US2530089A - Ultra high frequency resonant circuit - Google Patents

Description

NOV. 14, 1950 c, sMlTH 2,530,089

ULTRA HIGH FREQUENCY RESONANT CIRCUIT Filed June 28. 1946 INVENTOR ATTORNEY Patented Nov. 14, 1950 ULTRA HIGH FREQUENCY RESONANT CIRCUIT Lester C. Smith, Westmont, N. .l., assignor to Radio Corporation of America, a corporation of Delaware Application June 28, 1946, Serial No. 679,973

This invention relates to resonant circuits, and particularly to a resonant circuit of the concentric line type.

Among other objects of the invention are:

To provide a resonant circuit for use above 300 megacycles which has a frequency tuning range greater than two and somewhat less than 3;

To provide a concentric line resonator which eliminates the need for sliding contacts to engage the inner and outer conductors and which has a wider tuning frequency range than known concentric line resonant circuits which do not have sliding contacts.

' To provide a concentric line type of resonator which is more compact than a cavity resonator for the same frequency of operation and which requires a relatively smaller mechanical motion to cover the entire tuning range than known types of concentric line resonators.

A more detailed description of the invention follows in conjunction with a drawing wherein:

Fig. 1 illustrates a tunable resonator circuit in accordance with one embodiment of the invention, and

7 Figs. 2a, 2b and 2c illustrate the resonant circuit of Fig. 1 for three different conditions of tuning.

Referring to Fig. 1, there is shown a concentric line type of resonant circuit in accordance with theinvention, having an outer conductor 8 and a concentric inner conductor composed of a member ll] terminating without conductors, as shown, and a cup-shaped member I2. The cup-shaped member [2 comprises a hollow inner conductor H which is slightly larger than the diameter of the member l8, and a surrounding outer hollow conductor I3 which is slightly smaller in diameter than the hollow conductor 8. If conductor 8 and conductor ID are circular in cross-section then it is preferred that conductors H and i3 of cup [2 also be circular in cross section. If, on the other hand, conductors 8 and H] are rectangular in cross-section, it is preferred that conductors l l and I3 also be rectangular in crosssection.

Rod I is supported at its center and maintained in spaced relation with respect to the outer conductor 8 by means of an insulating ring c t-spider. =1 fixedlypositioned between the con- 4 Claims. (Cl. 178-44) ductors 8 and It]. The conductor f l' is sum-- ciently large to permit movement of this conductor over the right-hand end of conductor I0 without making contact with this rod. The capacity between the conductor I0 and the conductor H is designated by C2. The hollow conductor [3 of the cup 12 is similarly spaced from. the surrounding outer conductor 8 and forms therewith a capacity 01. The cup I2 is movable along the axis of the resonant circuit in the direc-- tion of the arrows by means of an insulating rod 9 which is secured to the cup and which is slidable within an end plate [4 attached to the surrounding outer conductor 8. It should be noted that as the conductor [2 is moved toward or away from the right-hand end of the conductor III, the capacity C2 will vary from a very low value to a relatively high value.

In practice, the length of the cup [2 which is designated Ll is approximately one-half the length of the rod II] which is designated L2. The length L2 of the rod I0 is one-half wave length at the highest frequency of operation, while the length LI of cup I2 is less than one-fourth of a wave length at the highest frequency of operation.

An inspection of Fig. 1 will show that when the cup [2 is arranged in the position shown, the inductance of conductor [0 will be in series with the capacitor C2, in series with the inductances of hollow conductors l l and I2, and in series with the capacitor Cl. Capacitor CI is a relatively large capacity and has a value appreciably larger than capacitor C2.

Fig. 2a illustrates the voltage distribution curve of the resonant circuit when the cup I2 is pulled away from the right-hand end of rod It! so as to have no effect on the resonant circuit. In this condition, the voltage curve of the resonant circuit is indicated by V which represents the distribution of the voltage along the rod 10. In this position, the frequency of the resonant circuit is the highest. Capacitors Cl and C2 do not enter into consideration in the particular position of Fig. 2a because of the large spacing between conductor l0 and cup l2.

Fig. 2b illustrates the condition where'the cup 12 is suficiently close to rod I0 so that the capacity between the rod l0 and cup l2 tunes out the inductance of the conductors of cup l2. In this.

condition, the system is so tuned that the frequency of operation corresponds to rod being one-fourth of a wave length. This frequency is an intermediate frequency for the tuning range of the resonant circuit.

Fig. 2c shows the condition of tuning which is opposite to that of Fig. 2a; namely, where the tuned circuit is resonant to the lowest frequency of operation in the tuning range. An inspection of the voltage curve distribution V of Fig. 2c will show that the entire length of LI plus L2 corresponds to one-fourth of a wave length at the operating frequency.

As an illustration only, assuming that the resonant circuit of the invention is designed to operate over a frequency range of 300 megacycles to 850 megacycles, L2 will be slightly larger than one-sixth of a meter long, while Ll will be slightly less than one-twelfth of ameter, both LI and L2 together having a total length of onefourth of a meter. It will thus be seen that L2 is approximately twice as long as LI.

In practice, it is necessary to move the cup I2 only a very short distance in order to overlap the right-hand end of rod for obtaining the lowest frequency of operation in the tuning range. This is shown in Fig. 20. Any further increase in capacityof C2.causedby further movement of the cup 12 over the rod 10 will cause further shortening of the effective length of the resonant circuit'and hence, increase in frequency. In using the resonator, of-the invention, itis preferred that the point of -minimum frequency not be passed by moving cup 12 too far to the left over theadjacent end of rod 10-. The preferred modusoperandi-is-to have the tuning range extend from the condition shown in Fig. 2a where L2 equalsone-half wave length at the highest frequencytothe condition shown in Fig. 2c where L! plus L2 equals one-fourth of a wavelength at the lowest frequency. In the variation between 'thehighest frequency to the lowest frequency in the tuning range, the operation issmooth and \continuous.

The resonant circuitof the present-invention eliminates the needfo'r sliding mechanical contacts betweeninner and outer conductors and :requires only a. small mechanical motion to cover the entire tuning. range of frequencies. By way of illustration, the motion of cup 12 needbe only one-half inch to cover a range from 300 megacy-* cles to 850 'megacycles with suitable dimensions of the. conductors and .suitablespacing' of these conductors to provide condensers C2 and Cl,

whereas the conventional type of resonant circuit may needa movement of. five inches or more to cover a considerably-narrower band of frequencies. It has: further been found that the system of the invention is much morev compact than a cavity resonator designed to operate at an equivalent frequency of operation.

A feature of the present invention lies in the fact that the resonant circuit decreases in frequency with decreasing overall effective length which is a condition opposite to concentric line tuned circuits wherein'aa decrease in frequency requires an increase in effectivelength of the tuned circuit.

The resonant circuitof the invention is not limited to any particular rangeof frequencies and may be designed to operate anywhere in the range upto10,000megacycles. The tuning range of the resonant circuit of the invention is about'2.8 to 1 which provides a frequencycoverage considerably "greater than -khown=--types of concentric assume 4 lines resonators which do not use sliding contacts. Thus a resonator in accordance with the invention can be constructed over a range of 300 megacycles to 850 megacycles or a range of 1,000 megacycles to 2,800 megacycles.

The load may be coupled to the resonator of the invention by connecting the grid and anode of a vacuum tube to the left-hand end of the outer conductor 8 and the rod H! where the resonant impedance is large, or by inserting a loop between the outer conductor 8 and the rod ID at an intermediate position where the-current at resonance is relatively'large'.

What is claimed is:

1. A tunable resonant circuit comprising a coaxial line having an outer conductor and an inner conductor terminated therewithin, and a tuning structure axially movable with respect to said line and comprising a tubular outer conductor spaced from and capacitively coupled to said first outer conductor and a coextensive tubular inner conductor within said outer conductors, a direct connection between said tubular conductors at one end, said tubular inner conductor having a hollow end portion at the end opposite said direct connection and facing. the termination of said first'inner conductor, the cross section of said tubular inner conductor being sufficiently different from said-first inner conductor'topermit overlapping without contact therebetween with a consequent capacitive couplin therebetween.

2. A-tunable resonant circuit comprising a coaxial line having an outer conductor and an in-' ner conductor terminated therewithin, and a tuning structure axially movablewith respect to saidline and comprising a tubular outer conductor insulatingly spaced from and within said first outer conductor and capacitively coupled thereto and a substantially coextensive tubularzinner-conduc tor within said first-outer conductor and having a physical length approximately one-half that of said first inner'cond-uctor, said tuning; structure having: a:member short circuiting said tubularconductors'at one end, said tubular i1iner-=con-' ductor havingea' hollow end portion facing. the terminaticnof and of a different cross-section than said first inner conductor to capacitively. couple. said-sinner conductors byxvirtue of l. the spacing therebetween, the capa'citybetween said: outer conductors being greater than the capacity between said inner conductors over the entire tuning range;

3. The tunable resonant circuit claimedin claim 2, said hollow end portion havin'g an inner diameter-greater than the outer diameter oi said first inner conductor and said movable tuning structure being axially movable to brlngsaid hollow portion around the terminating portion ofsaid first'inner conductor and spaced therefrom;-

4.- A tunable resonant circuit comprising: a- 00-- axial line having :an' outer conductor and an in-' ner conductor-terminated therewithin, .said linehaving an: electrical length substantially equal toa half wavelength at the highestoperating fre quency, and a tuning -structure axially movablewithin said outer conductor and comprising a conductive end plate, a tubular outercondu'ctor physically spaced from and adjacent to said-first outer conductor to be capacitively coupled thereto and conductively connected to and extending from said end plate and a tubular inner-con ductorwithin said tubular outer conductorand conductively. connected to and' extending from said end plate, one of said inner conductors hav-' ing a hollow terminating portiorr within vvl'iichs 2,530,089 5 6 the terminating portion of the other said inner REFERENCES CITED conductor may be received and in which when so received the one end portion is closely spaced from the other end portion received by it to The following references are of record in the file of this patent:

capacitively couple together said inner conduc- 5 UNITED STATES PATENTS tors, said tubular inner conductor having a length Number Name Date slightly less than a quarter wavelength at the 2,285,662 Hutcheson June 9, 1942 highest operating frequency, both inner conduC- 2,408,895 Turner Oct. 8, 1946 tors together having a total length of one-quar- 2,411,299 Sloan Nov. 19, 1946 ter wavelength at the lowest operating frequency, 10 the capacity between said outer conductors being greater than the capacity between said inner conductors over the entire tuning range.

LESTER 0. SMITH.

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