US2589259A - Radio-frequency circuit - Google Patents

Description

March 18, 1952 F c, [SELY 2,589,259

' RADIO-FREQUENCY CIRCUIT Filed Aug. 15, 1948 INVENTOR. FRANK C ISELY ATTORNEY Patented Mar. 18, 1952 UNITED STATES PATENT OFFICE RADIO-FREQUENCY CIRCUIT Frank C. Isely, Washington, D. 0.

Application- August 13, 1948, Serial No. 44,210

(Granted under" the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 5 Claims.

The present invention relates to an improvemen't'of the invention disclosed in my co-pending application Serial No. 353959, filed June 29, 1948, now U'. S. Patent'2,520,147, and is in particular concerned with the problem of eliminating unwanted harmonics in line controlled radio frequen'cycircuits.

One serious disadvantage often experienced in the'use'of 'line'controlled radio frequency circuits (distributed constant circuits) is that such circuits have a tendency to produce spurious" responses or to'break into random modes of operation. For example, a line out for one-quarter wave operation will operate, spuriously inany odd-quarter-wavelength mode. Likewise, a line out for one-half wave resonance will operate spuriously, in any integral number of half wavelengths greater than one.

It is accordingly'an object of this invention to providea distributed constant radio frequency circuit which is substantially free of the above defects.

Itis" another object of this invention to construct a simple, inexpensive distributed constant radio frequency circuit offering exceptionallywide range tunin and mode stabilization at frequencies above 300 megacycles.

Other objects and features of the presentinvention willbecome apparent upon a careful consideration of the following detailed description when taken together with the accompanying drawings, in which:

Fig; l is a partial cross-sectional view taken along thelongitudinal axis of one embodiment of the present invention;

Fig; 2 is'a cross-sectional View taken through the plane 22 of the embodiment illustrated in Fig. 1.

In accordance with the basic concepts of the present invention, a distributed constantradio frequencycircuit is constructed which comprises aparallel line section dimensioned to operate, fundamentally, in a greater than unity odd quarter wavelength mode. In the single; specialized embodiment herein exemplified a threequarter was/elength mode is selected. The. line section is constructed of a pair of dissimilar section so dimensioned, electrically, that the discontinuity formed at their juncture produces Wave reflections which combine'with the: wave reflectionsproduced at one end of the line in an additive manner when the line isexcited in its selectedmode'. This action assures eflicient, stable operation of the circuit.

Conversely, the phase relationship between the wave reflections from the: end of: the. line and 2. from the discontinuity is such, when the line is excited at any frequency other than the three quarter wave resonant frequency, that substantially all the harmonics are either cancelled or so thoroughly attenuated as to produce negligible effect on theoperationof the circuit.

A suitable resistance such as a coating of aquadag may be applied to the line at any convenientpoint therealong to further damp out spurious modes of operation.

Tuning of'the line may, as will. hereinafter be described, be attained by using the novel tuning principles disclosed in my co-pending application, supra.

Referring. nowin particular to Figs. 1 and 2 there is-disclosed the novel teachings of my invention as applied to a modified concentric line oscillatorwhich comprises avacuum tube component IE), and a line section including inner and outer conductors H and I2 respectively. In this particular embodiment th line is dimensioned to produce operation in a three quarter wave mode. The-inner conductor I l of the line is transversely slotted over a portion of its length Ila and is made rotatable fortuning reasons as will hereinafter be described. Inner conductor H is also made in two sections, one section Ila of a half wavelength long and the other lib of a quarter wavelength long. These dimensionsare taken at the-highest frequency of operation. The first section I la is made with a relatively large diameter whereby it cooperates with the outer cylindrical conductor I2 to form a half wave resonant line of low-characteristic impedance. The second section Ilb is made with a relatively smalldiameter whereby it may cooperate with the outer conductor I2 to form a quarter wave line section of relatively high characteristic impedance. The two line sections are effectively inseries with one end shorted through a radio frequency bypass Which comprises a fiber Washer l4 and a sheet of mica IE to the end wall l3 of the outer cylinder I2.

In operation in the three-quarter wave mode a voltage maximum'will exist at the juncture of line sections Ila and llb. Conversely a voltage minimumwill exist at the shorted end of section Hb. At-thisfrequency, line section Ila, will appear to be one-half wavelength long terminated in the infinite impedance reflected by the quarter wave section ll b. The wave energy reflected from the discontinuity in the line, that is from the juncture of line sections Ila and llb will be reflected without phase inversion since the half wave section I la is terminated in an infinite impedance. The energy reflected from the lower end, theshorted end of quarter wave section Hb,

Il therebetween is critical. 1%" slots with is" spacing gave a tuning range however, will be inverted since the line section is terminated in a short circuit. Now then since the propagation path extending from the juncture of sections Ila and MD to the shorted end of section I lb and back is equal to one-half wavelength in the desired mode of operation, and since the energy reflected from the shorted end of section llb is inverted the reflected energies from both the discontinuity and from the shorted end of the line will combine in phase. This phase relation is that which is required for efiicient, stable operation of the circuit.

In the quarter wave resonant mode the reflections occurring from the end of the line are out of phase with those occurring from the discontinuity. As a result of this difference in phase in the two reflections, the oscillations in this mode are seriously attenuated. Similarly, in the five quarter wave mode and all other odd quarter wave modes (except possibly the nine quarter wave mode), the two sets of reflections (one set from the shorted end of the line and the other from the discontinuity) are out-of-phase giving little chance for sustained or efiicient operation of the circuit except in the three quarter wave mode.

To further assist in the damping out of spurious modes of operation, I have found it desirable to coat a thin layer of resistance material l6 such as aquadag on one of the conductors at the juncture of the two line sections. In the three quarter wave mode, a current minimum exists at this point and little or no energy is dissipated in the resistance. In all other spurious modes some predetermined small current exists at this point thus giving rise to a voltage drop across the resistance which further accentuates the attenuation of these modes.

In order to permit tuning of the present circuit and in accordance with the principles disclosed in my aforementioned co-pending application, the inner conductor H is made rotatable relative to the outer conductor I2 and is disposed off-axis from outer conductor l2. Likewise the inner conductor is transversely slotted at uniformly spaced intervals I! along the length I la.

This construction provides a line wherein the distributed capacitance between conductors I2 and II is constant with rotation of conductor II, but the distributed inductance is variable with rotation of the conductor II. This characteristic arises by virtue of the fact that the fringe capacity between the teeth E8 of the slotted portion of conductor II and the inner wall of conductor I2 is essentially equal to the distributed capacity between the solid portion [9 of conductor II and conductor l2. Thus the distributed capacity of the line will remain the same with rotation of conductor II.

The mutual inductance between conductors II and I2, however, varies with the spacing between the solid portion I9 of conductor II and conductor l2. Accordingly, as conductor H is rotated the distributed capacity between conductors remains constant while the mutual inductance between conductors varies, it being a maximum in the position illustrated in Fig. 2. In this condition, the resonant frequency of the line section is a maximum. Rotation of conductor ll 180 minimizes the mutual inductance and also the resonant frequency of the line section.

In the construction of the inner conductor neither the width of the slots [8 nor the spacing In one embodiment of nearly two to one. 2" slots with spacing was also found to give a tuning range of substantially two to one. Similarly, the angle of the solid portion of conductor H is not critical and can be varied within wide latitudes without impairing the operation and performance of the circuit, since this angle only determines the value of self inductance of the conductor. The greater the angle, the smaller the self inductance and the higher the resonant frequency.

In this application .of the invention the inner conductor II is hollow, and is provided with a hollow reentrant sleeve 23 the extension of which forms the section ll'b. Sleeve 23 is positioned over a suitable center post 3 I. Spatially separating center post 3| and reentrant sleeve 23 is a fiber washer 24. Washer 24 as Well as the air dielectric between post 3| and sleeve 23 operate to provide a radio frequency connection between the center post and the inner conductor il. Post 3| is ofi-set from the axis of the outer cylinder l2 sufficiently to permit a good capacitive association between the inner conductor H and the inner walls of the outer cylinder 12.

A gear wheel 25 is securely attached to the lower end of the inner conductor ll for engagement with a fiber spur gear 26. Spur gear 26 is pinned or otherwise attached to a knurled knob 21 that projects from end wall 28 of the outer cylinder to provide rotation of the inner conductor II and therefore tuning of the line. Insulatingly separating the gears 25 and 26 from the end Wall 28 is a strip of mica insulation [5. The mica l5 coacts with the end wall 28 and the lower end of cylinder II to provide a capacitive short circuit between the ends of the inner and outer conductors.

The upper end of the center post 31 is longitudinally slotted at 30 to provide a finger like contact engagement with the plate cap 31A of a light house tube [0. The lower end of the center post is anchored to end plate 28 through pin 42, which also serves as a connection for a source of plate supply potential.

The upper end of the concentric line is closed by a cup-shaped end cap 33 held to the outer cylinder l2 by screws 60 and apertured at 34 to permit entry of the tube into the line. Attached to the inner periphery of aperture 34 is a flexible annular contact ring 35 adapted to contact the grid ring 36 of tube l0 thereby to provide grounded grid operation of the tube.

Surrounding the cathode ring terminal 39 of tube 10 is an annular support 31 held in position :by a plurality of stand-off insulators 43. Attached to the ring support 31 is a flexible annular contact ring 38 adapted to provide contact with the cathode of the tube I0.

To provide regenerative feedback for the production of oscillations, a capacitative feedback strap 40 is attached to the cathode support ring 31 and lead down through an aperture 4| cut in cap 33 to capacitatively engage the anode of tube Ill. The output may be derived from the oscillator by means of an inductive loop 50, for example, disposed on the end walls 28.

From the foregoing it becomes apparent that I have provided a distributed constant circuit which is of simple and inexpensive construction and which is easily tunable by a simple rotation motion. It is further apparent from the foregoing that numerous modifications may be made of the present invention without exceeding the spirit thereof. For example, the principles of this invention may be applied with equal advantages to other types of line controlled circuits such as the two wire line circuit now commonly in use. Also the invention may be used as a wave trap, or for radio frequency amplification or mixing as well as for the production of mode stabilized oscillations. Accordingly, it must be understood that this invention is not to be limited except as far as defined by the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. A radio frequency circuit comprising, a pair of substantially coextensive, parallel conductors dimensioned to form a resonant line of an odd multiple, greater than unity, quarter wave lengths long, said resonant line being shorted at one end and opened at the other and being formed from a pair of serially connected line sections having dissimilar characteristic impedances, the line section adjacent the shorted end of the resonant line having a characteristic impedance higher than that of the other line section, one of said line sections being shorter than the other by an odd number of quarter wavelengths.

2. A radio frequency circuit comprising, a pair of substantially coextensive parallel conductors dimensioned to form a three quarter wavelength resonant line shorted at one end and open at the other, said line being formed by the series connection of a quarter wave section of relatively high characteristic impedance and a half wave section of relatively low characteristic impedance, the free end of said quarter wave section being shorted.

3. A radio frequency circuit comprising, a pair of substantially coextensive, parallel conductors dimensioned to form a resonant line of an odd multiple, greater than unity, quarter wavelengths long, said resonant line being shorted at one end and open at the other, and formed from a pair of serially connected line sections having dissimilar characteristic impedances, the line section adjacent the shorted end of the resonant line having a characteristic impedance higher than that of the other line section, one of said pair of line sections being shorter than the othor by an odd number of quarter Wave-lengths and a coating of resistance material applied to said line at a point intermediate the ends thereof.

4. A radio frequency circuit comprising, a pair of substantially coextensive, parallel conductors dimensioned to form a resonant line of an odd multiple, greater than unity, quarter wavelengths long, said resonant line being shorted at one end and opened at the other, and formed from a pair of serially connected line sections having dissimilar characteristic impedances, the line section adjacent the shorted end of the resonant line having a characteristic impedance higher than that of the other line section, one of said pair of line sections being shorter than the other by an odd number of quarter wavelengths and a coating of resistance material applied to said line at the juncture of said pair of line sections.

5. A radio frequency circuit comprising, first and second transmission line sections having different characteristic impedances connected in cascade, each of said sections being at least a quarter wavelength long at a predetermined frequency, means terminating the free end of the second line section in an impedance other than its characteristic impedance to form a resonant line from said sections, the characteristic impedance of said line sections having relative values operative to produce a reflection at their junction point which reinforces the reflection which occurs at the end of the second line section when the line is excited at a predetermined frequency.

FRANK C. ISELY.

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

UNITED STATES PATENTS Number Name Date 1,963,723 Sterba June 19, 1934 2,270,949 Hulster Jan. 2'7, 1942 FOREIGN PATENTS Number Country Date 428,258 Great Britain May 5, 1935

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