US2443109A - Super high frequency attenuator - Google Patents

Description

June 8, 1948. E. G. LlNDER 2,443,109

v SUPER HIGH FREQUENCY ATTENUATOR Filed May l, 1945 Gtorneg Patented June S, 1048 SUPERl HIGH FRE QUENGY ATTENUATOR lErnest G. Linder, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May l, 1943, Serial No. 485,357v

(Cl. 178r- 44.)

2 Claims. 1

invention relates generally to super-high frequency attenuators, and more particularly to attenuators of a type employing semi-conductive plastic materials as dielectrics in the super-high freflllIlC/y eld. The term super-high frequency isused to Icover approximately 3,000,000 kilocycles to 30,009,000 kiloeyeies. The invention provides `an ,efficient and economical means for obtaining predetermined attenuation `of Vsuperhigh frequency energy in wave guides or concentric .transmission lines, as well as an effective means for preventing leakage of super-high frequency energy through openings in shielding enclosures therefor.

A convenient form of attenuator for a wave guide vvcomprises a short section of concentric line inserted within the guide. The line should preferably include ycoupling loops at each end thereof tov provide suitable coupling and impedance matching to the wave guide.

The concentric line section preferably includes a central conductor separated from the vouter concentric conductor by .a dielectric comprising energy absorptive material such, lfor example, as conductive rubber oi relatively low D.C. resistivity.A High direct current insulation may be provided conveniently by separating the high loss dielectric from the outer conductor by a relatively thm layer of varnished cambric or other low conductivitymateral.

The attenuation characteristics of `a lter of the type .described heretofore may be readily calculated for energy absorptive materials of known D.C resistivity and dielectric constant. The specific super-high frequency resistivity of the particular energy absorptive material employed maybe computed from the propagation function.

v=\/ R+jwL G+jwc 1) where R is the series resistance, L is the `series inductance, G is the shunt conductance, and C is the shunt capacitance, all per unit length. In the present case R is always negligibly small, and.

in the case of the lower resistivity dielectric materials, I@also is small compared to G. Hence,

where a is the attenuation and is the phase constant. Since this becomes where the units are in the MKS system, a and b are the inner and outer radii, respectively, and fr is the permeability. Also Inserting these expressions in (2) yields a- Zp n (3) This-is identical with the expression for the attenuatiQn. 0f a plane Wave in a medium o sufiiciently high conductivity so that Pea) where e is the permittivity (see Slater, Microwave Transmission, page 11,1). In the present case this condition is adequately met. Hence, Formula 3 may be applied here to both the concentric line andwave guide measurements.

The same calculations apply to an attenuator comprising a simple plug orl diaphragm of energy absorptive material inserted Within a wave guide using the H01 mode of transmission.

For an energy absorption material comprising a conductive synthetic rubber such as neoprene, having sufficient carbon content, such as acetylene blaclctol provide a D.C, resistivity of the order of 5 ohm-centimeters, a convenient concentric line type filter may be constructed to provide an attenuation ot the order of 'decibels per centimeter, and to provide a D.C. insulation effectiye for at least 172,000 volts. By way of example, the diameter of the outer conductor masl be approximately .6 centimeter while that of the inner conductor may be ofthe order of .l centi'- ,meten A typical formula for conductive rubber having a resistivity of the order of ohm-centimeters is:

Cured 45/290 F.

If an energy absorptive material of the same general type, but having a D.C. resistivity of the order of 200 ohm-centimeters, is employed, an attenuation of the order of 3 decibels per centimeter may be readily provided with a line having an outer conductor diameter of .9 centimeter and an inner conductor diameter of .15 centimeter. The foregoing calculations vare based upon the assumption that the input and output elements of the attenuator are properly matched to the Wave guide. In actual practice, such filter sections Would probably be mismatched intentionally, thereby providing attenuations higher than the calculated values.

Such filters may be employed for providing effective attenuation of super-high frequency energy in power leads to super-high frequency apparatus, by utilizing the power lead as the central conductor of a short section of concert; tric line which includes energy absorptive material of the type described heretofore. Similarly, the energy absorptive material, such as the conductive synthetic rubber described heretofore, may be employed in the form of gaskets to prevent leakage of super-high frequency energy through openings in shielding enclosures.

Among the objects of the invention is to provide an improved method of and means for attenuating super-high frequency energy. Another object is to provide an improved method of and means for preventing leakage of superhigh frequency energy through openings in shielding enclosures therefor. A further object is to provide an improved attenuator for superhigh frequency energy in power leads to superhigh frequency apparatus. Another object is to provide an improved super-high frequency attenuator comprising a concentric line section which includes a. dielectric of relatively low D.C.

resistivity and relatively high energy absorptive characteristics at super-high frequencies. An-l other object of the invention is to provide an im proved attenuator fon super-high vfrequencies which provides relatively uniform attenuation over a wide band of frequencies. A still `further object of the invention is to provide an improved means for sealing apertures or joints in superhigh frequency shielding enclosures.

The invention will be described in further detail by reference to the accompanying drawing, of which Figure 1 is a cross-sectional view of one embodiment of the invention, Figure 2 is across-sectional view of a second embodiment thereof, Figure 3 is the section III-III of Fig. 2r Figure 4 is a family of graphs illustrativeV of the invention, and Figures 5 and 6 are VI'nodii'ications. of a third embodiment of the invention. Similar' reference numerals. are applied to similar elements throughout the drawing.

Fig. 1 includes a wave guide I having metallic aperture devices 3, 5 transversely disposed there for example, as varnished cambric.

giriamo 4 in and separated axially a distance predetermined by the calculated length of the attenuator. A cylindrical outer conductor 1 connects the aper tures of the conductive aperture devices 3. 5 and is supported thereby substantially coaxially with the axis of the wave guide I. An inner conductor 9 is disposed substantially coaxially with the outer conductor I and is separated therefrom by an energy absorptive dielectric II and a relatively thin layer of D.C. insulating material I3 such, The ends of the inner conductor 9 are terminated in coupling loops I5, I1 which are connected, respectively, to the ends of the outer cylindrical conductor l. If desired, the characteristics of the coupling loops I5, I 'i may be selected to provide desired impedance matching with the wave guide I at the operating frequency. The lengths and diameters of the inner and outer conductors 9, l, respectively, and the characteristics of the dielectric materials I I, I3 may be readily calculated from the formulae discussed heretofore, to prof vide desired attenuation at the selected operat-v ing frequency. Figs. 2 and 3 illustrate attenuators of the general type described in connection with Fig. f1, which may be employed to attenuate effectively super-high frequency currents on, for example, a power conductor 9 Which'enters a shielding enclosure I9. The concentric line section consists of the power conductor 9 and an outer cylindrical conductor l, separated by an energy absorptive dielectric II and a D.'C. insulating dielectric I3. The outer cylindrical conductor 'I Vmay be fastened to the shielding enclosure I9 by means 'of brackets 2i which are preferably soldered to the cylindrical conductor 'i and the shielding `enclosure I 9.

Optimum values of resistivity will obtain 'for filters of predetermined proportions and char.-

acteristics. follows:

If the resistivity of the semi-conductive rubber is sufficiently low that considerable current liowsl longitudinally on its surface, it may then be regarded as the inner conductor of the transmission line. This condition obtains when the ef#l f-ective skin thickness becomes less than the radius of the rubber. v regarded as a low impedance line having a good dielectric (egg, the varnished cambric insula- 'Ihis feature may be explained as tion). The attenuation a2 may be determined by the conventional formula: p

I-x/#wk Y l where 1c is the dielectric constant of the varnished cambric, a=41r109 henries per centimeter, w=21rf, p is the resistivity, and :r is the thickness I of the varnished cambric or other insulation.

If the resistivity of the rubber is high, so thatv The lter may then bel Fig. 4 shows theoretical curves of the relations between attenuation and resistivity in filters of the type described which employ three different thicknesses of varnished cambric insulation having a dielectric constant equal to 4.

Fig. 5 illustrates a convenient method of employing energy absorptive material to prevent leakage of super-high frequency energy around the edges of a door in a shielding enclosure for super-high frequency apparatus. An enclosure I9 is provided with a door or cover 29 having a tapered edge 23 which ts a complementarily tapered ja-mb 25 at the aperture of the shielding enclosure I9. Energy absorptive material 21 is inserted in the aperture between the tapered portions 23, 25 of the door and jamb, respectively. Due to the tapered form of the door and jarnb, the effective length of the attenuator may be relatively large in comparison to the cross-sectional area of the aperture in the shielding enclosure.

A bolt 29, fastened to the shielding enclosure I9, extends through a hole in the door 20. The door 20 may be secured in a closed position by means of a nut 3| threaded to the end of the bolt 29 extending through the door. As many bolts and nuts as desired may be disposed at regular intervals around the edge of the door. Due to the relatively long energy absorbing path in the dielectric 21, between the tapered portions 23,

25 of the door an-d jamb, there will be relatively low energy leakage along the bolt 29 between the door 20 and the jamb 25. Preferably all joints between the tapered portion 25 and the body of the shielding enclosure I9 should be soldered to eliminate energiT leakage around the gasket.

A modification of the super-high frequency gasket is illustrated in Fig. 6. In this arrangement a shielding enclosure I9 includes a door frame 26 extending wel] into the door opening in the enclosure I9. The door 29 abuts against the door frame 26 and is separated therefrom by a gasket of super-high frequency energy absorptive material 21. A bolt 29 is secured to the door frame 26 and extends through a suitable aperture in th'e door. The door may be retained in a closed position by means of a nut 3| threaded to the portion of the bolt 29 extending through the door. This modification has the distinct advantage that the door frame 26 must necessarily extend well into the door aperture in order to provide a gasket as effective as that described for the tapered door frame of the modification shown in Fig. 5. However, where space is not an important consideration, either modification may be employed to advantage.

It should be understood that the highly conductive rubber employed as a dielectric or energy absorber in the various super-high frequency attenuators described heretofore may be similarly employed to prevent leakage of super-high frequency energy around rotating or movable shafts extending into various other types of enclosures. Various modifications and embodiments thereof are disclosed and claimed in a copending U. S. application of George L. Fernsler, Serial Number 485,012, filed April 29, 1943, assigned to the same assignee.

I claim as my invention:

1. An attenuator for supeiwhigh frequency energy comprising a transmission line including two conductors, a leakage path for said energy including e. first dielectric having a predetermined proportion of energy absorptive material, and a second dielectric interposed between said first di electric and one of the conductors of said line, characterized by the relation Ee p k where p is the resistivity of the first dielectric, x is the thickness of the second dielectric, and lc is the dielectric constant of the second dielectric.

2. An attenuator for super-high frequency energy including a predetermined length of coaxial transmission line having a rst solid dielectric including a predetermined proportion of energy absorptive material, a second dielectric interposed between said first dielectric and one of the conductors of said line, the resistivity of said first dielectric being a value just sufficiently high effectively to prevent substantial current skin effect thereon adjacent to the surface of said second dielectric in contact therewith, and a pair of coupling loops each terminating a different one of th'e ends of said line.

ERNEST G. LINDER.

REFERENCES CITED The following references are of record in the f le of this patent:

UNLTED STATES PATENTS Number Name Date 2,238,915 Peters et al Apr. 22, 1941 2,283,895 Mouromt'seif et al. May 19, 1942 2,304,210 Scott et al Dec, 8, 1942 2,322,773 Peters June 27, 1943 2,409,640 Moles Oct. 22, 1946 FOREIGN PATENTS Number Country Date 456,722 Great Britain Nov. 11, 1936 526,895 Great Britain Sept. 27, 1940

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