US2470307A - High-frequency matching transformer - Google Patents

Description

May 17, 1949. G. GUANELLA HIGH-FREQUENCY MATCHING TRANSFORMER 3 Sheets-Sheet 1 Filed April 5, 1945 INVENTOR wim/ uanellci BY ATTORNEY May 17, 1949. e. GUANELLA HIGH-FREQUENCY MATCHING TRANSFORMER I5 Sheets-Sheet 2 Filed April 5, 1945 INVENTOR.

. guslcw Quanella ATTORNEY y 1949- G. GUANELLA 2,470,307

HIGH-FREQUENCY MATCHING TRANSFORMER Filed April 5, 1945 3 Sheets-Sheet 3 C4 V4 Gadeu BY Z! w ATTORN EY Patented May 17, 1949 HIGH-FREQUEN CY MAT CHING TRANSFORMER Gustav Guanella, Zurich, Switzerland, assignor to Radio Patents Corporation, New York, N. Y., a corporation of New York Application April 5, 1945, Serial No. 586,766 In Switzerland February 25, 1944 2 Claims. 1

Systems are known which have more or less evenly distributed line constants and which can therefore be used to form artificial transmission lines. Thus for instance a Lecher wire system can be wound on a cylinder in order to save space, so that the conductors are distributed over the surface of the cylinder and outgoing and return conductors follow each other alternately, as shown in my copending patent application Serial No. 512,969, filed November 27, 1943, entitled High frequency coils. By means of a suitable connection of such coil arrangements, for instance by a series connection of the input ends and parallel connection of the output ends of the coils, it is possible to form transformer systems, as disclosed in greater detail in my copending patent application Serial No. 575,238, filed January 30, 1945, entitled Conductor systems. A disadvantage of such systems is the fact that there is only a comparatively small space between neighbouring conductors, so that with higher voltages there is a danger of flash-overs.

The present invention relates to a transformer system with transmission line character, wherein the aforementioned disadvantages are avoided. This is achieved by an arrangement comprising at least one pair of coils which are wound in the same sense and are spaced a certain distance apart by an intervening dielectric, the input ends of both boils being connected to both transformer input terminals and the output ends of both coils to the transformer output terminals.

The objects and novel aspects of the invention will become more apparent from the following detailed description taken in reference to the accompanying drawings forming part of this specification, and wherein:

Fig. 1 shows diagrammatically a cylindrical transmission line coil constructed in accordance with the principles of the invention;

Figs. 2a and 2b are front and side views, respectively, the latter partly shown in cross-section, of a fiat or disc-shaped transmission line coil in accordance with the invention;

Figs. 3 and 4 are equivalent electrical circuit diagrams representing a transmisison line coil according to the invention;

Fig. 5 shows an electric transmission circuit embodying a transmission line coil according to the invention;

Figs. 6 and '7 show various combinations of transmission line coils according to the invention for use in impedance matching of electrical circuits or devices:

Fig. 8 illustrates a portion of a radio transmitter embodyin a transmission line coil according to the invention;

Fig. 9 shows a transmission line transformer construction according to the invention including an iron core;

Fig. 10 illustrates a construction of a multiple coil transformer arrangement of the type according to Figs. 6-8;

Fig. 11 is a part of a radio transformer circuit illustrating a further use of the invention; and

Fig. 12 is a cross-section through a modified transmission line transformer construction according to the invention.

In the example shown in Fig. l, a cylindrical coil W2 is located inside the insulatin cylinder I and a second cylindrical coil W1 is located on the outside of this cylinder. The input and output ends of the cylindrical windings are indicated by the numerals I, 2 and 3, 4 respectively. Current i1 indicated by the full-line arrows flows through both windings in opposite directions so that the corresponding flux in the coil axis is negligibly small. The flux which for instance results from the first turn of both coils and penetrates the more remote turns of the same coil is therefore negligibly small, that is the mutual inductance between the turns which are more remote from each other can be neglected in case of a symmetrical current distribution. A certain inductance remains between the adjacent turns of each coil. This mutual inductance may be replaced by a corresponding increase in the natural inductance of both coil conductors as long as the developed conductor length between the turns which still have a noticeable influence is small in comparison with /4 wave length. In case of an unsymmetrical current 112 which, as indicated by the dotted arrows, flows in the same direction through both coils, the latter may be assumed to be connected in parallel and under this condition it is clear that the coil pair represents a considerable inductance for such currents and acts like a choke coil. The currents which flow in the same direction for instance through the first turn of each coil mutually support each other and produce a corresponding magnetic flux in the direction of the coil axis which also penetrates the more remote coil turns, so that there is a considerable mutual inductance between all the turns.

For smaller operating voltages the insulating cylinder I can under certain conditions be omitted. Furthermore, a magnetic core may be located inside both coils in order to increase the choking effect during the flow of unsymmetrical currents.

Disc-shaped coils may also be used instead of cylindrical coils to form a coil pair as shown in Figs. 2a and 2b. In this case, the effects of opposing currents in the neighbouring turns of both coils also neutralize each other, so that the coil pair has the same effect as a line for such currents, while for currents flowin in the same direction, that is unsymmetrical currents, again a considerable choking effect is obtained.

In both cases, the transformer system may be represented in the manner shown diagrammatically in Fig. 3. The two coils are again indicated by W1 and W2, respectively. The input ends of both coils which are connected to terminals 1, 2 are directly adjoining so that their eflect on more remote coil turns is practically neutralized. The same applies to the coil ends connected to terminals 3, 4. Since the coil :pair represents a line for the symmetrical currents i1, it may be replaced by an artificial line A for the opposed currents ii in the equivalent-diagram shown in Fig. 4, the currents being transmitted by way of the ideal transformers T1, T2. For the unsymmetrical current iz, .the coil pair represents a choke coil. In the equivalent diagram this current is therefore passed over the centre taps 5, 6 of the transformers and a choke coil B. The inductance of A0 is very large when the number of coil turns is adequate, so that in many cases the choke coil B can be neglected.

If, for instance, as shown in Fig. 5, an unsymmetrical earth, for instance for terminal 2 is provided in the input circuit "of the coil pair and a symmetrical earth for the tap point 6, then the impedance 'Zb of choke coil B may be imagined to lie between the tap point 5 and terminal 2. This impedance may also be replaced by the fourfold impedance 4Z1; indicated by the dotted lines between terminals 1, 2. Hence, a transformer system, in passing from an unsymmetrical to a symmetrical arrangement, possessesan additional input impedance of 4213 which in most cases can be neglected.

If the impedance ZB is neglected, a seriesparallel connection of a plurality of coil pairs may be employed to produce a match between unequal load resistances. The neighbouring coils of a coil pairpossess a definite surge impedance for the symmetrical current-s according to Fig. 1, this impedance coinciding with that of the corresponding artificial line. This surge impedance Z must be taken into account when making connection with the input and output terminals. In the arrangement shown in Fig. 6, an impedance 2Z occurs between the input terminals I, 2, while at the output terminals 3, 4 there is an impedance Z/2. With a seriesparallel connection of n coil pairs S1 Sn with a surge impedance Z there is an input impedance n-Z and an output impedance Z/n. The series-parallel connection according to Fig. 7 can also be realized in several successive groups. With correct matching, the input impedance of the m coil systems S1 which at the input end are connected in series and have a surge impedance Z1 is:

The output impedance of the same group amounts to:

For a second coil group with n coils S2 and surge impedance Z2, the input impedance is:

while for the output impedance:

For a correct matching of both groups, Ra must equal R3, that is the resistance ratio of the complete arrangement is:

Simultaneously, the surge impedances of the individual systems are as follows:

An application of such transformer systems is shown in Fig. 8. A push-pull transmitter comprising a pair of output tubes V1, V2 and the oscillation circuit L, C is matched by means of four series-parallel connected coil pairs S1 to S4 with the concentric cable K. The internal impedance of the tube transmitter between the terminals l, 2 is comparatively large, for example 1000 ohm, While the cable resistance between terminals 3, 4 only amounts to about 60 ohm. The adjustment for a resistance ratio of 16:1 is achieved by the coil pairs whose surge impedance in this case amounts to 250 ohm In an analagous manner, it is possible to match low and high ohmic load impedances, whereby the input ends of the coil pairs have to be connected in parallel and the output ends are connected in series.

In many cases it is advisable to use a ferromagnetic core, for instance an ironclad core M as shown in Fig. 9, for increasing the effective coil inductance Zn.

Several coil pairs with a common axis may also be built together, as shown in Fig. 10. One coil pair consists of the outer cylindrical :coils W1, W2, While the other system comprises the inner cylindrical coils W3, W4. Coils W1, W2 are wound in the same sense this being indicated by the signs. Coils W3, W4 are also wound in the same sense. Both coil pairs must, however, have a different relative sense of Winding, as indicated by the negative sign for the coils W3, W4, so that the current i which flows over terminals l and l to coils W1 and W4 also induces a current 2 in coils W2, W3 and a current 22' occurs between the terminals 2 and 8.

A further application of the invention is shown in Fig. 11, wherein the transformer system is used in a transmitter arrangement in place of a modulation transformer. The amplified low frequency energy is supplied to the coil pair S of the tubes V3, V4 over condensers C3, C4. Choke coil D1 with a centre tap is provided for the direct anode current supply to tubes V3 and V4. The output terminal '3 of the coil pair is connected with the oscillation circuit coil L of the highfrequency output stage and the output terminal 4 is earthed. Choke coil D2 is provided for supplying the anode current for the high frequency output stage. The advantage of the tranformer system S when compared with an ordinary modulation transformer, is due to the fact that with correct matching also the higher signal frequencies are transmitted and no undesirable resonance phenomena occur.

If both the cylindrical coils of the coil pair have unequal diameters, as shown in Fig. 1, it is not certain that there is electrical symmetry between :both, because the conductor lengths of both are generally not equal. It is therefore advisable to make the inner cylindrical coil with a larger number of turns than the outer one. Another means of achieving symmetry is to employ a slightly thinner wire for the inner cylindrical coil.

The surge impedance of the coil pair is relatively large when the coils are closely wound, because the series inductance is considerably increased by the magnetic coupling of neighbouring turns when compared with straight wires, whilst the parallel capacity is normally not appreciably increased. The surge impedance can be reduced by increasing the parallel capacitance, if both coils are separated from each other by a material with a high dielectric constant, or if additional condensers are distributed between both coils as transverse capacitances.

The transformer systems may also be constructed for matching unequal load resistances by making the surge impedance vary along the coils, as is the case with known systems having a line with exponential taper. With cylindrical coils this object may be achieved by varying the winding pitch along the coils or by a variable distance between the coils. Such a wave coil transformer is shown in Fig. 12 which shows a cross-section through a cylindrical coil with relatively tapering windings W1 and W2 and a tapering insulating spacer I. The same object may be achieved by means of a variable wire thickness or a variable dimensioning or distribution of additional parallel capacitances. With such measures it is, however, necessary that the transit time of the coil pair for symmetrical currents has at least the same order of magnitude as the oscillation period of the transmitted waves. Apart from this, the same points of View and rules apply to the distribution of the surge impedance along the coils as for the corresponding distribution with the known exponential taper line systems.

An interesting application for such transformer systems consists in the formation of artificial retarding devices, the advantage being that no restrictive rules exist as regards the retardation of a coil pair, while with the known retarding devices with separate series inductances and parallel capacitances, measures have to be adopted so that the retardation of each individual circuit remains small compared with an oscillation period, because otherwise undesirable reflections and frequency-dependent transmission conditions occur.

When ferro-magnetic cores are employed, it is advisable to screen these cores with a conductive insulating covering having at least one longitudinal slot for the purpose of avoiding additional losses due to capacitive currents. In order to obtain the necessary symmetry, a further metallic screen may be provided outside both coils which is so dimensioned that the input terminals and the output terminals possess electrically symmetrical properties relative to the screen.

An additional screening between both coils may also be provided which has for instance earth potential. Also, with this additional screening, longitudinal slots or other means must be employed to ensure that the effective inductance for unsymmetrical coil currents is not appreciably reduced. This additional conductive arrangement between both coils may also consist of an additional coil, a cylindrical coil for cylindrical coils or a disc coil for disc coils. The starting point of this additional coil lying between the input turns of both coils represents the electrical point of symmetry between the input terminals, and the end of this additional coil represents the point of symmetry between the output terminals of the whole system. It is expedient to use such an additional coil when the output and/ or input terminals are not operated symmetrically with respect to earth potential. The input or output voltage may be made symmetrical with respect to earth or a definite reference voltage if the corresponding end of the additional coil is connected to earth or to the reference potential, whereby this latter can also be an alternating voltage.

The construction of the transformer system in accordance with the invention has, as a result of the symmetrising effect achieved thereby, the ad vantage that for instance with series-parallel connection of the coils it can be employed for voltage transformation. Due to its line character the transformer system possesses considerable freedom from resonance so that in all its applications there is an important technical advantage.

If very large series inductances are required with unsymmetrical currents, mainly at the lower frequencies, the coils may also be constructed with concentrated winding turns whereby in the case of cylindrical coils several successive turns lie one on top of the other or in the case of disc coils next to each other. The coils can for instance be wound with multiple layers.

I claim:

1. A matching transformer comprising a pair of conducting wires each wound into a helical coil, one of said coils being located concentrically and inside the other coil and both coils having a relative taper such that adjacent coil winding turns are spaced at a progressively varying distance to form a continuous coiled two-wire transmission line having a wave impedance varying from one end to the other end of said line, and a tapering dielectric spacing member occupying the intervening space between said coils.

2. A matching transformer comprising a first conducting wire wound into a cylindrical helical coil, 2. second conducting wire wound into a conical helical coil and mounted concentrically with and inside said first coil such that the adjacent wires of said coils are spaced from each other by progressively varying distances and form a continuous coiled-up transmission line having a Wave impedance varying from one to the opposite end of the coil, and a tapering dielectric spacing member occupying the intervening space between said coils.

GUSTAV GUANELLA.

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

UNITED STATES PATENTS Number Name Date 1,852,605 Carlton et a1 Apr. 5, 1932 1,964,048 Gerth June 26, 1934 2,102,410 Fyler Dec. 14, 1937 2,362,470 De Rosa Nov. 14, 1944 2,379,168 McClellan June 26, 1945 FOREIGN PATENTS Number Country Date 685,701 France Apr. 1, 1930

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