US2920323A - Broad-band impedance matching - Google Patents

Description

Jan. 5, 1960 P. M. DUNSON 2,920,323

BROAD-BAND IMPEDANCE MATCHING /9a /70 I 25 25 /340 360 /4a l 240 3/0 320 33 I617 \2067 2557a) 5 I I 0 ml? INVENTOR.

Philip M. Dunson .8) F 5 E m ATTORNEYS.

Jan. 5, 1960 P. M. DUNSQN f 2,920,323

BROAD-BAND IMPEDANCE MATCHING Filed Sept. 16, 1953 2 Sheets-Sheet 2 4 v Flg 3 40 High f X x V5WR=3-7 Lowf 40 Highf X 43 X g 44 Lowf Lowf I I I l I I "2 l INVENTOR.

Philip M. Dunson da 7&1 W

ATTORNEYS.

United States Patent BROAD-BAND IMPEDANCE MATCHING Philip M. Dunson, Columbus, Ohio Application September 16, 1953, Serial No. 380,444 6 Claims. (Cl. 343850) This invent-ion relates to broad-band impedance matching. It has to do, more particularly, with means for providing impedance matching over a wide band of frequencies between a radio-frequency device, such as a transmitter or receiver, having substantially a constant impedance over the desired frequency range and another radio-frequency device, such as an antenna, having an impedance that varies materially with frequency over the desired band.

Impedance-matching circuits according to the present invention Were developed primarily to provide voltage standing wave ratios not greater than 3 to 1 in a transmission line between a whip-type antenna and a transmitter or receiver over a frequency range of approximately 2.2 to 1 (such as from megacycles to 22 megacycles, also commonly referred to as 120% band width). A whip-type antenna .was specified because of its lightness and reasonably low drag when used on a vehicle or aircraft. The vehicle or aircraft provides a ground plane from which the whip antenna protrudes and extends substantially perpendicularly. I

It is an object of the present invention to provide an impedance-matching device that is light in weight, compact, and inexpensive. An object of the impedancematching device is to provide good power transfer and low transmission-line losses over a wide band by providing voltage standing wave ratios not greater than about 3 to 1 over a wide frequency band (at least approximately 2.2 to 1). It is a further object to avoid the use of power-consuming components in the impedancematching device.

To obtain good broad-band impedance characteristics in an antenna system, it has been customary to develop the antenna itself so as to have a desirable impedance characteristic. For a given frequency range, this usually requires a more cumbersome, heavier antenna than a whip, which would produce considerably more drag than a whip antenna, except in the case of flush-mounted antennas, which have no drag but are generally heavy. Desired standing wave-ratio characteristics can be obtained by the use of circuits including resistors, but the power dissipated in the resistors reduces the effectiveness of the radiating system, sometimes substantially nullifying the beneficial elfects of the improved impedance match. Many' antenna systems generally termed broad band operate over considerably narrower bands than that for which the present equipment was developed. Lower standing wave ratios have been obtained for narrower band operation; the narrower the band, the lower the standing wave ratios obtainable in general. As far as was known at the time of the present development, the results sought' had not been obtained in any other way with a light, low-drag antenna, such as a whip.

In known antenna systems, the antenna itself generally is heavier or has more drag than a whip antenna, or

preferably in 2,920,323 Patented Jan. 5, 1960 both, for the same general frequency band. Some impedance-matching devices involve loss of power, while other impedance-matching devices using only transmission-line elements do not provide low standing wave ratios with a whip-type antenna or equivalent over such broad band widths as provided by the present equipment. The present invention overcomes these disadvantages by providing a novel circuit combination comprising a capacitor in parallel with the antenna, a short, high-impedance transmission-line section connected to the antenna as an impedance transformer, and at least one series-resonant circuit connected in parallel with the transmission line at the end of the impedance-transformer section away from the antenna.

It is a primary object of this invention, therefore, to provide impedance-matching circuits comprising as many of the elements of the above-described circuit combination as are required to provide broad-band impedance matching of various antennasand other radio-frequency devices, in accordance with the disclosure herein.

Other objects and advantages will be apparent from the following detailed description thereof.

In the drawing:

Fig. 1 is a sectional view, largely schematic, of a preferred embodiment of the present invention as developed for use with a whip-type antenna operating against a finite ground plane;

Fig. 2 is a resistance-reactance diagram in rectangular coordinates illustrating a typical impedance characteristic curve of a whip-type antenna;

Fig. 3 is a similar resistance-reactance diagram illustrating the impedance curve of Fig. 2 as modified by ineluding a shunt capacitor in parallel with the base of the antenna as in Fig. 1;

Fig. 4 is a similar diagram illustrating the impedance curve of Fig. 3 as further modified by a short high-impedance transmissionine impedance transformer section connected as shown in Fig. 1;-

Fig. 5 is a similar diagram illustrating the impedance curve of Fig. 4 as still further modified by series-resonant circuits connected in the impedance-matching circuit as shown in Fig. 1;

Fig. 6 is a graph in rectangular coordinates of reactance and susceptance of the series-resonant circuits of Fig. 1 against frequency; 1

Fig. 7 is a schematic diagram of an unbalanced antenna and impedance-matching circuit of the type shown in Fig. 1, and

Fig. 8 is a schematic diagram of a balanced antenna and impedance-matching circuit according to this invention.

Referring to Fig. 1, which illustrates a preferred embodiment of the invention as developed for use with a whip antenna operating against a finite ground plane, a whip antenna 10 is provided with a metal (conductive) base-support member 11 and a metal (conductive) basemounting member 12 spaced therefrom by a base insulator 13 of suitable dielectric material. The support member 11, insulator 13, and mounting member 12 toport means for the antenna 10. The mounting member 12 is both mechanically and electrically connected to a.

metal (conductive) matically at 16. p

The inner conductor 17 of a coaxial transmissionline impedance transformer section 18 is connected to the metal base-support member 11 of the antenna 10 the center as is indicated at 19, and the outer conductor 20 of this transmission-line section 18 is ground plane 15 asis indicated scheconnected to the metal base-mounting member 12 and thereby to the ground plane 15 as is indicated at 16. This transformer section 18 comprises a transmission line preferably having a characteristic impedance of at least about 2.34 times the impedance to which the antenna system is to be connected, and an effective length of approximately 0.067 wave length at the low-frequency end of the operating band and approximately 0145 Wave length at the high-frequency end of the operating band where the band width is approximately 2.2 to 1. At the end of the transmission-line section 18 away from the antenna 11 the outer conductor 20 is connected, as is indicated at '21, to a metal (conductive) shield box 22 containing a first series-resonant circuit 23 comprising an inductance 24 and a capacitance 25 in series between the center conductor 17 of the transmission-line system, as is indicated at 26, and the metal shield 22, as is indicated at 27, and resonant just below the low-frequency end of the operating band, as from about 1% to about 8% below the low-frequency end of the band. Also connected between the center conductor 17, as is indicated at 28, and the metal shield 22, as is indicated at 29, is a second series-resonant circuit 30 comprising an inductance 31 and a capacitance 32 in series, and resonant just above the high-frequency end of the band, as from about 1% to about 8% above the high-frequency end of the band.

The outer conductor 33 of a coaxial transmission line 34, having a characteristic impedance substantially equal to the impedance presented by the equipment with which the antenna system is to be employed is connected to the metal shield can 22, as is indicated at 35, and the inner conductor 36 of this transmission line 34 is connected inside the metal shield 22, as is indicated at 28, to the inner conductor 17 of the transmission-line impedancetransformer section 18. The other end 37 of the transmission line 34 is connected to a transmitter or receiver or other equipment (not shown) with which it is desired to employ the antenna 10.

Figs. 2 through are R-( (resistance-reactance) diagrams in rectangular coordinates on which impedance curves are plotted. The circle 40 in each diagram indicates the area in which voltage standing wave ratios are not greater than 3 to 1. Fig. 2 illustrates a typical impedance curve 41 of a thin, cylindrical or whip-type antenna 10, approximately one-quarter wave length long near the middle of the frequency band covered by the curve, on a finite ground plane 15, as in Fig. 1, over a frequency range of approximately 2.4 to 1, in the absence of any impedance-modifying elements. Fig. 3 illustrates, in the curve 42, the impedance curve of Fig. 2 as modified by including a shunt capacitor 14 of suitable capaci tance in parallel with the base 11 of the antenna 10, as shown in Fig. 1. Fig. 4 illustrates, in the curve 4-3, the impedance curve of Fig. 3 as further modified by a short high-impedance transmission-line impedance-transformer section 13 connected to the antenna 19 and the ground plane 15, as illustrated in Fig. 1. Fig. 5 illustrates, in the curve 44, the impedance curve of Fig. 4 as still further modified by the series- resonant circuits 23, 31 connected in the impedance-matching circuit, as illustrated in Fig. l.

The first series-resonant circuit 23 connected in shunt with the transmission lines 18, 34 inside the metal shield '22, which is resonant just below the low-frequency end of the operating band, affects only the low-frequency end 45 of the impedance curve 44, while the second seriesresonant circuit 30, which is resonant just above the high-frequency end of the band, affects only the highfrequency end 46 of the impedance curve 44. If the low-frequency series-resonant circuit 23 were omitted, the low-frequency end 45 of the impedance curve 44- would remain outside the 3 to 1 VSWR circle 40, as is indicated at 47 in Fig. 4, while, if the high-frequency series-resonant circuit 30 were omitted, the high-fre- 4 quency end 46 of the impedance curve 44 would remain outside the 3 to l VSWR circle 40, as is indicated at 48 in Fig. 4. In other words, the two series- resonant circuits 23, 30 are independent of each other and each affects the impedance of the impedance-compensating circuit only at its end of the operating band.

Either or both of the series- resonant circuits 23, 30 can be used depending upon the desired band width. In one embodiment of this invention, the desired 2.2 to 1 band width w s obtained using the low-frequency seriesrcsonant circuit 23 only, and it was unnecessary to use the high-frequency series-resonant circuit 30, although this could have been used to obtain still greater band width. The low-frequency circuit 23 was chosen because this perm tted the length of the whip to be reduced even below t. limit set in the specifications under which the development was made. The use of this series-resonant circuit 23 is what finally provided the required band width in conjunction with the other elements of the irnpedancematching device.

The circuit constants, inductance 24 and capacitance 25, for the low-frequency circuit 23 are chosen so as to provide resonance just below the low-frequency end of the desired frequency band and to have a relatively low inductive reactance over the lower end of the desired band where the impedance curve 4-3 extends beyond the 3 to l VSWR circle 40. The shunt inductive reactance provided over this frequency range causes this portion 47 of the curve ,13 to swing in a counterclockwise direction so as to move inside the 3 to 1 VSWR circle 40, as shown at 45 in Fig. 5. A further requirement in choosing the circuit values is that the inductive reactance rise to a very high level just beyond this frequency range, so as to have no measurable effect on the remainder of the impedance curve. Otherwise, the matching already obtained in the middle portion of the desired frequency range would be destroyed, that is the impedance curve would be pulled outside the 3 to 1 VSWR circle.

The principle of operation of the high-frequency circuit 30 is similar. The circuit constants, inductance 3i and capacitance 32, are chosen so as to provide series resonance just beyond the high-frequency end of the desired band, providing a fairly low capacitive reactance in the frequency range at the high end of the band Where the impedance curve is outside the 3 to 1 VSWR circle 4c. The shunt capacitive reactance pro ided over this frequency range causes this portion 48 of the impedance curve 43 to swing in a clockwise direction so as to move inside the 3 to 1 VSWR circle 40, as shown at 46 in Fig. 5. A further requirement in the choice of circuit values is that the capacitive reactance increase to a very high level at lower frequencies below this high-frequency end of the band so as to have no measurable effect upon the impedance in the middle portion of the band. Otherwise, the effect would be to swing portions of the impedance curve already inside the 3 to l VSWR circle 40 back out of the 3 to 1 VSWR circle 40.

Principles of broad-band impedance matching, some of which are applied in the present impedance-compensating circuit, are explained in the following publications:

Proc. IRE, v. 33, No. 10, p. 671, Oct. 1945, Design of Broad-Band Aircraft Antenna Systems, Bennett, Coleman, and Meier.

Very High-Frequency Techniques, v. I, McGraw-Hill, 1947, Harvard University Radio Research Laboratory Staff, Herbert J. Reich, Editor, chapter 3, pp. 53-92, Impedance Matching, Transformers, and Baluns, l. A. Nelson and G. Stavis.

Tole-Tech, v. IX, No. 1, pp. 19-20, January 1950, Design Charts for Transmission Line Matching Systems, Russell L. Linton, Jr-

Electromagnetic Waves and Radiating Systems, Prentice-Hall, Inc., New York, 1950, chapter 13, pp. 452-509, Impedance Characteristics of Antennas, Edward C. Jordan.

Electronics Engineering Manual, v. III, pp. 210-216, -Type Impedance Transforming Circuits, Phillip H. Smith.

The shunt capacitor 14 of the present impedancematching system moves each point on the impedance curve 41 in a clockwise direction along a circle through the point and tangent to the origin. The amount of movement of the point increases with frequency, since the susceptance of the shunt capacitor 14 is directly proportional to the frequency. The transformation from the curve 41 of Fig. 2 to the curve 42 of Fig. 3 is provided in this manner. The capacitance 14 is chosen such as to provide the indicated transformation, on the basis of computations well known in the art, so as'to enable further transformation as disclosed herein.

In a roughly similar manner, the high-impedance transmission line transformer section 18 provides the transformation from the impedance curve 42 of Fig. 3 to the impedance curve 43 of Fig. 4. The path over which each point is moved is generally similar to that described for the transformation from the curve 41 of Fig. 2 to the curve 42 of Fig. 3, but provides a different and more complicated transformation as needed. The amount of transformation increases with freqeuency since the length of the transmission-line section 18 in terms of wave lengths is directly proportional to the frequency. The optimum characteristic impedance and length of the trans mission line 18 for a specific embodiment may be determined by computations of a type well known in the art using a Smith Chart.

The movement of the curve provided by the high.- frequency series-resonant circuit 30 in the high-frequency end of the band is of the same type as that described for the capacitor 14 at the base of the antenna, since a shunt capacitive reactance is provided in this frequency range. Similarly, the low-frequency series-resonant circuit 23 provides movement of the same type but in a counterclockwise direction, since this circuit 23 provides a shunt inductive reactance in the low-frequency region of the operating band.

Fig. 6 is a graph in rectangular coordinates of reactance X and susceptance B against frequency f. The reactance and susceptance curves illustrate the fact that each seriesresonant circuit 23, 30 affects the impedance only at its respective end of the operating frequency band. The vertical line f indicates the lowest frequency in the desired operating band, while the vertical line f indicates the highest frequency of the desired operating band. The broken vertical line i indicates the resonant frequency of the low-frequency series-resonant circuit 23, while the vertical broken line f indicates the resonant frequency of the high-frequency series-resonant circuit 30. The vertical line f indicates the upper frequency at which impedance compensation is to be provided by the lowfrequency series-resonant circuit 23, while the vertical line f represents the lowest frequency at which it is desired to provide impedance compensation in the highfrequency end of the band by the high-frequency seriesresonant circuit 30. The curve X represents the reactance of the low-frequency series-resonant circuit 23, and the curve B represents the susceptance of this circuit 23, which is the negative reciprocal of the reactance X The curve X represents the quency series-resonant circuit 30, and the curve B represents the susceptance of this circuit 30.

It is apparent from the curve B, that a shunt inductive susceptance is provided in the circuit between the frequencies f and f over which inductive compensation is desired, While the magnitude of this shunt susceptance falls off rapidly with increasing frequency so that it is negligible in the lower middle portion of the operating band where the circuit is already inductive and where any additional inductive effect would hinder rather than help the impedance characteristic. Similarly, it is apparent from the curve B that a shunt capacitive suscepreactance of the high-freresonant circuits have an extra degree tance is provided between the frequencies f and f where additional capacitive efiect is beneficial, while the magnitude of the capacitive susceptance falls off rapidly in the upper middle portion of the operating band where the impedance characteristic is already capacitive and where further capacitive effect would be detrimental rather than beneficial.

Thus, an important principle of the present invention is to provide impedance compensation at each end of the operating band, as described, without detrimentally affecting the impedance characteristic in the middle portion of the operating band. This ordinarily cannot be accomplished with the usual transmission-line element, because such an element designed to provide a compensation at one end of the band may destroy the impedance match already obtained in other portions of the band. Similarly, this cannot be accomplished simply by using a shunt inductance to compensate at the low end of the band and a shunt capacitance to compensate at the high end of the band, because the impedance characteristics of such elements as functions of frequency are such that detrimental effects are encountered in other portions of the band. It is recognized that broad-band impedance matching has been obtained with transmission-line elements from impedance characteristics apparently similar to the curve 43 in Fig. 4. In the book Very High Frequency Techniques, listed above, an example of such compensation is described in pages 79 through 81. While the transmission-line element of that example provides compensation at each end of the operating band in a manner resembling that of the series-resonant circuits of the present invention, it should be noted that the impedance characteristic was Very good to start with and that the impedance curve for the middle portion of the band was well within the desired standing wave ratio circle, such that the problem of adverse effects in the middle of the band, which is a severe limiting factor in the impedance curve 43 of Fig. 4, is not a limiting factor in the example referred to. The-series-resonant circuits of the present invention are advantageous primarily to provide compensation in the difiicult cases where the impedance curve in the middle of the band liesvery close to the maximum permissible standing wave ratio circle, as in Fig. 4 of the present application, that cannot be handled by the technique described in the example referred to. Even in situations where a transmission-line element theoretically will providethe desired impedance compensation, the required characteristic impedance for the compensating element often is not available in any standard coaxial transmission line, and, particularly where very high characteristic impedances are required, the construction of the necessary transmission line in a rugged, useful form may be expensive, diflicult, or, in some cases, virtually impossible. In other cases, particularly for lowfrequency and medium-frequency operating bands, where the desired compensation could be obtained with either a transmission-line element or the series-rmonant circuits, substantial reductions in cost and in size and weight often can be obtained by the use of the series-resonant circuits of this. invention rather than a coaxial transmission-line element. Besides the foregoingv advantages, the seriesof freedom that can be availed of in difficult matching situations in that the resonant frequency and the slope of the reactance curve can be selected for the series-resonant circuit used at each end of the band independently of the resonant frequency and the reactance characteristics of the circuit .used for compensation at the other end of the band. The impedance of a length of transmission line is a periodic function of frequency and the impedance at one end of the band is a function of the impedance at the other end of the operating band, so a compromise between the two ends often is necessary. With the series-resonant circuits,

the optimum impedance characteristic required for matching can be provided at each end of the band with no need for compromise.

By providing impedance matching such that voltage standing wave ratios are not greater than 3 to 1 over the operating frequency band, the present invention provides good power transfer between the transmitter, receiver, or other equipment and the antenna 10, with losses in the tansmission lines 18, 34 kept at a low level, throughout the operating frequency band. Theoretically, there are no power-consuming components. Although the inductances in the series-resonant circuits necessarily have a small resistance, the power dissipation is negligible.

Various alternative forms of construction may be employed, if desired. For example, the transmission-line impedance transformer section 18, as well as the seriesresonant circuits 23, 30 may be enclosed in a container :with the transmission-line section 18 coiled to conserve space and perhaps with the series- resonant circuits 23, 30 located inside the coil. Normally, however, the construction indicated in Fig. l is preferable since the components required to provide the series- resonant circuits 23, 30 may be small and the metal shield 22 in which they are contained need not be much thicker than the transmission lines 18, 34. Thus, the impedance-matching circuit reduces the length of transmission line 34 required to con nect the antenna 10 to the transmitter, receiver, or other equipment, below the length that would be required if the transmission-line impedance transformer section 18 were coiled in a can. In other words, the impedancematching components may, if desired, merely replace a portion of the transmission line 34 that would be needed between the antenna 10 and the equipment if no matching system were employed. Obviously, only a negligible increase in weight is involved with such an arrangement.

Other forms of the shunt capacitor 14 may be used, of course, but the construction indicated in Fig. 1 appears to be desirable from mechanical considerations. The capacitor 14 of Fig. l achieves a very good conservation of space and weight by providing mechanical support for the thin whip, as well as providing the shunt capacitance utilized to transform the impedance curve of Fig. 2 to the impedance curve of Fig. 3. The metal base-support member 11 at the lower end of the antenna 10 preferably should provide a smooth transition between the small cross section of the antenna 10 and the larger cross section of the condenser plate formed by the lower surface of the support member 11, to avoid any discontinuity in the geometry of the radiating system that might adversely affect the current distribution and the impedance characteristics. The shape of the cross section of the metal base-support member is not critical. It may be round, or, as is preferred for vehicles or aircaft, it may be steamlined.

The combination illustrated in Fig. l is shown schematically in Fig. 7. In order to employ the invention with a dipole antenna, a folded dipole, or other form of balanced antenna, the circuit may be changed in an obvious manner to a balanced circuit, as is indicated schematically in Fig. 8.

In Fig. 8, the two halves of a balanced-type antenna are indicated at 10a and 10b. A condenser 14a is connected between the antenna members 10a, 10b as is indicated at 1% and 16a. A parallel-wire transmission-line impedance-transformer section 18a, comprising the conductors 17a, 20a, is connected to the antenna members 19a, 10]) as is indicated at 19a and 16a. The parallel wire transmission-line impedance-transformer section 18a corresponds to the coaxial impedance-transformer section 18 of Figs. 1 and 7, and comprises a high-impedance transmission line, preferably having a characteristic impedance of at least approximately 2.34 times the impedance to which the antenna system is to be connected, and an eflective length of approximately 0.067 wave length at the low-frequency end of the operating band and approximately 0.145 wave length at the high-frequency end of the operating band (approximately 0.1 wave length in the middle of the band) where the band width is approximately 2.2 to 1. A first series-resonant circuit 23a, comprising an inductance 24a and a capacitance 25a in series, 5; connected between the conductors 17a and 20a of the impedance-transformer section We at the end of the transmission-line section 18a away from the antenna 10a, 10b as is indicated at 26a and 27a. The circuit 23a is resonant just below the low-frequency end of the operating band. Also connected between the conductor 17a, as is indicated at 280, and the conductor 20a, as is indicated at 29a, is a second series-resonant circuit 30a comprising an inductance 31a and a capacitance 32a in series, and resonant just above the high-frequency end of the band. A parallel-wire transmission line 34a, having a characteristic impedance substantially equal to the impedance presented by the equipment with which the antenna system is to be employed, and comprising the conductors 36a, 33a, is connected to the transmission-line impedance transformer section 18a at the end of the section 18a away from the antenna 10a, 10b, as is indicated at 28a and 29a. The other end 37a of the transmission line 34a is connected to a transmitter or receiver or other equipment (not shown) with which it is desired to employ the antenna 10a, 10b. The balanced impedance-matching circuit of Fig. 8 operates in the same manner as does the unbalanced impedance-matching circuit of Figs. 1 and 7.

For antennas, whether balanced or unbalanced, having somewhat difierent impedance characteristics from those illustrated in Figs. 2, 3 or 4, various changes within the skill of the art may be made to provide transformation of the impedance characteristic to one approximately as shown in one of those figures, and, from there on, further transformation can be made in accordance with the disclosure herein.

It will be understood, of course, that, while the forms of the invention herein shown and described constitute preferred embodiments of the invention, it is not intended herein to illustrate all of the possible equivalent forms or namifications of the invention. It will also be understood that the words used are words of description rather than of limitation, and that various changes may be made without departing from the spirit or scope of the invention herein disclosed.

What is claimed is:

1. In combination: a cylindrical-type antenna having a conductive base-support member, a conductive basemounting member, and a base insulator between said base-support member and said base-mounting member, said base-support member, base insulator, and basemounting member together forming a shunt capacitor as well as a support means for said antenna, means connecting said base-mounting member both mechanically and electrically to a conductive ground plane, a coaxial transmission-line impedance-transformer section having a characteristic impedance of at least about 2.34 times a predetermined impedance to which said antenna is to be connected and having an effective length of approximately 0.1 wave length in the middle of. a predetermined band of operating frequencies, said impedance-transformer section comprising an inner conductor one end of which is connected to said base-support member and an outer conductor one end of which is connected to said base-mounting member and thereby to said ground plane, a conductive shield box connected to the other end of said outer conductor and containing a first series-resonant circuit comprising a first inductance and a first capacitance connected in series between the other end of said center conductor and said shield box, said circuit being resonant just below the low-frequency end of said operating band, a second series-resonant circuit contained in said shield box, comprising a second inductance and a second capacitance connected in series between said other end of said inner conductor and said shield box, said second series-resonant circuit being resonant just estimate above the high-frequency end of said operating band, a coaxial transmission line having a characteristic impedancesubstantially equal to said predetermined impedance, comprising an inner conductor having one end connected to said other end of said inner conductor of said coaxial transmission-line impedance-transformer section and an outer conductor having one end connected to said shield box, and means for connecting the opposite end of said inner conductor and the opposite end of said outer conductor to equipment having said predetermined impedance.

2. A broad-band impedance-matching system useful to provide impedance compensation between a cylindricaltype antenna and equipment having a predetermined substantially constant input impedance over a predetermined frequency band, comprising a two-conductor transmission-line impedance transformer section having a characteristic impedance of at least about 2.34 times said substantially constant impedance, at least one conductor at one end of said section being connected to said antenna, a capacitor connected in parallel with said impedance transformer section at said one end thereof, a first series-resonant circuit resonant just below, and within about eight percent of the frequency at, the low-frequency end of said frequency band, connected across said impedance transformer section at the end away from said antenna, a second series-resonant circuit resonant just above, and within about eight percent of the frequency at,

the high-frequency end of said frequency band, connected across said impedance transformer section at the end away from said antenna, the resonant frequency of said second series-resonant circuit being at least substantially twice the resonant frequency of said first seriesresonant circuit, said impedance transformer section having an effective length of substantially one-tenth wave length at a frequency midway between the resonant frequencies of said first and second series-resonant circuits, and means for connecting said end of said impedance transformer section away from said antenna to said equipment.

3. A broad-band impedance-matching system useful to provide impedance compensation between a cylindricaltype antenna and equipment having a predetermined sub stantially constant input impendance over a predetermined frequency band, comprising a two-conductor transmission-line impedance transformer section having a characteristic impedance of at least about 2.34 times said substantially constant impedance, at least one conductor at one end of said section being connected to said antenna, a capacitor connected in parallel with said impedance transformer section at said one end thereof, a series-resonant circuit resonant just beyond, and within about eight percent of the frequency at, one end of said frequency band, connected across said impedance transformer section at the end away from said antenna, the frequency at the high-frequency end of said predetermined frequency band being at least substantially twice the frequency at the low-frequency end of said band, said impedance transformer section having an effective length of substantially one-tenth wave length at a frequency midway between the frequencies at the ends of said frequency band, and means for connecting said end of said impedance transformer section away from said antenna to said equipment.

4. A broad-band impedance-matching system useful to provide impedance compensation between a cylindricaltype antenna and equipment having a predetermined substantially constant input impedance over a predetermined frequency band, comprising a two-conductor transmismission-line impedance transformer section having a characteristic impedance of at least about 2.34 times said substantially constant impedance, at least one conductor at one end of said section being connected to said antenna, a capacitor connected in parallel with said impedance transformer section at said one end thereof, a

first series-resonant circuit, comprising an inductance and a capacitance in series, resonant just below, and within about eight percent of the frequency at, the low-frequency end of said frequency band, connected across said impedance transformer section at the end away from said antenna, a second series-resonant circuit, comprising an inductance and a capacitance in series, resonant just above, and within about eight percent of the frequency at, the high-frequency end of said frequency band, connected across said impedance transformer section at the end away from said antenna, the resonant frequency of said second series-resonant circuit being at least substantially twice the resonant frequency of said first seriesresonant circuit, said impedance transformer section having, aneifective length of substantially one-tenth wave length at a frequency midway between the resonant frequencies of said first and second series-resonant circuits, and means for connecting said end of said impedance transformer section away from said antenna to said equipment.

5. A broad-band impedance-matching system useful to provide impedance compensation between a cylindricaltype antenna and equipment having a predetermined substantially constant input impedance over a predetermined frequency band, comprising a two-conductor transmissionline impedance transformer section having a characteristic impedance of at least about 2.34 times said substantially constant impedance, at least one conductor at one end of said section being connected to said antenna, a capacitor connected in parallel with said impedance transformer section at said one end thereof, a series-resonant circuit, comprising an inductance and a capacitance in series, resonant just beyond, and within about eight percent of the frequency at, one end of said frequency band, connected across said impedance transformer section at the end away from said antenna, the frequency at the high-frequency end of said predetermined frequency band being at least substantially twice the frequency at the low-frequency end of said band, said impedance transformer section having an effective length of substantially one-tenth wave length at a frequency midway between the frequencies at the ends of said fre quency band, and means for connecting said end of said impedance transformer section away from said antenna to said equipment.

6. In combination: a cylindrical-type antenna, a twoconductor transmission-line impedance transformer section having a characteristic impedance of at least about 2.34 times a predetermined impedance to which said antenna is to be connected and having an effective length of approximately one-tenth wave length in the middle of a predetermined band of operating frequencies of which the frequency at the high-frequency end is at least substantially twice the frequency at the low-frequency end, at least one conductor at one end of said section being connected to said antenna, a capacitor connected in parallel with said impedance transformer section at said one end thereof, a first series-resonant circuit comprising a first inductance and a first capacitance connected in series across the end of said impedance transformer section away from said antenna, said circuit being resonant just below, and within about eight percent of the frequency at, the low-frequency end of said predetermined band, a second series-resonant circuit comprising a second inductance and a second capacitance connected in series across said end of said impedance transformer section away from said antenna, said second circuit being resonant just above, and within about eight percent of the frequency at, the high-frequency end of said predetermined band, and a two-conductor transmission line having a characteristic impedance substantially equal to said predetermined impedance, having one end thereof connected to said end of said irnpendance transformer section away from said antenna, and means for connecting the opposite end of said last-mentioned transmission line to equipment having substantially said predetermined impedance.

References Cited in the file of this patent UNITED STATES PATENTS Kirkland Mar. 21, 1944 12 Bishop June 3, 1947 Wehner Jan, 30, 1951 Dishal July 7, 1953 Schlesinger Aug. 25, 1953 Seeley Nov. 10, 1953 Scholten Oct. 23, 1956 OTHER REFERENCES Practical Analysis of Ultra High Frequency, by Meagh- 10 er and Markley, copyright 1943, RCA Service C0,, Inc.

pp. 4 t0 7.

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