US2951999A - Constant impedance attenuator - Google Patents

Description

T. J. MARCHESE CONSTANT IMPEDANCE ATTENUATOR Sept. 6, 1960 s Sheets-Sheet 1 Filed April 21, 1958 THEODORE 1/. MARC/ ESE I n venior Attorney Se t. 6, 1960 'r. J. MARCHESE CONSTANT IMPEDANCE ATTENUATOR 3 Sheets-Sheet 2 Filed A ril 21, 1958 I n ventor THEODORE d. MARC/I656 A tforney Se t. 6, 1960 -r. J. MARCHESE CONSTANT IMPEDANCE ATTENUATOR 5 Sheets-Sheet 3 Filed April 21, 1958 I Inventor Till-000 65 d. MARC/I635 By%'b%% Attorney Unitc i CONSTANT IMPEDANCE ATTENUATOR Filed Apr. 21, 1958, Ser. No. 729,685 9 Claims. 01. 33381) This invention relates to electromagnetic energy attenuators and more particularly to broadband electromagneticenergy attenuators having a constant input impedance. Attenuators of radio frequency energy are widely used in themicrowave art today and their application in systems and test equipment is such that as microwave techniques replace the more conventional techniques, micro- Wave power attenuators'will come into still wider use. Prior art attenuators, even though they are relatively narrow band devices, have been acceptable up to the present time because most systems wherein they are utilized have also been relatively narrow band.

, .Because the. majority of applications for prior art attenuators have been relatively narrow band, the fact that theirinput impedance does not remain constant has not proved a troublesome factor. However, because systems, arebeing designed for broadband operation, the fact that the input impedance does not remain constant beqomes an important consideration, particularly when it is recognized that the input impedance does not renrain constant even in the relatively narrow band prior art attenuators. Thus, with the advent of broadband input sources such as traveling wave tubes, which in turn lead to broadband system applications, the need broadband, constant impedance, attenuators to complement these sources is clearly seen.

It is therefore an object of this invention to provide an improved attenuator.

, Another object of this invention is to provide a broadband attenuator.

Another object of this invention is to provide a broadband variable attenuator which has a constant input impedance over the operating bandwidth and has application, for instance, as a broadband modulator.

A feature of this invention is the provision of a broadband attenuator which utilizes a helical transmission line to propagate electromagnetic energy therealong coaxial of. a given axis. The attenuator further utilizes an element disposed coaxially of a given axis which has at least first and second portions. The first port-ion of the element carries electromagnetic energy absorption means while the second portion of the element is in electromagnetically coupled relation with the electromagnetic energy at all times. This broadband attenuator also provides for the utilization of means to couple the first portion having electromagnetic energy absorption means disposed thereon to interact with electromagnetic energy to absorb it.

Anotherfeature of this invention is the utilization in an. attenuatorof a dielectric element which provides a constant input impedance over the operating bandwidth of the attenuator.

Another feature of this'invention is the utilization of electromagnetic absorption means disposed either externally or internally on a surface of the dielectric element to absorb said electromagnetic energy.

"parent Patented- Sept. 6, 19ccv Another feature of this invention is the utilization of rings or helices of ferromagnetic material to provide for broadband attenuation of electromagnetic energy.

Another feature of this invention is the utilization of coolant means in the region of the electromagnetic energy absorption means to permit operation under relatively high power conditions. t

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

Fig. l is a longitudinal partial sectional view of a variable attenuator having a constant input impedance;

Fig. 2 is a longitudinal partial sectional view of another embodiment according to the principles of this invention of a broadband variable attenuator having a constant input impedance;

Fig. 3 is a longitudinal partial sectional view of a variable attenuator which utilizes ferromagnetic materials biased at gyromagnetic resonances;

Fig. 4 is a longitudinal partial sectional view of a variable attenuator which operates in accordance with the principles of this invention and consists of a plurality of ferrite rings biased by a longitudinal magnetic field;

Fig. 5 is a longitudinal partial sectional view of an attenuator which utilizes ferrite helices biased by a variable magnetic field to provide variable attenuation over a wide bandwidth; and

Fig. 6 is a longitudinal partial sectional view of an attenuator utilizing ferrite helices internal of the radio frequency helix biased at gyromagnetic resonances to provide attenuation.

It should be noted, in the followingfigures, that like portions of each figure have been numbered alike unless otherwise designated in the description which follows.

In Fig. 1 variable attenuator 1 consists of a conductive metallic envelope 2 having input and output terminals 3 and 4, respectively. Connected between terminals 3' and 4 is a'helical transmission line 5 coaxial of a given axis which in the embodiment shown is the longitudinal axis of attenuator 1. Helix 5 may be made of copper or some other conductive metal and should be of sufiicient stiffness and gauge to maintain the helical shape and to support its own weight between terminals 3, 4. In lieu of this technique, helix 5 may be supported by dielectric rods. Dielectric element 6, which may be a low loss ceramic or sapphire, is disposed parallel to the longitudinal axis of attenuator 1 within and coaxial with helical transmission line 5. Dielectric element 6 is composed of two portions. A first portion 7 of element 6 carries or has disposed thereon electromagnetic energy absorption means 7a. In the embodiment of Fig. 1, the electromagnetic absorption means 7a may be finely divided graphite disposed uniformly on the outer surface of dielectric element 6 and it is tapered either geometrically as shown in Fig. 1 or by varying the density of the absorptive material to insure a of unwanted reflections due to sharp discontinuities in boundaries seen by the electromagnetic wave. Dielectric element 6 further contains a second portion 8 which is characterized by the fact that there is no energy absorption means disposed thereon aud is further characterized by the fact that it is in electromagnetic-ally coupled relation with the electromagnetic energy at all times as opposed to portion 7 which may be coupled to the electromagnetic energy only by actuating drive means 9. In accordance with the teaching of this invention, dielectric element 6 may be translated by drive means 9 longitudinally of the given axis to provide variable attenuation over the broad range of frequencies which is supported by a helicaltype transmission line. Drive means 9 may be any one of a variety of electrical or mechanical devices wellknown in the Therefore, in order to obtain attenuation over the band, dielectric element 6 must be translated longitudinally of the given axis along helical transmission line 5 by drive means 9 such that portion 7 with electromagnetic energy absorption means 7a disposed thereon is coupled to the electric field of the electromagnetic wave and interacts with said energy to absorb or attenuate said energy. When no attenuation is required, dielectric element 6 is withdrawn from helical transmission line 5 such that energy absorption means 7a is decoupled from the electromagnetic field and only portion 8 of dielectric element 6 remains in the region of the electromagnetic field. Since the environmentto which the electromagnetic wave couples at the input remains substantially unchanged with the exception of the addition of a proper amount of attenuating material when attenuation is desired, a substantially constant input impedance is seen by the electromagnetic wave at all times.

In order to propagate a wave along helical transmission line 5 from coaxial-type input element 3, a transition region is required to match the impedance of the coaxial line to the usually higher impedance of the helical transmission line 5, and this is provided by tapered portions 2a of envelope 2 which provide for a transition from coaxial line operation to wire-over-ground-plane mode of operation in the area 2b and from thence to a helical transmission line TEM mode operation. Dielectrio element 6 in Fig. l is positioned and supported on the given axis by rollers 10 which further permit element 6 to be easily translated longitudinally along helical transmission line 5.

In Fig. 2, dielectric element 6a is a hollow annular cylinder coaxial with helical transmission line 5, but is disposed externally of transmission line 5 as opposed to the internal disposition of element 6 in Fig. l and is translatable along the given axis by drive means 9. Dielectric element 6a contains a slot 11 parallel to the longitudinal axis of the envelope 2 which permits translation longitudinally along this axis of dielectric element 6a without interference with the leads of input and output terminals 3 and 4. Dielectric element 6a has two portions 7b and 8b one of which, 7b is covered with an electromagnetic energy absorption means 7c. Fig. 2 shows electromagnetic energy absorption material 70 disposed on portion 7b of the inner surface of annular dielectric element 6a and tapered to provide a good match. Portion 8b shown as 8 in Fig. l, is characterized by the fact that it contains no energy absorption means. As in Fig. 1, under zero attenuation conditions an input wave sees a certain impedance and with the exception of the addition of attenuating material 70 when attenuation is desired, by actuating drive means 9, the environment remains substantially unchanged for an electromagnetic wave and thus the input impedance remains substantially constant. In Fig. 2, bushings 12, made of a low coefiicient of friction and a low dielectric constant material such as Teflon, permit the translation of dielectric element 6a along helical transmission line 5 with a minimum of effort and electrical interference. Further, bushings 12 contain keyed portions 13 which engage slot 11 in dielectric annular cylinder 6a to maintain the position of dielectric cylinder 6a fixed with respect to envelope 2.

Fig. 3 shows another embodiment according to the principles of this invention. Dielectric element 6b is a hollow cylindrical rod, composed of a material such as ceramic or sapphire, disposed coaxially of helical transmission line 5 and held in position internally of helical transmission line 5 by mounting dielectric element 6b in holes 14 and 15 in end plates '16 and 17 of attenuator 1. Dielectric element 6b has first and secend portions 7d and 80, respectively. Portion 7d has disposed thereon or carries rings of ferromagnetic material 7e which, when properly biased by an external axial magnetic field, act as attenuators for electromagnetic energy over a broad range of frequencies. Attenuating means 7e on portion 7d may consist of a single ring of ferromagnetic material or may consist of a plucrality of such rings. The ferromagnetic rings 7e of attenuating portion 7d may be disposed serially along dielectric element 6b such that they are immersed in a magnetic field which is supplied by magnetic coupling solenoid 9a parallel to the longitudinal axis of attenuator 1. Portions of dielectric rod 6b are cut away such that the outer diameter of the ferrite rings 7e and the outer diameter of dielectric rod 6b are the same. Dielectric element 6b has portions which are in electromagnetically coupled relation with the electromagnetic energy at all times and provide a constant environment for the electromagnetic energy when theapplied magnetic field H has a value such that the ferrite rings 7e are not eifeotively coupled to the electromagnetic field for absorption thereof. In this manner, a constant input impedance is seen by the electromagnetic field and the variations in the dielectric constant of the ferrites are.

such that reflections due to discontinuities are minimized.

In operation, a magnetic field is set up in the ferrite rings 7e which is parallel to the longitudinal axis of the attenuator by applied field H. This component of field set up in the rings is known as the Z component. The Z component of magnetic field interacts with a circularly polarized magnetic component of the helical mode electromagnetic energy. By applying the magnetic field H in the proper direction, attenuation of the electromagnetic energy will occur for one direction of propagation along the helical transmission line. In the embodiment shown, attenuation would be required for a wave which proceeded from input terminal 3 to output terminal 4. This attenuation takes place due to the phenomenon of gyromagnetic resonance which occurs for a positively circularly polarized wave and is now well-known to one skilled in the art. By applying a constant field to the ferrite rings, attenuation will occur, but only over a very narrow bandwidth. By changing the composition of materials of the ferrite rings 7e, however, it is possible to change the saturation magnetization of the ferrites and thereby the frequency at which resonance occurs for a' constant applied field H. This action is discussed in an article by S. Sensiper entitled Resonance Loss Properties of Ferrites in the 9KMC Region, pub-. lished on page 1327 of the October 1956 issue of the Proceeding of the IRE. Examples illustrative of this are MnFe O having a saturation magnetization of 5200. g-auss and CoFe O having a saturation magnetization of 5000 gauss. Further examples may be found in an article by Bloember-gen in the October 1956 issue of the Proceedings of the IRE entitled Magnetic Resonance in Ferrites, page 1266. Thus, by changing the saturation magnetization of each of the ferrite rings 72, it is possible to obtain attenuation over a large frequency range with a constant applied field H. Further, by varying the magnetic field H within certain limits by varying the current in solenoid 9a, it is possible to vary the attenuation over a large bandwidth in discrete steps. A reduction in magnetic field, for instance, for a ferrite having a certain saturation magnetization and biased at gyromagnet-ic resonance causes a change from the resonant point on the permeability curve for a positively circularly polarized wave and results in a reduction in the value of attenuation.

At resonance, ferrites are capable of absorbing large amounts of heat provided the heat generated is capable of being dissipated rapidly. In the embodiment of Fig. 3, dielectric element 6b is a cylindrical hollow ceramic rod and'a coolant may be introduced into hollow rod 6b at point 18 to carry heat which has been dissipated on portiori 7d to outlet 19.v In this manner high power attenu-.

ation may be obtained over a broad band of frequencies with an attenuator which has a constant input impedance.-

or variable, is obtained over a wide band of frequencies by coupling portion 7f with attenuation means 7g disposed thereon to the electromagnetic field by means of coupling solenoid 9a. 7

Fig. 5 is a further embodiment set down according to the principles of this invention. The embodiment of attenuator 1 as shown in Fig. 5 is constructed exactly as described for the embodiment entitled Fig. 3 with those exceptions. Instead of rings of ferrite material, a helix or a plurality of helices 20 offerrite material are inlaid onthe outer surface of portion 7d of dielectric element 6d, which in this embodiment is shown as a solid'rod. This configuration, in conjunction with a longitudinal field which is supplied by solenoid 9a, provides attenuation of electromagnetic energy for waves which proceed from input terminal 3 to output terminal 4. When amagnetic field H is applied to a ferrite which has a helical configuration, a so-called 0 component of magnetic field is set up which interacts with a so-called circularly polarized 0 component in the magnetic field of the helical transmission line mode. Again, as in Fig. 3, the ferrite helices 20 are biased at gyromagnetic resonance by a constant magnetic field H or the magnetic field H may be varied to vary the value of attenuation and a broadband fixed or variable attenuator is thus obtained. The broadband feature is, again, obtained by using a plurality of ferrite helices, each having a different saturation magnetization and, therefore, a resonant frequency different from every other helix when immersed in a constant value of applied field H.

The embodiment of Fig. 6 is constructed exactly as the embodiment shown in Fig. 4 with the exception that the ferrite rings 7 are now shown as a ferrite helix or helices 20 inlaid on the inner surface of dielectric element 60. Interaction of the so-called 6 component of magnetic field with the ferrite material by applying a magnetic field H by means of coupling solenoid 9a again causes attenuation of a wave proceeding from input terminal 3 to output terminal 4.

In connection with the embodiments of Figs. 4, 5, 6, and 7, it should be noted that unless the magnetic field H is applied, no attenuation takes place other than that due to the dielectric losses in the ferrites themselves. Thus, the ferromagnetic materials do not effectively attenuate an electromagnetic wave unless some coupling means to provide for interaction between the electromagnetic wave and the ferromagnetic material is used. In all the embodiments using ferromagnetic materials, this coupling means is solenoid 9a which has its mechanical analog drive means 9 utilized in connection with Figs. 1, 2.

The application of a coolant internally of hollow dielectric element 6b has been described in connection with Fig. 3 for the dissipation of heat from ferrite materials when the ferrites are biased to absorb electromagnetic energy.. The singling out of this figure to indicate this scheme is not to be construed as limiting the cooling technique to these embodiments. The dielectric element 6d of Fig. 5, for instance, could be a hollow cylinder for conducting a coolant. The other embodiments, while they are not conveniently cooled by passing a liquid coolant adjacent the attenuating portion 7, may be cooled by forced air cooling through perforations, not shown in the drawings, in the envelope 2 of attenuator 1 or by blowing air internally of dielectric element 6a of Fig. 2 for instance. These perforations referred to above are made in a region preferably remote from matching portion 2a.

6 The cooling feature as applied to all the embodiments, of course, increases the power handling capacity of the attenuators and consequently their utility as system components.

While the above-described embodiments have been shown as either fixed or variable attenuators, this is not to be construed as a limitation upon the application of this type of device. In connection with the embodiments wherein ferromagneticelectromagnetic Wave absorption means are used, it is apparent that these devices may also be used as non-reciprocal isolators, switches, equalizers, or modulators. In connection with the embodiments wherein ferromagnetic electromagnetic wave absorption means are used, it is apparent that these devices may also be used as non-reciprocal isolators, switches, equalizers,

or modulators. In connection wtih the above figures as Well as the attenuators of Figs. 1 and 2 which use the more conventional means for the absorption of electro: magnetic energy, that is, finely divided graphite particles deposited on the surface of the dielectric element, it may be seen that only a single terminal would be required to act as an input to a termination which would totally attenuate electromagnetic energy. Such a termination could be used in conjunction with any type of known transmis sion line provided the proper transitions are used. In

this regard, it may also be seen that the structures described herein are not limited to use only with I EM mode transmission lines, but in conjunction with the proper type transitions which are well-known to those skilled in the art, this device may be used in systems which use circular waveguide, rectangular waveguide, strip transmission lines, etc.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. A broadband attenuator for electromagnetic energy comprising coaxial conductors having coaxial leads connected thereto, the inner conductor being a helical transmission line to propagate electromagnetic energy therealong coaxial of a given axis, a dielectric element disposed coaxially of said helical line having at least first and second portions, electromagnetic energy absorption means carried on said first portion, said second portion of said element being in electromagnetically coupled relation with said electromagnetic energy at all times to thereby present a constant impedance to said electromagnetic energy, and means to produce relative movement between said first portion and said helix to control the interaction with said electromagnetic energy for absorption thereof, said means being withdrawn from the helical line to provide zero attenuation.

2. A device according to claim 1, wherein said element is a dielectric rod disposed internally of said helical trans mission line and has means for the absorption of electromagnetic energy disposed on the outer surface thereof, said means being terminated to provide impedance matching to the helical transmission line.

3. A device according to claim 2, wherein said di electric rod is hollow for receiving a coolant for said attenuator.

4. A device according to claim 1, wherein said element is a hollow dielectric rod disposed externally of said helical transmission line and has means for the absorption of electromagnetic energy disposed on the outer surface thereof and means for cooling said hollow rod and helix.

5. A device according to claim 4, wherein said electromagnet energy absorption means consists of finely divided graphite particles, and is configurated to match the line impedance.

6. A broadband variable attenuator for electromagnetic energy comprising a pair of coaxial conductors including a helical transmission line to propagate electromagnetic energy therealong coaxially of a given axis, an elongated dielectric member disposed coaxially of said given axis having at least first and second portions, electromagnetic energy absorption means carried on said first portion, said second portion of said element being in electromagnetically coupled relation with said electromagnetic energy at all times to thereby present a constant impedance to said electromagnetic energy, and means to produce relative movement between said first portion and said helix to control the interaction with said electromagnetic energy for absorption thereof.

7. A device according to claim 6 wherein said means to vary the coupling includes means for translating said elongated dielectric member'along the given axis.

, 8. A broadband variable attenuator for electromagnetic energy comprising a pair of coaxial conductors, the interior conductor being'a helical transmission line of constant diameter to propagate electromagnetic energy therealong coaxially of a given axis, said helical transmission line having input and output terminals coacting with transition means to match said input and output terminal to said helical transmission lines, an elongated dielectric rod having at least first and second portions disposed coaxially of said given axis, finely divided graphite particles carried on said first portion to absorb electromagnetic energy, said second portion being in electromagnetically coupled relation with said electromagnetic energy at all times to thereby present a constant impedance to said electromagnetic energy, mechanical means for translating said dielectric element along said given axis to vary the coupling between said first portion andcoupled to said helical line, said rod having an attenua-' tion section matched in impedance to said helical line and an integral dielectric section coupled electromagnetically to said helical line, means for withdrawing said rod fromv said helix to vary the attenuation and provide a substantially constant input impedance.

References Cited in the file of this patent UNITED STATESPATENTS 2,510,614 Weber 'June 6, 1950 2,524,857 -Secker Oct. 10, 1950 2,669,674 Diemer Feb. 16, 1954 2,720,609 Bruck Oct. 11, 1955 2,742,588 Hollenberg Apr. 17, 1956 2,771,565 Bryant Nov. 20, 1956 2,806,972 Sensiper Sept. 17, 1957 2,811,673 Kompfner Oct. 29, 1957 FOREIGN PATENTS 165,097 Australia Sept. 8, 1955

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