, Where the high frequency energy is produced by a magnetron in which case the output amplitude cannot be varied between wide limits by any known arrangements.

Since the attenuation of the high frequency energy by the matrix 11 will decrease with increased magnetic field in either direction, if an A.-C. current alone is applied to the coil in Figure l or the coil 31 in Figure 3, an amplitude modulation of the high frequency energy at twice the frequency of the A.-C. current will result. This elfect may be greatly reduced, by this invention, by a circuit such as shown in Figure 5 in which the control voltage is applied to terminals 33 and 34 with the electromagnet coil 15 and a battery 35, or other source of constant voltage, connected in series between the terminals 33 and 34. The constant voltage thus provides a bias such that variations in the voltage applied to the terminals 33 and 34 of either positive or negative potential will produce corresponding variations in the intensity 4 of the magnetic flux and the attenuation of the high frequency energy.

Another manner of using; the attenuators of this invention is illustrated in Figure 6 in which a pair of the attenuators 10 are disposed in co-axial lines 36 and 37 connected in parallel to an input co-axial line 38. One

terminal of the coil of each attenuator 10 is connected to one terminal of a battery 39. The other terminal of the battery 39 is connected to the movable contact 40 of a single pole-double throw switch 41 and the other terminals of the coils of the attenuators 10 are connected to stationary contacts 42 and 43 of the single poledouble throw switch 41. By actuation of the switch 41, accordingly, the coils of the attenuators 10 are selectively energized to selectively reduce attenuation in the lines 36 and 37. This arrangement thus provides a switching structure for co-axial lines with no moving parts in the high frequency circuit.

The matrix 11 may be of any desired insulating material with magnetizable particles dispersed therein and may preferably be a ceramic matrix of fused earth-like materials such as the silica, kaolin, feldspar and the like ingredients of porcelain containing finely divided particulate iron adapted to be magnetized while embedded in such matrix. The amount of iron may range from the minimum amount (i. e. about 0.5 weight percent of the ceramic matrix) capable of imparting an appreciable attenuating effect to the maximum practical amount (i. e. about 50 weight percent of the matrix) which might be used without destroying the insulating properties of the matrix, and preferably about 1-5% is used.

Table I below sets forth the preferred ranges of magnetic properties for compounds preferred for use in the invention and those properties of compound A (available commercially as Ferramic-B) which has been found to give optimum results in the practice of the invention.

Table I Property Range Compound Initial permeability at 1 melsecond 70 to 125 Maximum permeability 150-230 183 Saturation flux density (Gauss) 1, 500-2, 500 1, 000 Residual magnetism (Gauss 6504, 830 Coercive ioree (Oerstnd) 2-4 Volume resistivity (ohm-cm.) 1. 53X10 2X10 Curie point (G.+) 200-300 260 Loss factor at 1 Ind/second 0. 0001-0. 0002 0. 00016 Loss factor at 5 me./seeond 0. 0005-0. 0015 0.0011

The superior results obtained by the instant invention may be demonstrated by providing a co-axial line having an inner conductor of inch diameter and an outer conductor of approximately one half inch diameter, placing a matrix of compound A with a diameter of one-half inch and a length of one-half inch in the line, removing a pontion of the outer conductor around the matrix and replacing the same with an annular dielectric member, a shorting wire and a capacitance of 0.006 mfd. in the manner as illustrated above in Figures 1 and 2. With no magnetization of the matrix, the loss in the line due to power absorption by the matrix has been found to be approximately 20 db, while, with the matrix magnetized by a magnet as illustrated in Figures 1 and 2 and a magnetic field of approximately 10,000 Gausses, the attenuation loss is reduced down to approximately 1 db. This arrangement is effective over a range of high frequencies from 100 to 1,000 megacycles.

It will be understood that the compositions, dimensions and structural arrangements as described above, although peculiarly and highly advantageous, are given by way of illustrative example only and that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of the present invention.

I claim as my invention:

1. A variable attenuator for high-frequency electromagnetic energy, comprising: an inner conductor, a pair of aligned axially spaced outer conductors in radially spaced relation to said inner conductor and insulated therefrom, an annular member of dielectric material aligned between said outer conductors, a matrix of annular form within said annular member and around said inner conductor, a shorting wire at one side of said annular member and connected at opposite ends to said outer conductors, a capacitor at the opposite side of said annular member and connected between said outer conductors, said shorting wire, matrix and capacitor cooperating to define a low pass filter section with a substantially con stant characteristic impedance over a wide range of operating frequencies, said matrix being of insulating material and having magnetizable particles dispersed therein, and means external to said annular member for inducing a magnetic flux therethrongh and in said matrix for controlling attenuation of high-frequency energy.

2. A co-axial line attenuator comprising: an inner conductor, a pair of aligned axially spaced outer conductors in radially spaced relation to said inner conductor and insulated therefrom, a matrix of annular form around said inner conductors and having opposite end portions disposed within said outer conductors for supporting said outer conductors in axially spaced relation on said inner conductor, said matrix being of magnetic material arranged to attenuate high frequency energy to an extent dependent upon uni-directional flux there-through, and

an electro-magnet having opposite poles on opposite sides of said matrix and aligned between the facing ends of said axially spaced outer conductors for inducing a transverse flux in said matrix and controlling attenuation of high frequency energy, and a direct current conductive connection between said outer conductors at only one side of the field path between said poles.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES NBS Magnetic Attenuator, Nat. Bureau of Standards Technical News Bulletin, vol. 35, No. 8, pages 109411, August 1951.

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