Dec. '3, 1946. H. R. MEAHL ‘ I ' ‘ ATTENUATOR FOR ULTRA HIGH FREQUENCY SYSTEMS ' 2,412,055 ‘Filed Jan. 25, 1945 ‘N Inventor‘: vHa‘rrg; R. Meahl, Patented Dec. 3, 1946 2,412,055 UNITED STATES PATENT‘ OFFICE 2,412,055 ATTENUATOR FOR ULTRA HIGH FRE QUENCY SYSTEMS Harry R. Meahl, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application January 25, 1943, Serial No. 473,501 6 Claims. (01. 178—44) 1 2 My invention relates to ultra high frequency to a coaxial transmission line and is associated systems and more particularly to an attenuator with oppositely disposed walls or sides of the guide for utilization therein. It has been appreciated for some time that ultra high frequency electro in order to excite the guide in response to the en magnetic waves, now referred to as microwaves, discharge device comprising a part of the input or excitation means is displaced an appreciable dis tance from a closed end of the guide so that the ergization of the concentric line. may be transmitted dielectrically through guides of the hollow-pipe type, that is through guides de ?ned by conductive members. The types of elec The electric attenuation characteristic of the guide is not af fected appreciably by the re?ections from the closed end, and an axially adjustable output elec tromagnetic waves which may be transmitted through guides of this nature have been generally trode means which may be attached to an adjust classi?ed as transverse electric, TE, and trans verse magnetic, TM, depending upon the nature of the electric and magnetic ?elds constituting the electromagnetic waves. In accordance with the teachings of my inven tion described hereinafter, I provide a new and improved attenuator for systems of this nature able plunger is positioned at the other end of the guide and serves to derive from the guide ac curately predetermined voltages dependent upon the distribution of the guide energization, and more particularly upon the attenuation charac teristic of the guide. F01‘ a better understanding of my invention, reference may be had to the following description taken in connection with the accompanying draw wherein accurately controllable and determinable voltages may be derived upon energization or ex citation of the guide. It is an object of my invention to provide a new ing, and its scope will be pointed out in the ap pended claims. Figs. 1 and 2, which are respec and improved system for the utilization of ultra high frequency electromagnetic waves. It is another object of my invention to provide‘ tively a plan view and .a cross-sectional vertical view, represent one embodiment of my invention a new and improved attenuator for ultra high 25 as applied to an attenuator excited from a co axial transmission line, and wherein an electric discharge device is employed for converting the It is a further object of my invention to pro energization of the transmission line into the de vide a new and improved attenuator for an ultra high frequency system which comprises a dielec sired energization of the attenuator. tric guide of the hollow-pipe type which is excited 30 Referring now to Figs. 1 and 2 jointly, my in at a frequency below the critical or cut-off fre vention is there illustrated as applied to an ultra. quency, and in which means are provided for de high frequency system for controlling the trans-l riving therefrom voltages of predeterminable mission of energy through a dielectric wave guide magnitude. of the hollow~pipe type which comprises a con It is a still further object of my invention to 35 ductive or metallic guide I which may have a rectangular cross section. For example, the top provide a new and improved attenuator for a high frequency system which comprises a dielectric and the bottom of the guide may be de?ned by wave guide of the hollow-pipe type which is en conductive or metallic plates 2 and 3 and the side ergized or excited at a frequency below the cut walls by plates 4 and 5. One end of the guide is off frequency and in which the exciting means for closed by means of a metallic end plate 6. the guide is positioned with respect to a closed In order to excite the guide, I provide an elec tric discharge device 1 comprising a plurality of end thereof in order to minimize re?ections there from, so that accurately determinable output volt enclosed electrodes such as an anode 8 and a cathode 9 arranged in relatively close proximity in ages may be derived from the guide. frequency systems. order to utilize effectively ultra high frequency Brie?y stated, in accordance with the illus trated embodiment of my invention, I provide in currents. A cathode heating element (not shown) lies within the cylindrical cathode 9 and heating guide of the hollow-pipe type which is excited at a current is supplied thereto by means of lead-in conductors and terminal‘ posts Hi. Terminals H frequency below the critical or cut-off frequency in order to produce a predetermined attenuation 50 and I2 may be employed for connecting the cath of the excitation within the guide. The exciting ode to an external source of uni-directional cur an ultra high frequency system a dielectric Wave rent for-energizing the discharge. device.‘ The electrodes of the discharge device are enclosed, means for the guide may comprise an electric dis charge device, such as a diode. peculiarly adapted for the utilization of ultra high frequency energy, and the discharge device, in turn, is connected 55 within a vacuum-tight envelope, and the anode _B_ . is provided with an extension 13 to‘ serve as an. 2,412,055 3 4 adaptor for connection to the central conductor ductivity of the conductive material de?ning the of a concentric transmission line to be described guide. Stated in other words, the waves are prop agated with a predetermined attenuation. In accordance with the present invention, the presently. In order to facilitate the insertion of the dis charge device ‘I into operative relationship with respect tolthe guide and to maintain the dis charge device ?rmly in position after insertion, critical frequency de?ned by Equation 3 above,_ I employ a pair of metallic adaptors III and I5 designed to afford appreciable resilience for en attenuated along the longitudinal axis of a guide; that is, the propagation constant 7 contains no gaging parts of the discharge device. Discharge device ‘I, and particularly the elec tric discharge path between anode 8 and cathode 9, is controlled in response to the potential pro vided by a concentric transmission line I6 com prising an internal conductor I ‘I, which is con nected to the anode adaptor I3, and a tubular wave guide is excited at a frequency below the so that the electromagnetic waves are rapidly 10 imaginary component. Furthermore, in accordance with my invention, the excitation means for the guide including the electric discharge device 'I and the concentric transmission line I6 are positioned with respect to the end wall 6 so that there is little or no re?ection therefrom, in this way increasing the outer conductor I8 which is inserted into adaptor accuracy of the attenuated energization proceed I4 and which is electrically connected to one side ing from the electric discharge device toward the .of the wave guide such as plate 2. right of the guide. I have found that prevention In accordance with my invention, the wave 20 of undesirable reflections from the closed end guide is designed to attenuate the excitation in wall Ii of the wave guide may be obtained by posi troduced into the guide by the concentric trans tioning the electric discharge device 'I from the mission line I6 and electric discharge device ‘I. end Wall 6 at a distance which causes an attenua More particularly, the height a and the width b of the guide are chosen with respect to the fre quency of excitation so that the electromagnetic energization is not propagated in wave form along the longitudinal axis of the guide but is rapidly tion therebetween of at least 10 decibels so that the reflected excitation from the closed end 6 will be attenuated at least 20 decibels when it arrives at the electric discharge device ‘I. Considering now the attenuation of the elec tromagnetic excitation in a dielectric wave guide 'Each type of electromagnetic wave, that is each 30 operated below the cut-off frequency, it may be type of transverse electric or transverse magnetic stated generally that the energy of the excitation attenuated. wave in a given dielectric wave guide can be is attenuated in accordance with the exponential propagated at a frequency only above the critical 64%, where or represents the attenuation constant or cut-off frequency. Considering a rectangular of the guide and 2 represents the axial or longie type wave guide, the propagation constant 7 may 35 tudinal distance along the guide. This relation be expressed as follows: ship will be appreciated when it is considered that the energy of the excitation which may pass along the guide is a function of the cross product of the electric ?eld intensity and the magnetic where w is the angular velocity of the wave under 40 field intensity, and inasmuch as both. the electric consideration, 5 is the dielectric constant of the ?eld intensity and magnetic ?eld intensity are medium through which the wave is propagated, each attenuated in accordance with 6-12. the a is the permeability, m is the mode of propaga energy follows the squared law. Furthermore, tion of the wave, and n is the order of the wave, from the above it will be appreciated that the a is the height of the guide, and b is the width magnitude of the electric field of the energiza of the guide. . tion within the guide is a function of r“? For frequencies below the critical frequency, the ?rst term of Equation 1 is smaller than the sum of the other two terms, and Y is real. Con sequently, under this condition of operation the electromagnetic wave is attenuated. For frequen cies greater than the critical frequency, the ?rst term is greater than the sum of the other two terms; consequently, ‘Y is imaginary, thereby re sulting in a real propagated wave. The critical or cut-off frequency f0 for a rectan gular wave guide may be expressed as: I provide within the guide output electrode means which is adjustable along the longitudinal axis of the guide in order to derive therefrom voltage of predeterminable magnitude in accord ance with the attenuation characteristic of the guide. For example, the output electrode means may take the form of a loop of Wire I9 constitut ing an extension of an inner conductor 2| of a concentric transmission line 20 which com prises the inner conductor 2! and an outer tubu lar conductor 22. As a means for positioning the loop I9. the concentric transmission line 2!! and consequently the loop I9 may be supported by an axially 0r longitudinally positionable In order to simplify still further the presenta plunger 23 adapted to engage closely the top, tion of the subject matter, it will be assumed bottom and'side walls of the guide. The plunger that the dielectric is air and that the system is 23 may be actuated by means of a suitable me arranged for the transmission of an H01 type chanical expedient such as a rod 24. wave where the electric component of the ?eld 65 If it be considered that the guide is energi'led is perpendicular to the base I), in which case the at a substantially constant excitation by means critical frequency may be expressed as: of the concentric transmission line I6, there will be established within the guide an excitation of constant value, the electric discharge device (3) where c is the velocity of light. Of course, in any practical: wave guide operated at a frequency above the cut-off frequency, electromagnetic waves are propagated with attenuation, the mag nitude of thelatter being determined by the con 70 serving to periodically control the potential dif ference between plates 2 and 3 of the guide tov establish this excitation. The attenuation of the excitation within the guide is determined in ac cordance with the above described principles so that by the positioning of the loop I9 with re 2,412,055 5 6 While I have shown and described my inven tion as applied to a dielectric wave guide of par ' excitation of said member at a frequency below conductive wave guide having a closed end, ex spect thereto, I provide a system for obtaining citation means displaced from said closed end accurately and precisely determinable ultra-high for exciting said guide at a ‘frequency below the frequency voltages. The magnitude of the volt critical cut-off frequency, and means displaced age supplied to the concentric transmission line from said excitation means for deriving pre 20 is, of course, determined by the position of determinable amounts of energy from said guide the loop I 9. As the loop I9 is moved toward determined by the position thereof with respect the electric discharge device 1, the magnitude to said excitation means. of the voltage obtained is increased; and the 4. In an attenuator, the combination compris relationship of this voltage with respect to the longitudinal dimension of the guide follows the 10 ing a hollow conductive member having a closed end, excitation means connected to said guide exponential relationship r”, where a is measured and displaced from said closed end for e?ecting from discharge device 1. , ’ the cut-off frequency thereby imparting to the ticular con?guration and as employing various 16 electro-magnetic energization of said member a elements diagrammatically illustrated, it will be obvious to those skilled in the art that changes and modi?cations may be made without depart-. ing from my invention and I, therefore, aim in the appended claims to cover all such changes 20 predeterminable attenuation characteristic, and adjustable means comprising a plunger having output electrode means for deriving predeter minable voltages from said member. 5. In an attenuator, the combination compris ing a hollow conductive member having a closed end, means for exciting said member at a fre quency below the critical cut-off frequency com prising an electric discharge device including a and modi?cations as fall within the true spirit and scope of my invention. What I claim as new and desire to secure by Letters Patent of the United States, is: 1. A dielectric wave guide system of the hol 25 plurality of enclosed electrodes, said discharge device being positioned from said closed end at a distance to minimize the re?ection therefrom, and axially adjustable output electrode means at the other end of said member for deriving from prevent the re?ection of appreciable energy 30 said member a voltage of predetermined magni tude determined by the excitation and the at therefrom, and output electrode means opposite tenuation characteristic of said member. the excitation means for deriving energy from 6. In an attenuator, the combination compris said guide. ing a hollow conductive member having a closed 2. In an attenuator, the combination compris ing a hollow conductive member having a closed 35 end, means for exciting said attenuator at a fre quency below the critical cut-off frequency and end, means for exciting said member at a fre- . positioned from said closed end at a distance to quency below the critical cut-off frequency and prevent appreciable re?ection therefrom, and positioned a distance from said closed end to axially positionable means comprising an adjust- ' minimize the reflection therefrom, and axially. positionable means at the other end of said 40 able plunger supporting output electrode means at the other end of said member. member and including output electrode means. low-pipe type comprising a conductive member having a closed end, means for exciting said member below the critical cut-oil frequency and positioned a distance from said closed end to 3. In an attenuator for a dielectric wave guide system, the combination comprising a hollow HARRY R. MEAHL.