US2897460A - Transmission-line impedance-matching apparatus - Google Patents

Description

July 28, 1959 R. LA ROSA 2,897,460

TRANSMISSIONLINE IMPEDANCE-MATCHING APPARATUS Filed June 25, 1954 SIGNAL SOU ROE LOAD SOURCE iwoa #2511 aob j25b LOAD.

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TRANSMISSION-LINE EDANCE-MATCHING APPARATUS 'Richard La Rosa, 'Levittown, N.Y., assignor to Hazeltine Research, Inc., Chicago, 111., a corporation of Illinois Application June 25, 1954,8eriai No. 439,285

9 Claims. (Cl. 33333) General This invention relates to transmission-line impedance- "matching apparatus and is particularly useful with transmission lines of the conductor-aboveground-plane'or socalled strip-line type.

Various types of adjustable impedance-matching devices for use with transmission lines have been heretofore proposed. Some of these utilize sections of short-circuite'd transmission line connected across the maintransmission line for introducing the proper susceptance in the main transmission line to achieve the desired impedance matching. The susceptance introduced by such devices is determined by the position of a shorting bar, thewposi-' tion of which is adjusted by means of aplunger. Other 'types of impedance-matching devices utilize slidable "metallic slugs to' introduce susceptance at the desired points along the transmission line. Still other impedance- 'matching' devices utilize metallic stubs, the depth of insertionof which into the electric field of the transmission line determines the impedance-matching susceptance intro'duced. The mechanical motion required 'for-adjust- ""ment' of these previously proposed devices "is" frequently inconvenient for smooth adjustment, slight adjustment, or "control-panel operation of the devices. 'Also, t-he range "of impedance which may be matched is often more limited than is desirable.

"It isan object -of 'the invention; therefore, to provide a newand improved transmission 'line-impedance-match- 'ing apparatus whichavoids one or more of the foregoing limitations of such devices heretofore proposed.

"It is another objectof the invention toprovide' a new and improved transmission-line impedance-matching apparatus having convenient rotary motion of' adju'stment. It is a further object-ofthe invention to'prov-idea' new and improved transrnission line impedance-matching apparatus'ofrelatively simple eonstruction=for matchin'gan "impedance of any value. 1

It is an additional object of the invention to provide a new and improved transmission-line impedancevmatching -apparatus' for a conductor-above-ground-plane transmission line.

*lnaccordance with the invention, --a -=transmission line impedance-matching apparatus comprises a transmission 'linefor translating a signal of predetermined operating frequency and impedance-matching means including. a single "rotatable "element 'for introducing a variable susceptance' at*either of .only two distinct pointsulocated substantially an odd number ofon'e-quarter wave len'gths-apartalong thetransmission line at the operating frequency thereof. 'The-impedance-matching. apparatus also'includes meansfor rotating the element about afixed point adjacent tothe transmission line or selecting one'of "sa'id-two-points' for the introduction 'of the variable susceptance, thereby to double the range of susceptance variation at eitherof the pointsalong the transmission line.

For a" better understanding of the present invention, together with' other and further-objects --thereof,'-.reference is -'had-tcf -'the following description taken 1 in.'.connection I transmission line.

with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring to the drawing:

Fig. 1 is a circuit diagram, partly schematic, of a transmission-line impedance-matching apparatus in accordance Description of impedance-matching apparatus of Fig. '1

Referring to Fig. 1 of the drawing, the transmissionline impedance-matching apparatuslt) there represented comprises a transmission line 12 which maycomprise, for

example, a transmission-line conductor 13 positioned above an extended conductive surface or ground plane 14. The conductive surface 14 maybe covered with a layer of insulating material 15 and the transmission-line conductor may consist of, for example, a strip of copper foil laid on the insulating material '15. One end of'the transmission line is coupled to a signal source 16 while the other end is coupled to a load 18.

Theimpedance-matching apparatus 10 also. includes a rotatable element for introducing a variable susceptance at either of two distinct points located substantially .an odd number of one-quarter wave lengths apart along the The rotatable element may be, 'for example, a segment ofcurved transmission lineLZfi having a first end'Zila for introducing a variable susceptance at one point along the transmission line .anda second end 20b for introducingsusceptance at another =pointalong the"'transmission line one-quarter of a wave Iength'removed from the: first point. The curvedtransmission-line and oneend of the segment of-curved transmission line .20 to introduce susceptanceaatapoint.along the transmis- 'sion:line.

The impedance-matching"apparatus 10 :further includes -means for rotating the element ZG-about a fixed-;point adjacent-to the :transmissionline to introducesusceptance at either of the two points,- thereby to doublet-he range of su'sceptance variation at either of thesepoints alongthe transmission line. The means for rotatingthe element -20"rnay: include-a sector-shaped piece of insulatingfimaterial 21 connected to a shaft indicated diagrammatically by the dashed line22 which in turn is connected to a controlknoblli. The sector-shaped pieceZLis-mounted in azplane slightly above the plane of -t'hetransmission-line .COndllCiOI' 13"SO"illat the piece, 21 may slide. thereover. --Where the element 20 is mounted on 1 top of the sectorshapedzpieee 21,:thersector-shapedpiece 21 is preferably of minimum thickness to aifordmaximum capacitance venient, the element ZZO may-be mounted on the underside ductor 13 when the two=-are overlapped. If more cmof a thicker sector-shaped piece 21 and suitablyi-nsulated fromthetransmission-line conductor 13 by athinlayer of insulating material. The vcenterline of the 1shaft 22 represents the fixed point adjacent to the transmissionline about-whichthe. element 20 rotates.

Since one element may not provide impedance matchingover a sufliciently wide. range of values for some applications, the impedance-matching apparatus, .10 v.preferablyincludes asecond rotatable elementNZS mounted .on: asuitablemeans-Zfi for rotating the element-abont a fixed point adjacent to the transmission line. The rotating means 26 is a piece of sector-shaped insulating material which is connected to a shaft indicated by the dashed line 27 which in turn is coupled to a second control knob 28. The fixed point about which the rotatable element 25 rotates preferably is spaced by substantially an odd number of one-eighth wave lengths from the fixed point about which the element 20 rotates to enable rotation of the elements 20 and 25 to match the impedance on one side of the elements to the impedance on the other side of the elements, for reasons more fully explained subsequently.

Operation of impedance-matching apparatus of Fig. 1

Considering the operation of the transmission-line impedance-matching apparatus just described and assuming for the moment that the impedance of the load 18 attached to one end of the transmission line does not match the characteristic impedance of the transmission line, part of the signal energy that is originally transmitted down the line towards the load is reflected back towards the input end of the transmission line. As a result, maximum energy is not transferred to the load. Also, a standing wave is produced along the transmission line which causes additional energy loss through energy dissipation in the resistance of the transmission line. Associated with the standing wave, there is an apparent impedance or admittance variation along the transmission line, that is, the mismatched load may be. thought of as reflecting impedance or admittance variations back down the line, the impedance or admittance variations being similar to the voltage standing wave. As it is more convenient to speak in terms of admittance, rather than impedance, such will be done, it being remembered that admittance is simply the reciprocal of impedance.

Admittance reflection back down the transmission line may be prevented by introducing susceptance across the line at the proper point, provided the sign of the introduced susceptance is of opposite polarity to the sign of the susceptance component of the reflected admittance. In this manner, the introduced susceptance cancels out the reflected susceptance component. The susceptance is properly introduced at a point where the conductive component of the reflected admittance corresponds to the characteristic admittance of the transmission line. Thus, looking down the transmission line from the input end, the admittance presented by the line and load on the other side of the point at which the susceptance is introduced appears to match the characteristic admittance of the transmission line. As a result, maximum energy is transferred past this point and no energy is reflected back past this point. Standing waves still exist between this point where the susceptance is introduced and the load, but the losses in the line are small and substantially all of the energy is dissipated in the load.

It is frequently inconvenient to introduce susceptance at a specific distance along the transmission line from the -load. An alternative method is to introduce susceptance at two arbitrary points along the transmission line. For this method, the susceptances need not be introduced at any particular location with respect to the load. The susceptance introduced at one point causes the conductive component of the reflected admittance at the second point to be equal to the characteristic admittance of the transmission line. The susceptance introduced at the second point is then adjusted to be of such magnitude and polarity as to cancel the susceptance component of the reflected admittance.

Where susceptance is introduced at two arbitrary points, the most effective spacing between these points is an odd number of one-eighth wave lengths. Spacing at because then the susceptances are eifectively in parallel,

' even multiples of one-eighth wave length is undesirable,

The same difficulty arises if the susceptances are introduced at closely spaced points.

Referring to Figs. 1 and 2, the effect of positioning the transmission-line conductor or strip 13 above the ground plane 14 results in the effective positioning of an imaginary conductor or strip, represented by the dotted strip 13a of Fig. 2, an equal distance away from the ground plane 14 on the other side thereof. That is, the electric field configuration between strip 13 and ground plane 14 is the same as though a parallel conductor 13a were present and the ground plane 14 were removed. Thus, the strip transmission line is electrically equivalent to a two-wire transmission line. The same consideration holds for the curved one-quarter wave-length sections of strip lines 20 and 25.

Considering the curved segment 20, the segment is preferably one-quarter of a wave length long and is opencircuited at either end 28a or 2012. Considering for the moment 20a as the input end and 20b as the output end, then looking into the section of the line 20 at the input end 20a the section of line 20 behaves like a seriesresonant circuit near its resonant frequency. This is represented by the electrically equivalent resonant circuit 20 of Fig. 2. Because the circuit 20 is at resonance, it presents very nearly a zero impedance with respect to the ground plane. Now, as the line section 20 is rotated so that the end 20a comes close to the strip 13, the capacitance between the strip 13 and the line section 20 is increased. This coupling capacitance is represented by its electrical equivalent 20a in Fig. 2. Because the impedance of the one-quarter wave-length line section 20 is negligible, this coupling capacitance represented by 20a is effectively shunted between the conductor 13 and the ground plane 14. In this manner, capacitative reactance or positive susceptance is introduced across the transmission line at the point where the end 20a of the line section 20 overlaps the primary conductor strip 13.

Fig. 3 shows the electrical equivalent of the transmis' sion line and the various capacities introduced thereacross by rotation of the one-quarter wave line sections 20 and 25. One end of the line section 20 introduces capacitance represented diagrammatically by condenser 20a of Fig. 3, while the other end 20b introduces capacitance rep resented diagrammatically by condenser 20b of Fig. 3. These two condensers are shown as being ganged, as represented by broken line 50, because they are controlled by the same mechanical element. It will be understood that when the line section 20 is rotated so that the end 20a introduces capacitance at the point where this end overlaps the primary strip conductor 13, then substantially no capacitance exists between the end 2% and the primary conductor 13. Similarly, when capacitance is introduced by the end 20b, then substantially no capacitance exists between the end 20a and the conductor 13. These same considerations hold for the capacities introduced by the ends 25a and 25b of the line section 25.

As indicated by Fig. 3, the point at which capacitance is introduced by the end 20a of the line section 20 is located one-quarter of a wave length from the point at which capacitance is introduced by the end 20b. As a result, when capacitance is introduced by the end 20a, it may be considered instead that inductance is effectively introduced at the point spaced therefrom by one-quarter wave length and occupied by condenser 20b of Fig. 3. Thus, for the complete range of capacitance variation available at the point of condenser 20a, there effectively is a corresponding range of inductance variation available at the point of condenser 20b. Similarly, when the line section 20 is rotated so that capacitance is introduced at the point of condenser 20]), then inductance may be considered as effectively introduced at the point of condenser 20a. In this manner, at the point of either of the condensers 2% or 200, there is a complete range of capacitance and inductance variation. Accordingly, the range of susceptance variation at either of the points is double What it would be were capacitance introduced at only one of the points. Because there is a complete rangefor both inductive and capacitat'ive susceptance variation and because these susceptance variations occur at'two electrically different locations along the transmission line, 1

a single impedance-matching element such as line section 20, constructed in accordance with the present invention, has improved impedance-matching capabilities.

Where two impedance-matching elements are used,

such as the line sections and 25, susceptance may be introduced at two different points at the same time in order to enable the impedance-matching elements to'match the transmission line to the load impedance irrespective 'of the location of the matching elements with respect to the load. As mentioned, the two susceptances preferably are:

introduced at points separated by substantially an odd number of one-eighth wavelengths. This requirement is fulfilled if the fixed points about which the elements 20 and rotate are separated by one-eighth of a wave length. Thus, the center points of the shafts 22 and 27, indicated by the dashed lines of Fig. 1, preferably are spaced by substantially an odd number of one-eighth wave lengths along the transmission line.

From the foregoing description of the invention, it

-will be apparent that transmission-lineimpedance match-ilms ing apparatus constructed in accordance with the present invention is apparatus of relatively simple construction for matching an impedance of any value and is particularly useful with transmission lines of the conductor-aboveground-plane type.

While there has been described what is 'at'present considered to be the preferred embodiment of this invention, it will be obvious to'those skilled in the art that various changes and modifications may be made thereinwithout departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim is:

1. Strip-transmission-line in1pedance-matching apparatus comprising: a strip transmission line for translating a signal of predetermined operating frequency; impedancematching means for individually introducing susceptance at one or the other of two distinct points along the main transmission line, said means including a pair of curved approximately one-quarter wavelength sections of strip line at the operating frequency thereof; and a pair of means for individually rotating the one-quarter wavelength sections of strip line about fixed points adjacent to the main transmission line to match the impedance on one side of the one-quarter wave-length sections to the impedance on the other side of the one-quarter wavelength sections.

2. Strip-transmission-line impedance-matching apparatus comprising: a strip transmission line comprising a primary conductive strip positioned adjacent an extended conductive surface for translating a signal of predetermined operating frequency; impedance-matching means for introducing susceptance at one or the other of two distinct points along the main transmission line, said means including a pair of curved approximately one-quarter wavelength sections of strip line at the operating frequency thereof positioned adjacent said conductive surface for varying the capacitance between said primary conductive strip and one end of each of said curved sections of strip line and providing negligible impedance between each of said curved sections and said conductive surface; and a pair of means for individually rotating the curved sections of strip line about fixed points adjacent to the primary conductive strip to match the impedance on one side of the one-quarter wave-length sections to the impedance on the other side of the one-quarter wave-length sections.

3. Transmission-line impedance-matching apparatus comprising: a transmission line for translating a signal of predetermined operating frequency impedance-matching means including; a-single rotatable element "foiifv troducing a variable-susceptanceat either ofonly two distinct points located substantially an odd number of one-quarter -wave lengths apart along the transmission line at the operating frequency thereof; and means for rotating said :element about a fixed point adjacent to said transmission line for selecting one of said two points for the'introduction of said variable susceptance, thereby to double the range of susceptance variation at either of said points along said transmission line.

4.-Transrnission-line impedance-matching apparatus comprising: .a primary transmission line for translating a signal of predetermined'operating frequency impedance-matching means including; a rotatable segmentof curved transmission line for introducing a variable susceptance at either of two distinct points located substantially an odd-number of one-quarter wave lengths apart along the primary transmission line at'the operating frequency thereof; and means for rotatingsaid-segrnent ofcurved-transmission line-about a fixed point adjacent tosaid-primary transmission line to introduce'suscept- -ance at either of said two points, thereby to double the range of susceptance variation at either of said points along said primary transmission line.

5. Transmission-line impedance-matching apparatus -comprisingz-aprimary transmission line for translating a signal of predeterminedoperating frequency impedancematching means including; 'a rotatable approximately one-quarter wave-length segment of curved open-circuit transmission line for introducing a variable susceptance at either of two distinct points located substantially an odd' number of one-quarter wave lengths apart alongthe vprimary transmission line at the operating frequency thereof; and means forrota-ting-said segment of curved transmission line about a fixed point adjacent to said primary transmission line to introduce susceptance at either of said two points, thereby to double the range of susceptance variation at either of said points along said primary transmission line.

6. Transmission-line impedance-matching apparatus comprising: a strip line type of transmission line comprising a transmission-line conductor positioned adjacent an extended conductive surface for translating a signal of predetermined operating frequency; impedancematching means for introducing a variable susceptance at either of only two distinct points located substantially an odd number of one-quarter wave lengths apart along the transmission line at the operating frequency thereof, said means including a single rotatable element positioned adjacent said conductive surface to form a condenser therewith for varying the capacitance between said transmission-line conductor and said element and providing negligible impedance between said element and said conductive surface; and means for rotating said element about a fixed point adjacent to said transmission line for selecting one of said two points for the intro duction of said variable susceptance, thereby to double the range of susceptance variation at either of said points along said transmission line.

7. Transmission-line impedance-matching apparatus comprising: a strip line type of transmission line comprising a primary transmission-line conductor positioned adjacent an extended conductive surface for translating a signal of predetermined operating frequency; impedance-matching means for introducing a variable susceptance at either of two distinct points located substantially an odd number of one-quarter wave lengths apart along the transmission line at the operating frequency thereof, said means including a rotatable approximately one-quarter wave-length signal of curved transmissionline conductor positioned adjacent said conductive surface to form a condenser therewith for varying the capacitance between said primary transmission-line conductor and one end of said segment of said curved transmission-line conductor and providing negligible impedance between said segment of said curved conductor and said conductive surface; and means for rotating said segment of curved conductor about a fixed point adjacent to said transmission line to introduce susceptance at either of said two points, thereby to double the range of susceptance variation at either of said points along said transmission line.

8. Transmission-line impedance-matching apparatus comprising: a transmission line for translating a signal of predetermined operating frequency; impedance-matching means for introducing a variable susceptance at two distinct points located substantially an odd number of one-quarter wave lengths apart along the transmission line at the operating frequency thereof, said means including first and second rotatable elements coupled to the transmission line to form condensers therewith; and first and second means for individually rotating the elements about individual fixed points adjacent to the transmission line and spaced by substantially an odd number of one-eighth wave lengths at the operating frequency thereof, thereby to enable rotation of the elements to match the impedance at a point along the transmission line on one side of the elements to the impedance at a point along the transmission line on the other side of the elements.

9. Transmission-line impedance-matching apparatus comprising: a strip line type of transmission line comprising a primary transmission-line conductor positioned adjacent an extended conductive surface for translating a signal of predetermined operating frequency; impedance-matching means for introducing a variable susceptance at two distinct points located substantially an odd number of one-quarter wave lengths apart along the transmission line at the operating frequency thereof,

said means including first and second rotatable approxi- 1 mately one-quarter wave-length segments of curved transmission-line conductor positioned adjacent said conductive surface to form a condenser therewith for varying "the capacitance between said primary transmission-line conductor and providing negligible impedance between each of said segments of curved conductor and said conductive surface; and first and second means for individually rotating the segments of curved conductor about individualfixed points adjacent to the primary trans- '1nission-line conductor and spaced by substantially an odd number of one-eighth wave lengths at the operating frequency thereof, thereby to enable'rotation of the segments of curved conductor to match the impedance at a point along the transmission line on one side of the curved segments to the impedance at a point along the transmission line on the other side of the curvedsegments.

References Cited in the file of this patent UNITED STATES PATENTS 2,246,928 Schick June 24, 1941 2,424,982 Higgins et al. Aug. 5, 1947 2,427,100 Kihn Sept. 9, 1947 2,556,001 j Robertson June 5, 1951 2,757,344 Kostriza et al. July 31, 1956 OTHER REFERENCES Proceedings of the I.R.E.," vol. 40, pages 1644-1650, December 1952.

. Microwave Transmission Circuits, vol. 9 of Radiation Laboratory Series. Copyright May 21, 1948. McGraw- Hill Book Co.; pages 472-478 and 498-512.

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