US2395165A - High frequency transformer - Google Patents

Description

Feb- 19, 1946- J. COLLARD 2,395,165

HIGH FREQUENCY TRANSFORMER Filed NOV. 17, 1945 INV ENT OR.

ATTO/PNE Y Patented Feb. 19, 1946 HIGH FREQUENCY TRANSFORMER John Collard, London, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England Application November 17, 1943, Serial No. 510,637 In Great Britain June 3, 1941 3 Claims.

This invention relates to high frequency transformers for circuit arrangements operating at wavelengths of the order of a few centimeters. At frequencies corresponding to such short wavelengths the use of ordinary forms of iron or. air core transformers is impracticable due to the stray capacities which occur in such transformers and which act effectively as short circuits at such high frequencies.

The object of the present invention is to provide high frequency transforming means which will operate to transfer very high signal frequencies with minimum loss of power.

According to the present invention a high frequency transformer consists of a length of line substantially half a wavelength'long at the operatingfrequency and comprising outer and inner conductors short-circuited at their ends by conducting members through which pass conductors forming loops capable of being tuned and constituting coupling elements between which transformation is to be effected. The outer conductor may be of cylindrical form and may contain a plurality of inner conductors and the tuned loops are preferably capable of adjustment in relation to the outer and inner conductors.

In order that the invention may be more clearly understood and readily carried into effect, one form of high frequency transformer embodying the invention and certain modifications thereof will now be described in greater detail by way of example with reference to the drawing filed with the present specification, in which:

Fig. 1 is a side elevation in section of such a transformer;

Fig. 2 is an end elevation;

Fig. 3 shows the electrical equivalentlof the construction shown in Figs. 1 and 2; and

Figs. 4 and 5 are alternative networks equivalent to Fig. 3 under certain tuning conditions.

Referring to Figs. 1 and 2 of the drawing, a transformer is shown consisting of a hollow outer conductor or screen 0 one-half wavelength long and two inner conductors RI, R2, The ends of the conductors C, RI and R2 are short-circuited by end plates formed with central holes through which lengths of coaxial cable CC are introduced. The ends of the cables terminate in loops Ll, L2, each of which has a certain mutual coupling to the twin circuit formed by the inner and outer conductors, the magnitude of the coupling depending on the extent to which the loops protrude into the circult and also upon the angle a between the plane of the loops and that of the two conductors Rl, R2. The arrangement may, for the purpose of explanation, be considered as a three-winding transformer, as shown in Fig. 3.

In Fig. 1 it will be seen that the loops Ll, L2 are shown split so that a series capacity, represented by the condensers Cl, C2 in Fig. 3, is introduced into each loop. The self-inductance of the loops is represented by the coils SI, S2 in Fig. 3 and the mutual inductances between the loops and the twin circuit formed by the conductors C, RI are represented at Ml, M2. The twin circuit is represented as a resonant circuit having an inductance P, capacity K and resistance R.

One loop is assumed to be connected to a resistance Z2 and the impedance measured at the other loop is represented by Zl. It is further assumed that the gaps in the loops forming the capacities Cl and C2 are adjusted so that these capacities tune out the inductances represented bySl and S2, respectively. Similarly, it is assumed that the capacity K tunes out the inductance P. Under such conditions, the circuit may be represented as in Fig. 4 from which it will be clear that the value Zl is given y w MI ZZ R Z2+o=M2 Hence, if R is made small, the expression becomes ag e Zl Ml,

If WI represents the power fed into the loop Ll, equal to il Zl and W2 represents the power obtainable from loop 2, equal to 2 Z2, then it can be shown that Hence, if

R Z2 9M2 is small compared to unity, there will be no appreciable power lost in the transformer. The requirements for an eflicient transference of power are thus that It must be made as small as possible and the coupling as tight as possible.

Since the transformer described is a half wavelength of line rather than a simple resonant circuit, it may more accurately be represented as shown in Fig. 4, in which the network A, A, 3 represents the half wavelength of line. In general A=Z tanh 5 and so that when the line is half a wavelength long any impedance applied across one end of it can be considered as being transferred unaltered to the other end. This of course ignores the resistance of the line and in this case it will be obvious as before that The effect of the resistance in this case is to produce an attenuation term and in order to reduce the attenuation to a minimum it is clearly necessary to choose the optimum dimensions and spacing of the conductors in accordance with well known practice.

In the constructional example described, use is made of two inner conductors since, for mechanical reasons, it is sometimes desirable to have the loops central. Alternatively, a single central conductor could be used and the. loops placed oif center. Again, four equally spaced conductors may be used with the object of reducing the attenuation.

The dimension of half a wavelength for the length of the conductor C must be accurately chosen in order to give resonance at the desired frequency. If the dimension is not accurate, some form of tuning device can be provided.

The tuning of the loops is effected externally of the outer conductor C and their capacity and inductance are adjusted until the impedance of each loop is found to be substantially a short circuit.

Since, in certain circumstances it is desirable to keep the physical dimensions of the transformer to a minimum, the space inside the conductor C may be filled with a material having a high dielectric constant and a low loss. If the dielectric constant of the filler is, for example, four, the half wavelength dimension will be half of what would be required with air as the dielectrio.

In order to keep the loss of the transformer as low as possible, the coupling must be made close and it is an advantage to fix one loop at its maximum coupling and to adjust the other loop to obtain the desired transformation ratio.

While the loops LI and L2 are shown in Fig. l as unbalanced, that is to say, one side connected to earth, balanced loops may be used or one loop may be balanced and the other unbalanced.

The high frequency transformer device described may be used to produce a variable resistance at the high frequencies corresponding to centimeter wavelengths. It is difilcult to produce a reliable resistance apart from that given by an electrically long length of cable. Such a resistance is fixed, however, but by connecting it to one of the loops and varying the coupling between that loop and the twin circuit, a variable resistance can be produced at the other loop. An accurate attenuator is thus provided.

What is claimed is:

1. A high frequency transformer consisting of a length of line substantially half a wavelength long at the operating frequency and comprising an outer conductor and a pair of spaced parallel inner conductors, all of said conductors being connected together at their ends by conducting end members through which pass conductors forming loops constituting coupling elements between which transformation may be effected, said loops being located between said inner conductors.

2. A high frequency transformer consisting of a length of line substantially half a wave length long at the operating frequency and comprising an outer conductor and a pair of spaced parallel inner conductors, all of said conductors being connected together at their ends by conducting end members through which pass conductors forming loops capable of being tuned and constituting coupling elements between which transformation may be effected, said loops being lo-- cated between said inner conductors, each of said loops being split within said outer conductor to form a condenser, said condensers having such values as to tune out the inductances of said loops.

3; A high frequency transformer consisting of a length of line substantially half a Wavelength long at the operating frequency and comprising an outer conductor and a pair of spaced parallel inner conductors, all of said conductors being connected together at their ends by conducting end members through which pass conductors forming loops capable of being tuned and constituting coupling elements between which transformation may be eifected, said loops being located between said inner conductors, said conductors which form said loops constituting extensions of the inner conductors of coaxial lines, each of said loops being split within said outer conductor to form a condenser, said condensers having such values as to tune out the inductances of said loops.

JOHN COLLARD.

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