US2144029A

US2144029A - High frequency coupling device

Info

Publication number: US2144029A
Application number: US146239A
Authority: US
Inventors: Wladimir J Polydoroff
Original assignee: Johnson Laboratories Inc
Current assignee: Johnson Laboratories Inc
Priority date: 1937-06-03
Filing date: 1937-06-03
Publication date: 1939-01-17
Anticipated expiration: 1956-01-17

Description

Jan. 17, 1939. w. J. POLYDOROFF ,1 ,0

HIGH FREQUENCY COUPLING DEVICE Filed June 5, 1937 l N V E N TO R W1 40/ 1? 1 04 YDOROFf BMMIGM ATTO R N EY Patented Jan. 17, 1939 UNITED STATES 2,144,029 men FREQUENCY COUPLING nsvroa Wladimir J. Polydorofl', Johnson Laboratories poration of Illinois Wilmette, 111., asaignor to Inc., Chicago, 111., a cor- Application June 3, 1931, Serial No. 140,239

16 Claims.

This invention relates to improvements in highfrequency resonant systems, such, for example, as those which are generally employed between the output terminals of a first vacuum tube and the input terminals of a second vacuum tube, and which are intended for operation at a particular frequency, or over a limited range of frequencies, for example, in the intermediate-frequency am plifiers of radio receivers of the superheterodyne type.

Resonant systems of the above-mentioned type are of two general forms, one employing a single resonant circuit and the other employing two or more coupled resonant circuits, each circuit having an inductor and a capacitor, one or both of which elements is adjustable to align the circuits at a particular frequency or to permit varying the resonant frequency of each circuit over a limited range.- The present invention is particularly addressed to the second form.

Fixed air-core inductors shunted by adjustable tuning capacitors have been in common use in coupled resonant-circuit devices, the two inductors being so arranged with respect to each other as to provide, at some chosen frequency, a desired degree of inductive coupling between the two resonant circuits. The usual type of adjustable "trimmer" capacitor, however, is appreciably affected by changes in temperature, and also variations in capacitance value with time, thus making it difiicult if not impossible to maintain the desired performance characteristics in capacitively tuned fixed-inductance coupled resonantcircuit devices of the types now commonly em ployed.

The difficulty just described may be eliminated by employing fixed capacitors and accomplishing tuning by the use of variable inductors, preferably oi the type employing movable ferromagnetic cores for inductance adjustment. In such a device, the resonant circuits may be tuned by inductance variation to secure alignment at a desired frequency, and, once established, this alignment is substantially permanent.

In the design 01' devices of the class here under consideration. it is necessary to first determine the frequency at which they are to be operated, or the range of frequencies over which they are to be adjustable A particular device will. in general, be designed for use at a particular frequency, but will be adjustable and operative over a small range of frequencies. When the device is adjusted to operate at the particular frequency for which it was designed, either the capacitance or the inductance will be varied to compensate for deviations in the capacitances and inductances in the circuits with which the device is associated, in such a way as to bring the product of the total eilective capacitance and the total efiective inductance to the correct value for the particular 5 frequency. Thus, if the inductance value in a particular case is high, the capacitance value must be correspondingly lower, and vice versa.

Under the circumstances just described, it now becomes possible in accordance with the invention to design a device so that, for any adjustment of its variable elements, it will produce the same resonant gain or the same selectivity characteristic, as may be desired.

Since modern radio receiving apparatus usually has adequate gain and in addition has automatic gain control to compensate for any changes in gain or in the strength of the received signal, and since, on the other hand, variations in the selectivity characteristic result in variations in fidelity and may produce serious distortion, it is preferable to design the device so that it will maintain the selectivity constant at the desired value regardless oi the settings of its adjustable elements.

In general, there are two methods by which the over-all selectivity of such a device may be kept substantially constant. In the first method, each of the circuits isso designed that its individual selectivity will remain constant, and the coupling between the two circuits is so arranged that it remains constant, regardless of adjustments in either of the circuits. In the second method, the individual selectivities of the circuits are allowed to vary but the coupling between the two circuits is arranged to also vary in such a manner as to 85 substantially compensate for the individual circuit variations. The first method, and devices which enable it to be carried out, constitute the subject matter of the present application. My co-pending application, Serial Number 146,241, filed June 3rd, 1937, is addressed to the second method.

The condition to be fulfilled it each of the circuits is to have constant selectivity is that the ratio of its inductance to its resistance, or L/R, shall remain constant for any position of the adjustments necessary to secure resonance at the intended frequency. If, in addition, the arrangement is such that the degree of coupling between the two circuits remains constant for all positions of the adjustments, then the over-all selectivity will also remain constant.

It is an object of this invention to provide a high-frequency coupling device in which the coupling between the resonant circuits is reguiated to compensate for the effect of adjustments of the individual resonant-circuit inductance values.

Another object of the invention is to provide a high- "equency coupling device in which substantially uniform selectivity may be secured, regardless of inductance adjustment, without resort to expensive or complicated arrangements.

United States Patent No. 2,066,777 to Harnett shows and describes a coupling device including two coupled resonant circuits which are tuned by inductance variation. The inductance of the windings is adjusted by varying the spacing between the sections or pies of each winding, and the arrangement is described as being such that the degree of coupling between the two resonant circuits is maintained constant, as the inductance values are changed by relative movement of the sections of either winding to tune the circuits to the desired frequency, in order to achieve constant selectivity. It is essential, in the device of the Harnett patent, that the ratio of inductance to resistance, or L/R, of the individual circuits remain substantially constant throughout the range of their adjustability, if the degree of coupling between the circuits remains constant and the selectivity of the complete device is to be substantially constant for operation at a given fixed frequency. The Harnett patent, however, fails to recognize the necessity for maintaining L/R constant in the individual circuits, nor does it disclose how this result could be achieved.

Patent No. 2,066,777, above referred to, states that the principles of the invention may be practiced in various other suitable ways, as for example, by changing the self-inductances of the individual windings by adjusting the relative positions of magnetic cores employed in connection with the remote sections of the windings. The Harnett patent does not disclose, however, how these cores should be arranged to provide substantially constant L/R of the individual circuits and thus uniform selectivity over the range of adjustability, nor is there any mention of the efiect which movement of the magnetic cores is bound to have on the mutual inductance of the device. This efiect is appreciable and must be taken into consideration if the degree of coupling is to be maintained substantially constant.

In accordance with the present invention, the inductors are so designed that at the chosen frequency and regardless of any variation of inductance, the ratio of inductance to resistance, L/R, is kept constant. Each circuit then has constant selectivity throughout its range of adjustability. Additionally, in order to maintain the over-all selectivity of the two coupled circuits constant, the device may be so designed that the capacitive coupling between the circuits is negligible, and as to maintain the inductive coupling coeficient in constant, and this may be accomplished by providing that for any change in the inductance values, L1 or L2, of either of the inductors, the mutual inductance Lm is changed in the same direction, these quantities being related in the well-known equation It is possible to use my improved coupling de vice not only at a particular frequency as above described, but also at some other frequency within a limited range. For example, an intermediatefrequency coupling device in accordance with my invention may be made to operate at any freaieaoae quency within its range of adjustability in order to secure the best possible discrimination against a particular local image-frequency interference.

If the frequency change were to be efiected solely by change of inductance and it was desired to maintain the selectivity of the pair of coupled circuits substantially constant, one method of securing this result would be to so construct the inductances that the L/R ratio of each circuit remained constant throughout the range of frequencies and to so design the unit that the coupling coeflicient would be inversely proportional to the frequency. The inductances would each be inversely proportional to the square of the frequency, and the mutual inductance, therefore, would be inversely proportional to the cube oi the frequency and hence would change more rapidly than the inductances. If such a unit were to be associated with an amplifying tube, not only the selectivity but also the amplification would remain constant throughout the range of adjustability.

As will be clear from what is to follow, the coupling device of the present invention not only satisfies the conditions for constant selectivity at a'single frequency, but also produces 'approximately constant selectivity and constant amplification over a range of frequencies.

My improved coupling device, therefore, although particularly adapted for operation at a single frequency, may also be used successfully for tuning over a range of frequencies or to some frequency other than the particular frequency for which the device is primarily designed. The individual inductors are designed to maintain constant L/R at a particular frequency and so, when used over a frequency range, the increase in inductance for a lower frequency results in somewhat improved L/R due to the relatively low losses in the ferromagnetic cores and to the decreasing resistance of the coils. This, however, closely corresponds to conditions for maintaining the quantity Q=wL/R constant. For this case, as is known, the coupling coefiicient should remain constant, which, as already pointed out, is a feature of my invention. My improved coupling device, therefore, may be tuned throughout a range of frequencies and provides a pair of coupled circuits with substantially constant coeflicient of coupling, and although the selectivity will be somewhat improved as the frequency decreases, the fact that the coefficient of coupling remains substantially constant is of particular advantage when critical couplings are employed, as is usually the case.

In addition to inductive coupling between the two circuits of the coupling device due to mutual inductance, there may be some capacitive coupling between the circuits. The capacitive coupling may either aid or oppose the inductive coupling, depending upon its phase with respect to the phase of the inductive coupling. The degree of capacitive coupling may be expressed as follows:

where Cm is the mutual capacitance and C1 and C2 are the capacitances in the two circuits. Since Cm is usually very small compared with C1 and In a particular coupling device, Cm depends upon the spacing and the connections of the windings,

and C1 and Ca include not only the tuning capacitances but also the input and output capacitances of the vacuum tubes which may be associated with the device and the stray capacitance of the connecting leads.

United States Patent No. 2,06637'7, above referred to, also makes no mention of capacitive coupling between the two resonant circuits of its coupling device, nor does the patent show any means for utilizing or eliminating it. If a coupling device is to provide uniform performance at fixed frequency over a range of adjustability oi the circuit inductances and capacitances, it is necessary to take into consideration the capacitive coupling which aids or opposes the inductive coupling. This is especially important in the case of highly eflicient circuits where the degree of coupling is very small. Movement of a portion of each winding, as shown in the Harnett patent, for example, is bound to have an appreciable effect upon the capacitive coupling which exists between the two resonant circuits including the windings.

In one advantageous embodiment of my invention, I employ a combination of inductive and capacitive couplings which are so correlated as to maintain the degree of over-all coupling substantially constant at fixed frequency as the circuit inductances and capacitances are varied. In this case, the capacitive coupling is arranged to aid the inductive coupling, so that the total coupling coeflicient may be expressed as follows:

Substituting

Equations

1 and 3 in Equation 4, the result is:

m m k=:

1 2 1/ i 2 (5) From Equation 5, it is clear that the total coupling coeflicient may be maintained substantially constant either by maintaining each term of the right-hand side of the equation constant, or by so proportioning the inductive and capacitive couplings that the second term compensates for inconstancy of the first term.

If for instance the capacitances in each of the individual circuits are lower than they should he, say by about 10%, then the expression is likewise about 10% lower. Assuming that the accompanying decrease in the mutual capacitance is negligibly small, the total capacitive coupling is about 10% higher. Therefore in order that the total effective degree of coupling between the two circuits may remain unchanged and assuming kn and kc to be substantially equal, the increase in the capacitive coupling must be compensated by a corresponding (10%) decrease in the inductive coupling, when the inductances of the circuits are adjusted to re-establish resonance at the desired frequency.

At constant frequency, L1C1=Li'Ci' and L2C2=La'C2', the primes designating the changed values of inductance and capacitance in the individual circuits, brought about by the unintentional changes of capacitance and the compensating changes of inductance to reestablish resonance at the desiredfrequency. Therefore a 10% decrease of the value must be accompanied by a 10% increase of the value V 1 to JUL. and if this increase in the inductance values of the individual circuits is performed without noticeable change of the mutual inductance then the total efl'ective coupling will be the same in both cases, i. e.,

If, in addition, the inductance adjustment has been performed with a core which maintains the L/R ratio of the circuit constant as the inductance is changed, then the selectivity within and between the circuits, and hence the over-all selectivity, remains substantially constant, with no change in either the mutual inductance or the mutual capacitance.

Thus the total coupling cofficient is maintained substantially constant, in spite of the fact that the mutual inductance does not vary in direct proportion to the square root of the product of the individual circuit inductances. This advantageous utilization of capacitive coupling, in ac cordance with my invention, greatly simplifies the design of coupling devices capable of providing substantially uniform selectivity over a wide range-of adjustability.

The invention will be better understood by reference to the accompanying drawing, in which:

Fig. 1 shows a schematic wiring diagram of one form of the coupling device; and

Fig. 2 shows, partly in section, a preferred embodiment of a coupling device of the form of the invention shown in Fig. 1.

Referring to Fig. 1, vacuum tubes i and 2 are shown coupled by a coupling device contained within shield 3. The output circuit of vacuum tube I includes an input resonant circuit comprising serially connected

coils

4 and 4a and shunt capacitor 8. Input coil 4 is provided with a fixed ferromagnetic core 6. adjustable in its inductance value by means of movable ferromagnetic core 1. Similarly, an output resonant circuit comprising serially connected

coils

4 and 4a and shunt capacitor 8 is connected in the input circuit of vacuum tube 2. Output coil 4 is provided with a fixed ferromagnetic core 6, and output coil 411 has an adjustable ferromagnetic core 1. By adjustment of cores I relatively to coils 4a, the effective inductances of -the input and output resonant circuits, respectively, may be varied. Input coils 4 and 4a in series are inductively coupled to output coils l and la in series. The degree of capacitive coupling may be increased by employing a capacitor 20 shown dotted between the high-potential terminals of the input and output circuits.

A preferred embodiment of my improved coupling device is shown in Fig. 2, in which coils l and 4a are mounted on an insulating tube 5 vertically disposed in shield can 3. Cores 6 are fixedly mounted inside of tube 5 in a desired relation to coils 4, respectively. An internally threaded nut 9 rotates in insulating assembly plate I0, which is secured to shield can 3 by means of bolts II and nuts l2. Spring washer l3 secures nut 9 in plate l0. Sleeve H, to which is secured tube 5 and guiding member 15, is externally threaded to engage the threads of nut 9. Member l5 slidably engages bolts II and thus prevents rotation of sleeve I4. Assembly plate l6, which is secured to the open end of shield can 3 by means of mounting spade bolts ll, supports lower core 1 in fixed position by means of screw i8 inserted in rubber washer l9, to which the core is attached. Thus rotation of nut 9 imparts a vertical motion to tube 5 bearing lower coil 4a,

Input coil to is and the relative-position of lower coil 4a and lower core 1 is therefore changed. Upper core 1' is similarly secured to an adjusting screw l8, which engages the internal threads of sleeve I4,

5 thus permitting adjustment of its position with 1 bly being moved relatively to lower core I, so that in effect rotation of nut 9 merely adjusts the position of lower core I relatively to lower coil 4a, producing no other change. It will be readily understood by those skilled in the mechanical arts 15 that other arrangements to produce identical relative motions of the parts of the assembly can be devised. Since these alternative arrangements will be the exact electrical and magnetic equivalents of the arrangements which I have de- 20 scribed, they are to be understood to lie within the scope of my invention.

-Figure 2 may be taken as illustrative of a practical design of the embodiment shown, for use at a frequency of 460 kilocycles, and is approxi- 25 mately a full-size drawing of such a device. The

ferromagnetic cores

5, 6 and l, I may be, for example, in diameter and the tube 5, upon which the

coils

4, 4 and 4a, 4a are directly wound,

has a thin wall and provides a sliding fit for the 30

cores

6, 6 and I, 1. The

coils

4, 4 and 4a, 4a are of the universal-wound type, preferably of so- -called Litz wire having a plurality of individually insulated strands, and each pair may be wound to an inductance value of approximately 1300 35 microhenries with the cores fully inserted, and

may be adjusted to values approximately 20% lower by partial withdrawal of the cores.

The inner cores are adjusted to leave distances of about between them and the inner coils to leave a distance of 1%. The desired mutual inductance Lm may be secured by placing the outer coils from the inner coils. Such a selection of distances maintains a desired variation of Lm when the inductances are varied, and provides uniform operation of the coupling device over the range of adjustability of the cores.

Fixed capacitors 8, which may have a value of 70 micromicrofarads, are secured to members l5 and I6, respectively, and are connectedin shunt with each serially connected pair of

coils

4 and 4a to form the input and output resonant circuits, respectively.

In operation, moving the cores 1 toward the cores 6 gradually closes the air gaps and thus 55 materially shortens the magnetic paths of the two groups of

coils

4 and 4a. This in turn not only increases the total inductance in each circuit by a substantial amount, by virtue of the increased linkages between the two coils in that v60 circuit, but also increases the effective inductance of coils 4 as well as that of coils 4a. By appropriate choice of the coils and cores, the resonant circuits may be made to have substantially constant L/R at constant frequency and will have 65 nearly constant Q values over a range of fre- 75 their associated cores 1.

If the inductors are made in accordance with the disclosure of my United States Patent No. 1,982,690, the ferromagnetic core, when it is fully or partially inserted in the associated coil, in-

creases the L/R and hence the Q of the coil. Thus the use of properly designed inductors employing magnetic cores provides not only substantially constant selectivity throughout the range of adjustability, but also an improved degree of selectivity over that previously realized in coupling devices intended for the same purpose. This double advantage of employing magnetic cores in an inductance-tuned coupling device is one of the features of the present invention.

According to one form of the invention, the mutual inductance is arranged to increase proportionately to increases in theindividual circuit inductances in order to maintain the degree of inductive coupling substantially constant. {According to the invention, the mutual inductance between the input and output resonant circuits may be made to increase at the proper rate as the cores 1 are moved closer together by several simultaneous and cooperating effects. The increase in the individual inductances of coils 4a due to the introduction of cores 1 into them results in a greater contribution to the mutual inductance by these outer coils. Additionally,

v the magnetic path for the flux linking the coils of the two circuits is shortened by the movement of cores 1, with resultant strengthening of the mutual inductance.

It is within the scope of the invention to so design the coupling device that the mutual inductance increases at a rate which is not substantially equal to that required for maintaining a constant degree of inductive coupling. If a higher rate of increase is desired, coils 4a may be made to have an inductance value somewhat greater than that of coils 4. If the outer coils 4a have less inductance than the inner coils 4, the rate of change of mutual inductance will be lower than would be realized were the

coils

4 and 4a of similar inductance. The mutual inductance may be supplemented by mutual capacitive reactance of the proper phase and magnitude to provide, in conjunction with the inductive coupling, a desired degree of over-all coupling. If the windings are so connected that the outer turns of coils 4 are at low high-frequency potential, the capacitive coupling between the circuits is very low. By connecting the outer turns of coils 4 to the high-potential sides of the circuits, however, an appreciable degree of capacitive coupling is obtained. This capacitive coupling may be still-further increased by the use of capacitor 20 (see Fig. 1) connected between the high-potential terminals of the two resonant circuits. Whether the capacitive coupling thus obtained aids or opposes the inductive coupling depends uponthe relative winding directions of the two pairs of serially connected

coils

4 and 4a.

The scope of the invention is not limited to the embodiments shown in the drawing and described herein, but includes as well such further modified forms as will occur to those skilled in the art and embodying the principles hereinabove disclosed.

Having thus described my invention, what I claim is:

' l. A high-frequency coupling device including plural inductance coils and capacitors connected to provide a pair of coupled resonant circuits, and ferromagnetic cores adjustable relatively to said coils in such a manner that the selectivity of each of said circuits is maintained-constant, said 9,144,029 coils and said cores being so disposed that move-,

ment of said cores relative to said coils produces simultaneous and substantially proportional changes in the square root of the product of the eifective inductances of said circuits and in the mutual inductance between said circuits.

2. A high-frequency coupling device including plural inductance coils and capacitors connected to provide a pair of coupled resonant circuits, and ferromagnetic cores adjustable relatively to said coils in such a manner that the selectivity of each of said circuits is maintained constant, said coils and said cores being so disposed that movement of said cores into said coils increases the eiiective inductances in said circuits and increases the coupling between said circuits in such a way as to maintain the selectivity of said coupling device substantially constant.

3. A high-frequency coupling device including plural inductance coils and capacitors connected to provide a pair,of coupled resonant circuits, and ferromagnetic cores adjustable relatively to said coils in such a manner that the selectivity of each of said circuits is maintained constant, said coils and said cores being so disposed that movement of said cores relative to said coils alters the resonant frequency of said circuits while maintaining the degree of coupling between said circuits substantially constant.

4. A high-frequency coupling device including plural inductance coils and capacitors connected to provide a pair of coupled resonant circuits, and ferromagnetic cores adjustable relatively to said coils in such a manner that the selectivity of each of said circuits is maintained constant, said coils and said cores being so disposed that movement of said cores relative to said coils alters the effective inductance of said circuits while maintaining the degree of coupling between said circuits substantially constant.

5. A high-frequency coupling device including two groups of inductance coils, plural capacitors;

said coils and said capacitors beingconnected to provide a pair of inductively coupled resonant circuits, ferromagnetic cores fixedly positioned relatively to a first of said groups, and adjustable ferromagnetic cores movable relatively to the second of said groups, said coils and said cores being so disposed that movement of said adjustable cores relative to said second group produces simultaneous and .substantially proportional changes in the square root of the product ofthe effective inductances of said circuits and in the mutual inductance between said circuits.

6. A high-frequency coupling device including two groups of inductance coils, plural capacitors, said coils and said capacitors being connected to I provide a pair of coupled resonant circuits, ferromagnetic cores fixedly positioned relatively to a first of said groups, and adjustable ferromagnetic cores movable relatively to the second of said groups, said coils and said cores being so disposed that movement of said adjustable cores into said second group increases the efi'ective inductances in said circuits and increases the coupling between said circuits in such a way as to maintain the selectivity of said coupling device substantially constant.

7. A high-frequency coupling device including two groups of inductance coils, plural capacitors, said coils and said capacitors being connected to provide a pair of coupled resonant circuits, ferromagnetic cores fixedly positioned relatively to a first of said groups, and adiustable ferromagnetic cores movable relatively to the second of said,

groups, said coils and said cores being so disposed that movement of said adjustable cores relative to said second group alters the resonant frequency of said circuits while coupling between said onant.

8. A high-frequency coupling device including two groups of inductance coils, plural capacitors, said coils and said capacitors being connected to provide a pair of coupled resonant circuits, ferromagnetic cores fixedly positioned relatively to a first of said groups, and adjustable ferromagnetic cores movable relatively to the second of said groups, said coils and saidcores being so disposed that movement of said adjustable cores relative to said second group alters the effective inductances of said circuits while maintaining the degree of coupling between said circuits subcircuits substantially res- -stantially resonant.

9. A high-frequency coupling device including first and second resonant circuits, first and second inductive windings in each of said circuits, ferromagnetic cores fixedly positioned within said first windings, and adjustable ferromagnetic cores positioned within but movable relatively to said second windings, said windings and said cores being so disposed that movement of said adjustable cores relative to said second windings produces simultaneous and substantially proportional changes in the square root of the product of the eifective inductances ofsaid circuits and in the mutual inductance between said circuits.

10. A high-frequency coupling device including first and second resonant circuits, first and secand inductive windings in each of said circuits, ferromagnetic cores fixedly positioned within said first windings, and adjustable ferromagnetic cores positioned within but movable relatively to said second windings, said windings and said cores being so disposed that movement of said adjustable cores into said second windings increases the efi'ective inductances in said circuits and increases the coupling between said circuits in such a way as to maintain the selectivity of said coupling device substantially constant.

11. A high-frequency coupling device including first and second resonant circuits, first and second inductive windings in each of said circuits, ferromagnetic cores fixedly positioned within said first windings, and adjustable ferromagnetic cores positioned within but movable relatively to said second windings, said windings and said cores being so disposed that movement of said adjustable cores relative to said second windings alters the resonant frequency of said circuits while maintaining the degree of coupling between said circuits substantially constant.

12. A high-frequency coupling device including first and second resonant circuits, first and second inductive windings in each of said circuits, ferromagnetic cores fixedly positioned within said first windings. and adjustable ferromagnetic cores positioned within but movable relatively to said second windings, said windings and said cores being so disposed that movement of said adjustable cores relative to said second windings alters the effective inductances of said circuits while maintaining the degree of coupling between said circuits substantially constant. 7

13. A high-frequency coupling device including first and second resonant circuits, first and second inductive windings in each of said circuits, ferromagnetic cores fixedly positioned within said first windings, and adjustable ferromagnetic cores positioned within but movable relatively to said maintaining the degree of second windings, said windings and said cores being coaxiaily arranged and so disposed that movement of said adjustable cores relative to said second windings produces simultaneous and substantially proportional changes in the square root of the product of the eflective inductances positioned within but movable relatively to said second windings, said windings and said cores being coaxiaily arranged and so disposed that movement of said adjustable cores into said second windings increases the eiiective inductances in said circuits and increases the coupling between said circuits in such a way as to maintain the selectivity of said coupling device substantially constant.

15. A high-frequency coupling device including first and second resonant circuits, first and second inductive windings in each of said circuits.

ferromagnetic cores fixedly positioned within said first windings, and adjustable ferromagnetic cores positioned within but movable relatively to said second windings, said windings and said cores being coaxiaily arranged and so disposed that movement of said adjustable cores relative to said second windings alters the resonant frequency of said circuits while maintaining the degree of coupling between said circuits substantially constant.

16. A high-frequency coupling device including first and second resonant circuits, first and second inductive windings in each of said circuits, ferromagnetic cores fixedly positioned within said first windings, and adjustable ferromagnetic cores positioned within but movable relatively to said second windings, said windings and said cores being coaxiaily arranged and so disposed that movement of said adjustable cores relative to said second windings alters the efifective induct- WLADIMIR J. POLYDOROFF. 25