US2847670A - Impedance matching - Google Patents

Description

' IMPEDANCE MATCHING Filed Nqv. 12, 1953 7 2 Sheets-Sheet 1 Fvgl.

//Vl/ENTOR co, fmw

ATTORNEY Aug. 12, 1958 F. J. cox 2,847,670

IMPEDANCE MATCHING Fiied Nov. 12, 1953 2 Sheets-Sheet 2 Fig.3,

WM K/LWdiM ATTORNEY United States Patent IMPEDANCE MATCHING Frederickrlames Cox, Taplow, England, assignor to British" Telecommunications Research Limited, Taplow, England Application November 12, 1953, Serial No. 391,689

Claims priority, application Great Britain November 26,1952

6 Claims. (Cl. 343--821) The present invention relates to impedance matching in radio frequency circuits t and is concerned particularly but not exclusivelywith rnatching the impedance of a transmission line to the input impedance of an aerial. or aerial system.

The impedance of an aerial systemis sometimes substantially different from that of a transmission line for feeding power to the aerial system and hence to. ensure the optimum transfer of energy from the line into the aerial system it is necessary to employ an impedance transformer. Various impedance-transforming arrangements have been proposed, but these proposed arrangements have usually been such that their impedance transformation ratios cannot readily be varied.

The impedance of someaerial systems cannot always be calculatedwith accuracy and hence duringsinstallation it is necessary to vary either the impedance of the system in some way or other or totolerate some degree of mismatching between the aerial systemand the transmission line feeding the aerial system with consequent .loss of energy transfer.

For example in a conventional aerial system of the type known as a Yagi aerial. in which the fed element is folded, it is usual to match the impedance. of the aerial system to a. transmission line feeding it by varying the ratio of the. diameters of the two arms of the folded element. Such a procedure is sometimes not convenient.

According to the present invention a method of generating radio-frequency oscillations includes the steps of connecting a radio frequency oscillation source of output impedance Z to a load of input impedance Z through an impedance transformer comprising an outer cylindrical conductor and an inner conductor supported substantially coaxially within the outer conductor by means of spaced insulating members, and adjusting the impedance of the r a pr er t a alue su st t lv au N 2 g varying the mean dielectric constantsof the dielectr-ic between the inner and outerconductors. The mean dielectric constant of the dielectric between the inner and outer conductors is determined by the ratio or" the total volume of the insulating members to the total volume of the space between the inner and outer members and by the dielectric constant of the material from which the insulating members are formed. The mean. dielectric constant may be varied therefore by changing the number of insulating members employed, their spacing relatively to one another and by replacing one or more of the insulating members with one or more further insulating members of different dielectric constant. It will be appreciated that the impedance-transformer is in the form of'a. coaxial transmission line. The electrical length of the transmission line is varied in dependence upon the type of impedance presented by the load to the transformer. If theload presents a resistive impedance to the transformer the electrical :length of the transmission line is made rut/4. where n is an odd integer anda is the wave length of the oscillations lgenerated by thesource. If the imp an e pre n edlbnthe load is other 'thanresistive I ice 2 the electrical length of the transmission line is varied appropriately from nk/ 4 in known manner.

The invention also provides apparatus for carrying out the method according to the invention, the apparatus comprising. a. radio frequency oscillation source of output impedance Z connected to a load of input impedance Z through an impedance transformer whose impedance can be adjusted over a range including the" value /Z Z to enable the impedance of the load to be matched to that of the source, the impedance transformer comprising a tube of conducting material, and an inner conductor of substantially the same length as the tube supported substantially coaxially within the tube, the inner conductor being supported by spaced discs of insulating material, the planes of the discs being transverse of the longitudinal axis of the tube, each disc having an aperture therein through which the inner conductor is threaded, the dimensions of the apertures being such that the inner conductor is a push fit in the apertures, the diameters of the discs being such that the discs are a sliding fit in the tube, and the inner conductor being sufliciently rigid to ensure that when the inner conductor has the discs fitted theretoit can be slid into and out of the tube repeatedly without buckling. Thus the discs constitute the aforesaid insulating members and the impedance of the impedance transformer can readily be varied by sliding. the inner conductor with the discs out of the tube and removing or adding discs, the spacing of the discs being varied in accordance with the number of discs used and the required electrical length of the transformer; The transformation ratio may also be varied by using discs of different materials having different dielectric constants. It is preferred, however, to use identical discs of the same material whereby standard parts may be used. It will usually be preferred to use a rigid tube, but in some circumstances a tube having some degree of flexibility may be employed if practical considerations require it. By using discs which are a push fit on the inner conductor it is ensured that the spacing of the discs is not altered when the inner conductor with the discs thereon is slid into the tube.

According to a feature of the invention, the said source is in unbalanced form, the load is in balanced form and the impedance transformer is incorporated in an unbalance-to-balance transformer connected between the source and the load.

According to another feature of the invention the said sourceis in balance form, the load is in unbalanced form and the impedance transformer is incorporated in a balance-to-unbalance transformer connected between the load and the source.

The invention will nowbe described by way of example with reference to the accompanying drawings, in which Figure 1 is a diagrammatic cross-sectional elevation of one embodiment of the invention,

Figure 2 is a diagrammatic cross-sectional elevation of a second embodiment of the invention, and

Figure 3 is a diagrammatic cross-sectional elevation of athirdlembodiment of the invention.

In Figure 1 a dipole radiator 10 is fed from a coaxial transmission line 11 through an unbalancc-to-balance transformer 12 in which an impedance transformer 13 is incorporated. The transmission line 11 is of the type having a solid polyethylene dielectric and a characteristic impedance of ohms. The dipole radiator has an impedance between the inner ends of its two arms of less than 75 ohms.

The unbalance-to-balance transformer is of the socalled bazooka type and comprises a metal cylinder 14 having a length equal to M4, where A is the wavelength of the oscillations fed to the radiator from the line 11, and an extension of the line 11 also of a length equal to 1/4 arranged concentrically within and spaced from the cylinder 14. The extension of the line 11 is usually formed by a part of the line itself in known unbalance-to-balance transformers of the bazooka type, but in the present embodiment of the invention it includes the impedance transformer 13. The input end of the cylinder 14 is short-circuited to the outer conductor of the coaxial transmission line 11 by means of a shorting piece 15 in the form of a disc which completely closes the annular gap between the outer conductor of the coaxial transmission line and the input end of the cylinder 14.

The impedance transformer 13 comprises a rigid copper tube 16 and an inner conductor 17 supported coaxially within the tube 16 by means of discs 18 of insulating material such as polyethylene to form a coaxial transmission line. That part of the coaxial transmission line so formed lying between the input end and disc 19 is arranged to have a characteristic impedance of 75 ohms to match the characteristic impedance of the line 11 and forms no part of the impedance transformer when in operation. The impedance transformer is constituted by the section of the transmission line formed by the tube 16 and the inner conductor 17 between the disc 19 and disc 20. The number of discs used in this section and the spacing of the discs is made such that the electrical length of this section is equal to a quarter of a wavelength, as indicated by the dimension AM/ 4 in the drawing where M represents the factor by which the electrical length is greater than the physical length.

The discs supporting the inner conductor 17 have apertures therein through which the inner conductor is threaded and it is arranged that the inner conductor 17 is a firm push fit in these apertures. The outer diameters of the discs are made slightly less than the inner diameter of the tube 16 whereby the discs are an easy sliding fit in the tube 16. Any suitable means (not shown) are provided for enabling the transmission line 11 to be readily disconnected from the shorting piece 15 and the input end of the line formed by the tube 16 and the conductor 17. The transmission line 11 may for example be terminated by an adaptor which contains a socket for gripping the input end of the conductor 17 and a screwthreaded ferrule which makes a strong mechanical and a good electrical connection between the outer conductor of the transmission line 11 and the shorting piece 15. The tube 16 may be permanently connected to the shorting piece 15.

In order to ensure that the conductor 17 with the discs fitted thereto can be slid into and out of tube 16 repeatedly without the conductor 17 buckling, the conductor 17 is made of say No. 6 gauge copper wire.

In setting up the arrangement of Figure 1 for use the procedure may be somewhat as follows:

The arms of the dipole radiator are made slightly longer than is necessary and the lengths of these arms are first of all reduced until the radiator presents zero reactance between the inner ends of the two arms. The impedance between the inner ends of the two arms is then matched to the 75 ohm transmission line by varying the impedance provided by the impedance transformer 13 until a match is obtained. The impedance can be readily varied by removing the inner conductor 17 carrying the discs and either increasing or decreasing the number of discs and adjusting the spacing thereof accordingly.

Figure 2 shows a second embodiment in which the invention is employed in a feed to a dipole radiator. As in the embodiment of Figure 1 the variable impedance transformer is incorporated in an unbalance-to-balance transformer.

In the embodiment of Figure 2 the unbalance-tobalance transformer is of the type known as a Pawsey stub, and the coaxial cylinder of Figure 1 is replaced by a tube 14' disposed parallel with the tube 16 and having its input end short-circuited to the outer conductor of .4 the line 11. The method of setting upthe arrangement of Figure 2 may be the same as that already described with reference to Figure 1.

Figure 3 shows the coaxial transmission line 11 connected through an impedance transformer 13 to two further coaxial transmission lines 11' and 11" which are connected to two loads (not shown) each of which has an impedance approximately equal to the characteristic impedance of the line 11. The combined impedance of the two loads is therefore about half the impedance of the line 11 and the impedance transformer 13 is used to obtain a match. The impedance transformer is of the same construction as that already described with reference to Figure 1 and the impedance may be adjusted to the optimum value in the manner already described.

The two loads connected to the transmission lines 11 and 11 may be as shown for example in Figure 1 or Figure 2. Thus the arrangement of Figure 3 may be employed to ensure maximum transfer of energy from the line 11 to the lines 11 and 11", and the arrangement of Figure l or Figure 2 may be employed to ensure maximum transfer of energy into the dipole radiators.

The arrangements described are particularly suitable for use in feeding an aerial system such as an array of Yagi aerials, the fed elements of the Yagi aerials being the elements 10 in Figures 1 and 2. If the aerial system has two Yagi aerials the junction between the two feeders to the Yagi aerials may be arranged as shown in Figure 3. Where more than two Yagi aerials are used the arrangement of Figure 3 may be used with suitable modifications. For example if four Yagi aerials are used the transmission line 11 may be connected through a further arrangement as shown in Figure 3 to one pair of the Yagi aerials, and the transmission line 11" may be connected through a further arrangement as shown in Figure 3 to the other pair of Yagi aerials, the terminations at the Yagi aerials being as shown in Fig. 1 or 2.

Although the arrangements shown in Figures 1 and 2 are described as if energy is being fed from the line 11 to the aerial 10 it will be understood that the arrangements of these figures will also function in reverse, that is to say when the aerial 10 is being used for receiving oscillations which are required to be transmitted through the line 11 to a receiver. The cylinder 14 together with the extension 16 of the line 11 then functions as a balance-to-unbalance transformer. Similarly in Figure 2 the tube 14' and the extension 16 of the line 11 function as a balance-to-unbalance transformer when the aerial 10 is used for receiving oscillations to be transmitted through the line 11 to a receiver.

Iclaim:

1. Electric apparatus comprising a radio-frequency oscillation source of output impedance Z connected to a load of input impedance Z through an impedance transformer whose impedance can be adjusted over a range including the value /Z Z to enable the impedance of the load to be matched to that of the source, the impedance transformer comprising a tube of conducting material, and an inner conductor of substantially the same length as the tube supported substantially coaxially within the tube and being removable therefrom, the inner conductor being supported by spaced discs of insulating material, the planes of the discs being transverse of the longitudinal axis of the tube, each disc having an aperture therein through which the inner conductor is threaded, the diameters of the discs being such that the discs are a sliding fit in the tube, the inner conductor being sufficiently rigid to ensure that when the inner conductor has the discs fitted thereto it can be slid into and out of the tube repeatedly without buckling, and the dimensions of the apertures in said discs being such that the inner conductor is a push fit in the apertures to maintain said discs in set positions on said inner conductor during sliding thereof intoand out of said tube.

2. Apparatus according to claim 1, wherein the source is in unbalanced form and the load is in balanced form and the impedance transformer is embodied in an unbalance-to-balance transformer.

3. Apparatus according to claim 1, wherein the source is in balanced form and the load is in unbalanced form and the impedance transformer is included in a balanceto-unbalance transformer.

4. Electrical apparatus as claimed in claim 1, wherein said source comprises a coaxial cable connected to an oscillation generator, the said load comprises a dipole aerial, the said transformer has an electrical length of one-quarter wavelength, one end of said inner conductor being connected to one arm of the dipole and the corresponding end of the said tube is connected to the other arm of the dipole, the other end of the said tube and'inner conductor being extended to form a transmission line which is connected to the coaxial cable of the said source, a further tube surrounding the first said tube and extension thereof to form a further transmission line with the said tube and extension thereof, the further transmission line having an electrical length of one-quarter wavelength, and means to short-circuit the end of the said extension of the first said tube to the corresponding end of the second said tube.

5. Electrical apparatus as claimed in claim 1, wherein the said source comprises a coaxial cable connected to an oscillation generator, the said load comprises a dipole aerial, the said transformer has an electrical length of onequarter wavelength, one end of the said tube being conaxially withinthe tube and being removable therefrom,

the inner conductor being supported by flat discs of insulating material mounted in spaced relation on the inner conductor, the planes of the discs being transverse of the longitudinal axis of the tube, each disc having an aperture therein through which the inner conductor is threaded, the diameters of the discs being such that the discs are a sliding fit in the tube, the inner conductor being sufl'iciently rigid to ensure that when the inner conductor has the discs fitted thereto it can be slid into and out of the tube repeatedly without buckling, and the dimensions of the apertures in said discs being such that the inner conductor is a push fit in the apertures to maintain said discs in set positions on said inner conductor during sliding thereof into and out of said tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,510,358 Wolf June 6, 1950 2,518,665 Collard Aug. 15, 1950 FOREIGN PATENTS 572,739 Great Britain Oct. 22, 1945

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