US2617884A - Coupling arrangement between aerial and transmission line - Google Patents

Description

Nov. 11, 1952 E. O. WILLOUGHBY COUPLING ARRANGEMENT BETWEEN AERIAL. AND TRANSMISSION LINE Filed Aug. 2, 1946 By M 0%.

Attorney Patented Nov. 11, 1952 COUPLING ARRANGEMENT BETWEEN AERIAL AND TRANSMISSION LINE Eric Osborne Willoughby, Aldwych, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application August 2, 1946, Serial No. 688,163 In Great Britain August 24,1945

1 Claim.

The present invention relates to electrical transforming arrangements for use mainly at ultra-high frequencies.

The principal object of the invention is to provide a very compact transforming arrangement which can give relatively large transformation ratios for matching circuits of different impedance at high frequencies. I

A secondary object is to provide means for matching antennae of various types to feeder transmission lines without necessitating the use of bulky equipment.

The invention in its broadest aspect provides an electrical transforming arrangement for a given impedance ratio, said arrangement comprising a conducting-surface means, first and second transmission line conductors arranged in spaced parallel relationship with each other and in spaced relation with said conducting-surface means, and means for connecting two corresponding ends of said conductors to form with said conductors a transmission line section shortcircuited at one end for high frequency currents and open-circuited at the other end, the length of said section being made such that the section is quarter wave resonant at the mean operating frequency, and said conductors being so made that the square of the admittance ratio Yl/YO is ,equal to said impedance ratio, where Y1 is the admittance of both said conductors together with respect to said conducting-surface means and Y is the admittance per unit length of said first conductor with respect to said conducting-surface means.

The invention will be illustrated in terms of a single embodiment which will be described with reference to the accompanying drawings, in which:

Figure 1 shows a longitudinal section of a simple form of a transformer referred to in the explanation of the invention;

Figures 2 and 3 show two alternative transverse sections, the transformer of Figure 1 illustrating two slightly differing formsof the central conductor;

Figures 4 and 5 show equivalent circuit diagrams of the transformer of Figures 1, 2 and 3;

Figure 6 shows a longitudinal view of transformer according to the invention with the screen sectioned in order to showthe arrangement of the interior; and

Figure'l shows a transverse sectional view of x-Figure 6.

Figures '1. and 2v show a. simple form of a-high zfr'equency transformenwhich forms a convenient reference in explanation of the invention. The transformer comprises a cylindrical metal case I closed at the lower end by a plate 2 and a cylindrical central conductor having a main portion 3 electrically connected at the lower endto the plate 2. The conductor also has a longitudinal chordal slot 4 cut therein which separates a tongue 5 from the main portion ,3, thus dividing the conductor intotwo portions of segmented section. The lower end of the tongue 5 is insulated from the base plate 2. The case I and central conductor are tapered down at the upper end in order that they may be continued as a co-axial transmission line, part of which is shown at, 6. A small hole 1 is provided in the base plate 2 to permit a connection to be made to the end of the tongue 5.

The isolated tongue or leg 5 form with the remainder of the conductor 3 a transmission line which is short-circuited at the upper'end. The length of the slot 4 should be so chosen that the transmission line just mentioned is quarter-wave resonant at the mean frequency of the band over which the transformer is designed to operate. It will be noted that the case I forms with the central conductor a transmission line which is short circuited at the lower end. This line should also be quarter-wave resonant at the mean operating frequency, and should preferably have a characteristic impedance equal to the terminal impedance of the transmission line 6. The load impedance to which the impedance of the transmission line is to be matched is not shown, but will be connected between the lower endof the tongue 5 and the case I.

Figure 3- shows in transverse section an alternative form of the central conductor .of the device. In this case it consists of a hollow. tube having a sector detached by means of two longitudinal radial slots 4A and 43 forming an isolated tongue 5. The remaining portion of the tube forms the main conductor 3 which is connected to the case I at the lower end as in Figure l. The lower end of the isolated tongue 5 is insulated from the case and access thereto is obtained through a hole similar to I. This form behaves electrically in substantially the same way as that shown in Figures 1 and 2.

Th manner in which the transformershown in Figures 1 and 2 or 3 operates will be understood I of Figure 1, and conductors 3 and 5 correspond respectively to the main portion 3 and isolated tongue 5 of the central conductor.

The conductors l, 3 and 5 form in pairs three transmission lines which are respectively indicated by the condensers a, b and connectin the correspondin pairs of conductors. 1

The transmission line A formed by the conductors l and 5 constitutes the connection between the impedances Z1 and Z2. The transmission line B formed by the conductors l and 3 upper end of the transformer should be so has one pair of terminals connected to the imtheright-hand terminals of the line A as shown inFigure 5. Thus as regards transmission from Z1 toZz, the impedances of B and C can be neglected.

The transmission line A connecting Z1 and Z2 being a quarter wavelength long, operates in the well known manner as an impedance inverting arrangement, so that the impedance seen from Z1 will be Zo /Z2, where Z0 is the characteristic impedance of the transmission line A. Let Z2=m Z1, then the requirement for impedance matching at the left-handend of the line A is Z1=,Zo /m Z1,

so that Zo=mZ1. 7

Now it has been said that the characteristic impedance Z1; M the transmission line formed by the whole of the central conductor and the case i should preferably be equal to Z1. Since the characteristic impedance of a transmission line (neglecting dissipation) is equal to /L/C, where L .and C are the inductance and capacity per unit length, the ratio Zk/ZO will be substantially equal to the inverse ratio of the corresponding capacities per unit length, because with cylindrical conductors of the kind shown in Figures 1 and 3, the

ratio of the inductances per unit length is sub- ;stantially equal to the inverse ratio of the corresponding capacities. It follows, therefore, that the ratio Zk/ZU will be substantially equal to the ratio'of the capacities per unit length to the case I of the conductor 5 alone, and of the conductors 3 and 5 together. Referring to Figure 3, if the slots 4A and 43 be assumed to be very narrow, then, the numbers of lines of force terminating respectively on the tongue 5 and on the portions 3 and 5 together will be substantially proportional to 0 and 211' respectively where 0 is the angle subtended at the centre of the cross section by the tongue 5 (Figure 3), and these numbers of lines are proportional to the corresponding capacities.

Thus if N is the total number of lines of force terminating on the conductors 3 and 5 together, and n is the number terminating on 5 alone, then approximately Zo/Zk=Zo/Z1=N/n=21r/6 But since from above Zo:mZ1 and Z2:m Z1, it follows that m=N/n=21r/0, and the impedance transformation rati0=m =N /n (21r/ 0) It is not essential according to the invention that there should be any case or screen such as I surrounding the slotted conductor 3. If there is no screen, the operation is substantially the same, the ground plane taking the place of the screen. More generally therefore, if Yo is the admittance arranged that the characteristic impedance at any point of the tapered portion is equal to that of the line B (namely Z1).

'In the above description, it has been stated. that certain transmission lines should be quarter wave resonant at the mean operating frequency. The actual physical length of such lines will oftenbe very nearly equal to one quarter of the corre-* sponding wavelength, but when, as is sometimes the case, the open end of the line is shunted by a stray capacity, or when a condenser is connected between the conductors of the line at the openend or at some other point, the physical lengthcorresponding to resonance may be appreciably,- less than a quarter of the wavelength. The essential point is that the physical length should be such that a maximum impedance approaching infinity will be measured at the open end of the line. The term quarter-wave resonant is to be understood in this sense throughout this specification.

Clearly, an added condenser may be used to time a given transmission line to quarter-wave resonance.

Figures 6 and 7 show a transforming device according to the invention. The conductors 3 and 5 of Figure 1 are replaced by six parallel conductors or wires 8 symmetrically arranged parallel to the axis of the case I in a circular plan. These wires are stretched between a plate 9, and the base plate 2, and a conical metal member IE, to which the plate 9 is fixed, tapers the arrangement down to the central conductor of the transmission line 6. One of the wires 8 is insulated from the plate 2 and may pass out through the hole 7, as shown, for connection to the impedance Z2. Any number of wires may, of course, be used and it is not essential that they be arranged in a symmetrical pattern. Further, more than one of them may be insulated from the plate 2 and may be taken out for connection to the impedance Z2. It follows from what has been said above, that when there are N wires symmetrically arranged, and n of them are connected to the impedance Z2, the transformation ratio m will be approximately equal to N /n While the impedance Z1 has been represented by a transmission line 6 in Figures 1 and 6, the invention is, of course, not limited to the use of any particular type of impedances for either Z1 or Z2.

It will be seen that in the arrangement of Figure 6, the main portion of the parallel wire cage structure is acting as a partial screen for the isolated conductor shown extending through aperture 1 so that the capacity to the case of this conductor is greatly reduced. This enables the impedance Z0 of the transmission line A to be made high without the necessity for a large and bulky construction. As is well known, the impedance of a simple co-axial transmission line cannot easily be made much greater than ohms, and if in Figure 1 the conductor '3 were absent, the transmission line A would have the usual low impedance less than 100 ohms.

It has been pointed out that by arranging the conductors 3 and 5 of the transformer (in any of the forms which have so far been described) so that one of them screens the other, a transmission line of relatively high characteristic impedance can be produced without the use of bulky apparatus. In the broadest sense, the two conductors may comprise any two groups of one or more conductors arranged so that they form between them a quarter-wave resonant transmission line short circuited at one end, so that at the open end, the two groups can be regarded as substantially insulated from one another at ultra high frequency.

It will be evident also that greater screening effect will be obtained in the embodiment according to Figs. 6 and 7 by forming any number of parallel wires connecting the plates 2 and 9 into a cage, and by providing inside the cage an additional wire connected to the plate 9 and passing out through a hole such as 7 for connection to the impedance Z2.

In the claim which follows, unless otherwise stated, when it is specified that circuits or conductors are connected together, or that a transmission line is short-circuited, or when similar statements are made, it is to be understood that the connection is an effective connection for high frequency waves and may be either a direct connection or through a blocking condenser or like element of negligible impedance.

What is claimed is:

An electrical transforming arrangement for coupling a first unbalanced circuit of impedance Z1 to'a second unbalanced circuit of larger impedance Z2, said arrangement comprising first and second transmission line conductors arranged in spaced parallel relationship, one of said conductors being constituted by a plurality of stretched conducting wires connected at their ends to form a cage structure, a hollow metal screen surrounding said conductors and in spaced parallel relationship therewith means for connecting two corresponding ends of said conductors to form therewith a transmission line section short-circuited at one end and open-circuited at the other, the length of said section being made such that said section is quarter wave resonant at the mean operating frequency, means at the open-circuited end of said first section for connecting the free end of said second conductor to said screen, means for connecting the short-circuited end of said section to one terminal of said first circuit, means for connecting the other terminal of said first circuit to said screen at a point adjacent said short-circuited end, means for connecting the free end of said first conductor to one terminal of said second circuit, and means for connecting the other terminal of said second circuit to said screen at a point adjacent said free end, said conductors being so made that the admittance Y1 per unit of the two conductors jointly with respect to said screen corresponds to a characteristic impedance equal to Z1, and that the admittance per unit length of said first conductor is equal to Yo, where the ratio Y1/ Y0 is equal to the square root of the impedance ratio Z2/Z1, where Z1 represents the terminal impedance of the transmission line and Z2 is the impedance to which Z1 is matched.

ERIC OSBORNE WILLOUGHB-Y.

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

UNITED STATES PATENTS Number Name Date 2,006,950 Jelleff July 2, 1935 2,249,963 Lindenblad July 22, 1941 2,275,342 Brown Mar. 3, 1942 2,287,220 Alford June 13, 1942 2,344,171 Rote Mar. 14, 1944 2,385,783 Alford Oct. 2, 1945 2,417,895 Wheeler Mar. 25, 1947 2,419,800 Tomlin Apr. 29, 1947 2,421,784 Haeseler et a1 June 10, 1947 2,426,236 Page Aug. 26, 1947 2,479,227 Gilbert Aug. 16, 1949

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