US2512704A - Arrangement for coupling wide frequency band antennae to transmission lines - Google Patents

Description

June 27, 1950 E. o. WILLOUGHBY 2,512,704 ARRANGEMENT FOR COUPLING WIDE FREQUENCY BAND ANTENNAE TO TRANSMISSION LINES Filed Jan. 16, 1945 3.Sheets-Sheet l won Inventor t5 Ailorn June 27, 1950 v E. o. WILLOUGHBY 251 .704

ARRANGEMENT FOR COUPLING WIDE FREQUENCY BAND AN'I'ENNAE T0 TRANSMISSION LINES Filed Jan. 16, 1945 r 3 Sheets-Sheet 2 6 Inventor Ema OBBQRNE \JRLWGM BY June 27, 1950 E. o. WILLOUGHBY 1 .70

ARRANGEMENT FOR COUPLING WIDE FREQUENCY BAND ANTENNAE T0 TRANSMISSION LINES 3 Sheets-Sheet 3 Filed Jan. 16, 1945 l lnvenlor c BBMKNE W \umseamX A llor r v Patented June 27, 1950 ,UNITED STATES PATENT OFFICE ARRANGEMENT FOR COUPLING WIDE FRE- QUENCY BAND ANTENNAE TO TRAN S- MISSION LINES Eric Osborne Willoughb London, d,

signor, by mesne assignments, to Standard Electric Corporation, New N. Y., 'a corporation of Delaware International York,

Application January 1c, 1945, Serial No. 573,100 g In Great Britain December 6, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires December 6, 1963 3 Claims.

The present invention relates to arrangements may be usedfor transmission or reception of broad frequency bands in the ultra high fr quency range, for example in the art of television, radio location equipment utilising frequency sweep methods or for systems utilising the transmission and reception of pulses.

According to the invention arrangements of the type specified having a low degree of overall mismatch between the impedances throughout a broad band of ultra high frequencies comprise an end fed type antenna half wave 'resonant'at substantially the mean operating frequency of the band or adipole antenna having each element half wav resonant substantially at said,

mean operating frequency, a transmission line for coupling said antenna to a translation device,

' means for forming a combination with said transmission line a parallel tuned circuit resonant at substantially said mean operating frequency and a seriestuned circuit resonant at substantially said mean operating frequency coupling said antenna to said parallel tuned circuit, said parallel tuned circuit being so formed as to have substantially th same Q as the antenna and the peak resistance of saidparallel circuit at the point of coupling to said series tuned circuit substantially equal to. the peak resistance of the antenna at the point of coupling to said series tuned circuit so as to obtain substantial cancella tion or neutralisation of the sum of the reactances due to the antenna andparallel tuned circuit.

In a slight modification of the invention, the series tuned circuit may be replaced by a portion of transmission line half wave resonant sub-: stantially at the mean operating frequency connecting the antenna to-the parallel tuned circuit, and having such impedance characteristic as substantially to neutralise the reactances of the parallel tuned circuit and antenna Over the operating band of frequencies.

The physical length cf the antenna should not be artificially shortened by the use of loading coils or end capacity,- s-ince these-means in general reduce the band width of .the antenna bymain- .10 The arrangements according to this invention taining the magnitude of the reactive constants and at the same time reducing the damping. The resistance and reactance of .end fed antennae are relatively high .and these factors are only sufficiently low for cases wherepractical constants ar possible for the compensating series tuned circuit, in antennae of low characteristic impedance, for example, conical antennae.

The invention and the principles on which it is based will be made clearer inthe following description which also includes a description of several preferred embodiments of the invention which are given. by way of example only. Reference will be made to the accompanying drawings in which:

Figs. 1 and 2 are diagrammatic circuit arrangements which illustrate the principles on which the invention is based;

Fig. 3 shows various characteristic curves of the arrangement shown in Fig. l or Fig. 3A

which is an alternative form of representing the circuit shown in Fig. 1;

Fig. 4 shows various characteristic curves of the arrangement shown in Fig. 2 or Fig. 4A;

Figs. 5 to 9 inclusive illustrate several prac- ,tical embodiments of the invention;

Fig. 10 is a diagram used in the explanation Figs. 11 and 12 which illustrate further embodiments of the invention;

Figs. 13 and i l-illustrate several practical embodiments 0f the invention. a

In Fig. 1 of the drawings, the parallel tuned circuits I, 2, 3 and 4, 5,6 have equal Q, that Ali is ratio of reactance tojresistance, and equal maximum equivalentshunt resistances, represented by resistances 3 and 4 respectively. These two parallel tuned circuits, resonant at the mean operating frequency of the desired frequency band are connected together by means of the series tuned circuit 1 and .8 resonant at said mean operating frequency, the inductance "l and capacity 8 being appropriately chosen. The actionof these three circuits will be explained with reference to Fig. 3. As shown in Fig. 3A, the capacity 8 may be placed inone lead and the inductance I in the other. a

In Fig. 3, the ordinates represent resistance or reactance and the abscissae represent frequencies in terms of the mean frequency F0. The curves R; and X represent the variation of resistance and reactance respectively of a parallel of the circuit whose characteristic is X. The

series circuit having the characteristic X2 has larger series reactances than the series circuit The circuit hav- However, thelseries circuit have having the characteristic XI and the uncompensated or unneutralised reactances (X2-X) which remain in the combined parallel and series tuned circuit are shown shaded in Fig. 3 (the curve X having been reversed as shown in the broken line).

In regard to Fig. 3 it should be observed that since there are two parallel tuned circuits each having impedances represented by R-l-iX connected by a series tuned circuit, the series reactance curves should be doubled in ordinates to represent the complete reactance, compensation by the series tuned circuit. More precisely, a reactance characteristic of the type shown at :r, 'Fig. 3, should be added to z to represent the sum of the characteristics of the antenna and parallel tuned circuit across the transmission line 3. This implies that the reactances comprising the series tuned circuits whose characteristics are XI and 2 X2, Fig. 3, should have their ordinates approxi- "mately doubled, and the ordinates difierences (X2-X) of the shaded areas representing unneutralisedreactances will also be doubled.

If there are two parallel circuits each having reactance characteristics of the type shown'atX, that is they both have equal Q and each have a resistance characteristic such as R,.oi equal maximum value, the reactances of the series tuned circuit and the input parallel tuned circuit, and the reactances of the series tuned circuit and the output parallel tuned circuit very nearly neutralise each other and the resistances of the two parallel circuits are substantially equal, so that throughout the frequency range represented by the abscissae of the points FI, F2 in Fig. 3, the mismatch between the impedances of the two parallel circuits is represented by that due to the unneutralised reactances indicated by the ordinates of shaded areas in Fig. 3, which mismatch is very low throughout the frequency. range FI to F2.

In other words, to a first approximation and neglecting the slight asymmetry of the parallel circuit characteristics about the mean operating frequency, or resonant frequency due to its low Q, if the reactances I, 2, 5 and 5 are each of magnitude Xp and the reactances I, 8 are 'each of magnitude Xs. resistance 4 is R ohms and the transmission line 3 is tapped on the inductance I so as to produce the same Q for the parallel circuit on the left as the one on the right (Fig. 3) of the series tuned compensating circuit I, 8 and if substantially then reactance cancellation or neutralisation at the frequencies near the peaks of the reactance characteristics of the parallel tuned circuits will A length of transmission line halfwave rescnant at the mean operating frequency will match any two equal impedances at the mean operating frequency, but it is possible by appropriate choice of characteristic impedance of the transmission line to obtain over a frequency band good resistance matching and some degree of reactance cancellation but not so good as the series tuned coupling ircuit.

Consider a transmission line of length I with the frequency increasing from below the mean value to above the mean frequency, the length of the line thus varying from above )\/2 to below M2 where A is the mean operating wave length, and find the characteristic impedance Z so that an impedance R-Hx at its output terminals appears to be of magnitude R-7Zr at the input terminals: i. e.

(R.jx) cos g +jZ sin 20 cos oR- x sin Solving-this has only one real solution I Fig. 2 shows diagrammatically two equal impedances, parallel tuned circuits I, 2, 3, 4, 5, 6

connected by a. length of transmission line 9 and Fig. 4 shows in three sets of curves labelled G, H, K the results of viewing the impedance of Fig. 2 (i. e. a parallel tuned circuit of peak resistance R and Q=2.5) through the half wave resonant transmission line 9 at mean mean operating frequency in for difierent impedances Z0 of the line 9 equal to 1.4R, 156R and 2.2R. It will be observed by comparing G, H, K, Fig. 4, with X, Fig. 3, that while Zn=1.66R. (curves H, Fig.4) gives reactance compensation at the peaks of the reactance curve (i. e. at frequencies 0.81% and 1.21%) and reasonable resistance matching (curves K, Fig. 4, and R, Fig. 3), the characteristics K (Fig. 4) with Z0=2.2R gives much better average reactance cancellation than either of the lower characteristic impedance connecting portions of transmission lines having curves H and K as well as giving good resistance matching.

In order to determine the best characteristic impedance Z0 for the half wave transmission line one may work out the optimum values of Zn for several frequencies in the operating band. This will diiier from frequency to frequency and a value may be finally selected above the mean which provides reasonable average reactance compensation of'the reactances of the antenna and the parallel tuned circuit when the half Wave transmission line of appropriate Z0 is connected to the parallel tuned circuit.

The resistance and reactance characteristics of a low impedance antenna near resonance are, apart from slight asymmetry, closely analogous to the corresponding characteristics of a parallel tuned circuit, in the frequency range corresponding to the range between the peaks of the reactance-frequency characteristic. In general, therefore, low impedance antenna-e are the best for carrying out the invention and the characteristic impedance of the antenna must be sufiiciently low if use is to be made of practical circuit constants in forming the compensating circuits, i. e. transmission line elements of presticable realisable characteristic impedance. 'An expedient will be described hereinafter, however.

-ielements. imean operating wave length of the desired operating frequency range. actance-of antennae of this type are, however.

asi'ngm "which will enable antennae of-higher charac- .iuned circuit 4, 5, B and its 'frequency-reactance characteristic may be made more symmetrical about its resonant frequency by a, small amount of series inductance or shunt' capacity.

End fedantennaeof the half wavelength element type have the appropriate type of impedance characteristic for reactance neutralisation according to this invention. This is either a half wave resonant length vertical aintennaabovean earth plane, or a dipole 'of 'h'alfwave-resonant The half wave lengths :refer to the The resistance and. re-

relatively high and only those types of antennae, for example of the conical type, can be used which have sufliciently low characteristic impedance values if practical c'ircuit 'constantsare *to be possible for the compensating circuits.- A

cylindrical vertical antenna having a 'conica lend near the earth ,plane, or dipoles having adjacent conical ends may be employed, and in this case the overall characteristic impedanceof the cylindrical portion should preferably be substantially equal to the characteristic impedance of the conical portion. This-embodiment is shown in Figs. 13 and 14.

Referring again to Fig. -1- the parallel tuned circuit 4, 5, represents the end impedance of a half wave antenna and I, 2 represents apara'llel tuned circuit shunted across the input transmission line represented by resistance 3. The circuit I, 2, 3 is arranged to have substantially the same Q, as the antenna 4, 5, 6. It willbe understood. that the circuit I, '2 includes any .s'tray capacity introduced by the transmission line connection.

' ,As already stated, 1 and 8 'is a series tuned circuit coupling the antenna, to the paralleltuned . circuit 1,2, 3 terminating the transmission line and the circuits I, Z, 3, 4, 5, 6., and I, I8. are all resonant at the mean operating frequency of the desired. operating frequency band.

i To. apply the principles illustrated in Fig. l ef-' fectively, as already stated the characteristic impedance of the vantenna must be low, so that the maximum end radiation resistance is of the order of 300 ohms .forthe balanced dipole antenna and-half this amount for the vertical unbalanced antenna. This is also true .for the case of the half wave coupling transmission line '9 i1- lustrated in Fig. '2. The characteristic impedanceoif 9 becomes impracticably high even for balanced antennae fed by a balance'd transmis- 'sion line if the peak radiation resistance of the antenna exceeds the value given above.

Figure 5 shows a. dipole antennawith conical "elements 4, 5, 6' each half wave length resonant 'at'the'mean operatin'g'frequency. '3 .is the transmission line and I, 2 the parallel tuned'circuit "resonant at the mean operating frequency and 3 is connected at such points onthe inductance l f'so'as to give to circuit I, 2, (time same shunt peak resistance as that of the antenna' and "the "inductance and :capa city 1', ii are adjusted to" give the same 'Q'as thatof the antenna. 9 is the half wave length transmission line at the mean operating frequency coupling I, '2, 3* t'o 4, 5, 6 as "explained with reference to Fig. '2. The (Shameteristic'impedance of the antenna 4', "5, 6 issuf ficiently "low to enable a practical transmission line 9 to be used.

Fig. 6 shows the arrangement .of'ian unbalancedsntenna 4, 5, 5 located above the earth plane '21 and is the unbalanced version of the balanced arrangementshown in Fig. 5, u'tilising an unbalanced transmissionline for the half wave portion 9. Fig. 7 shows alsothearran'gement of 'anunbalanced antenna but utilising a concentric conductor or coaxial cable as the-portio'n 9.

Fig. 3A shows a slightly imodifled form of Fig. 1. In Figure 3A the inductance 1 is connected in' on'e lead between the two parallel tuned cir- -cuits and the capacity 8 in the other, whereas in f F'ig. 1 the inductance l and lcapacityt are in seform-of alength of high impedance transmission line 1 providing the series inductance I, -3, coupled to the antenna at its ends by capacityxin theform of open stubs formed by the conductors 8"; and the interior oi the antenna, 4, 5,15.

S'oia'r it has been assumed that the characteristic impedance of the antenna is sufficiently low. -In general however owing to the characteristic impedance of the antenna being too'highior practical "compensation by a single series tuned circuit a "portion of transmission line [1 having the '--necessary high characteristic impedance to provide the necessary inductance for the series turiedpircuit may not be practical audit will *be necessaryto provide part of the seriescom-"- pensating reactance 1,8 in the form of separate elements.

For example, a'200 ohm characteristic impedance concentric-or coaxial conductor'transmis- :sion line corresponds to a 28:1 ratio of outer to =inner "conductor diameters, and since "doubling this ratio merely means an increase 111 characteristic impedance to 242 ohrns, 200ohims may ibetaken as a reasonable practical upper ilimit'of coaxial line characteristic impedance.

- Then the required bandwidth is i20'%' of the 1 mean operating frequency, the :reactance compensation .for a short circuited' half wave line is $200 tan 36 1-145 'ohrms. This amount *of compensating impedance canv thus be obtained for each half wave length-of antenna, that is 290 shins-compensation for a balanced antenna,:so

-that if :the antenna has 350*ohms ,peale-resistancey-the-use of a'compensating line having the characteristic impedance of 200 ohms leaves only 350-2- 11l5=60 ohms for further compensation or vneu-tralisation to be' obtained. This may easily be done, for example, by a 1'50 ohm capac- "ity and inductive reactance in series.

The different "elements will be described. The first is :a closed stub loaded to be equivalent to a half wave length short :circuited stub. 'T his 'expedient is illustrated in Fig. 9 in which the coupling short length of transmission line I1 is cou-- pled to the lower end of the unbalanced antenna 4, 5, 6 by a capacity '3 and accommodated within the antenna is a closed stub I0 loaded to half wave resonance by a capacity 12 lying in a matchingfizone within onewquarter wave length (mean operating frequency) from either end of lower end of the stub, when it is connected to the antenna, zeroat the mean operating frequency. It will be realised that the stub is effectively in .series between the antenna and line I l and gives a good degree of reactance compensation between-the antenna and the reactances as seen ,throughtheportion of transmission line H. The characteristic impedance of the stub III is chosen, for examplebysuitable diameter, ofcentral conductor. It so that the rea'ctance seen acrossits lower end is substantially equal and opposite to twice the reactance at the base of the antenna, at frequencies near the outer limits of the operating frequency band, that is at these frequencies,'the reactance of stub I is equal to the sum of the reactance of the antenna at the base thereof and the combined reactances at the base of .the antenna of the parallel tuned circuit I, 2 and of the length of transmission line H and condenser 8.

:line IT, the other side of which would normally be earthed, it will be understood that another dipole element similar to that shown may be connected to the other side of line H, instead of earth potential, forming a balanced system.

- The other expedient which enables antennae of higher characteristic impedance and radiation resistance to be used is illustrated in Figs. 10, 11

and 12 and consists in adding the series reactance of a parallel undamped circuit at the end of a quarter wavelength of line resonant at substantially the mean operating frequency in series with the antenna. i

Fig. 10 shows diagrammatically the quarter wave line i 4 open at the end A which is connected at the base of the antenna, and loaded at its other end with the parallel tuned circuit [5 represented by capacity l6 and loop l1, and resonant,

at the mean operating frequency. In eifect the impedance of circuit I5 is thus in series with the antenna because the parallel tuned circuit when seen throughthe quarter wave line H resonant at the mean operating frequency has substantially the same reactance neutralisation chest or compensation as a series tuned circuit end A (Fig. 10).

at the open Let the parallel impedance of the circuit I5 at the end of the transmission line I4, Fig. 10, of

impedance Z0 be :"X, then the input reactance is given by so f? a where X is negative iorfrequencies above resonance where i ,is physicaljlength ofthefline l4. n

Since X is the impedance of a parallel tuned circuit this may be rewritten. 1

' tan wi=2ir$ frequency f1 I I w0=21rXm3I1 operating fre quency. Y I

Examination of this expression shows that the best reactance compensation occurs when the ratio of the correcting reactance at the input terminals 20 to thecharacteristic impedance of the is not large compared to 1. I

However, taking 'ior, example f1=,0.8.',fn and f2=0.9 in we have as follows:

f1 Ratio Z input Z input Zn/Zyx 4.012.; i. OZjZo :2525 3.0jZo 0.892110 0.290 2 0 jZo 0.687jZu 0.344 j 1 5120 0. 560jZc 0.373

showing that although the ratio of the reactance near resonance to that giving perfect compensation at the peaksof the reactance characteristic is low compared to the ideal to give perfect'compensation fora reactance characteristic convex outwards on both sides of the line of zero resistance resonable compensation can be obtained with neutralising reactances up to three times the characteristic impedanceof the line for band widths of the order of 1 2072; of the carrier freq n i I i It may be notedthat although two quarter wavelength transmission lines in series may be used as an impedance reducing mechanism, they generally result in considerable reduction of the band width, but the principles of the invention may be applied to the antenna as seen through two quarter wavelengths transmission lines in series. j"

Two embodiments utilising the expedient as described in relation toFig. 10 are illustrated in Figs. 11 and 12. In thesefiguresthe transmission line H takes the form of a quarter wave concentric line l4 andthe parallel tuned circuit comprises a portion of transmission line less thanone quarter wave length long at the mean operating frequency closed at itsouter end 18 and tuned into resonance by the variable capacity l9 connected between the two coaxial conductors at the'end of the quarter wave length M. The portion M1 is connected directly to the base of the antenna 4, -5, 6 and the parallel tuned circuit I, 2 is connected to the central conductor of the stubat the base of the antenna.- From what has already been statedit willbe clear. that the line arrangement l4, l8,. 19 has the effect of a series tuned circuit and this connects the parallel tuned circuit I, 2 to the antenna4, 5, B.

Fig. llshows a balanced antenna and an unbalanced. transmission line whose coaxial conduc- WIS. e tapp d a 1 a i r p t P nt on th ductance represented as loop i. In Fig. 12 is shown an unbalanced antenna and an unbalanced transmission line 3, the parallel circuit 1, 2 taking the form of a section of coaxial transmission line closed at one end and tuned to resonance at the mean frequency by a capacity 2 connected across the conductors at the other end to which the transmission line is connected, the tapping point of the inner conductor of 3 being suitably chosen on the inner conductor of 1.

While several embodiments of the invention have been described and illustrated, others falling within the scope of the invention as defined in the appended claims will occur to those skilled in the art. For example, cylindrical antenna with conical ends may be used, the overall characteristic impedance of the cylindrical portion being properly matched to the characteristic impedance of the conical portion at their junctions. Furthermore, balanced or unbalanced antennae may be used with balanced or unbalanced transmission lines.

References in the claims to a series-tuned circuit are intended in appropriate instances to include a portion of transmission line operating as such a circuit.

What is claimed is:

1. A system for coupling an antenna to an input transmission line so as to have a low degree of overall mismatch between the impedances throughout a broad band of ultra high frequencies comprising a coupling transmission line, an antenna including a pair of end-fed radiating elements half wave resonant substantially at the mean operating frequency of the band, a series tuned circuit resonant substantially at said mean frequency coupled to said antenna, a parallel tuned circuit resonant substantially at said mean frequency coupled to said series tuned circuit and said coupling transmission line and having sub stantially the same Q aS the antenna, the reactances of said series tuned circuit being dimensioned to neutralize the reactances of both said parallel tuned circuit and said antenna off resonance, said antenna comprising a pair of hollow elements and said series circuit comprising a quarter wave stub resonant substantially at the mean operating frequency accommodated within each hollow antenna element and connected to said antenna element at one end thereof and a second parallel tuned circuit resonant substantially at the mean operating frequency and connected to said stub at its other end.

2. A system as claimed in claim 1 wherein said second parallel tuned circuit is incorporated in a half Wave closed stub, provided with a capacity at an intermediate point and connected between the concentric conductors of the stub, arranged to tune the portion including the open end into resonance at substantially the mean operating frequency and to parallel tune the other portion including the closed end into resonance substantially at the mean operating frequency, and the said parallel tuned circuit formed with the transmission line is connected directly to said stub at its point of connection to the antenna.

A system according to claim 2 in which the said parallel tuned circuit comprises a wire loop connected on either side to the central conductor of the stubs and a capacity connected between said central conductors of said stubs.

ERIC OSBORNE WILLOUGHBY.

' nnrnannons orrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,842,916 Osnos Jan. 26, 1932 1,984,408 Giddens Dec. 18, 1934 2,111,743 Blumlein et a1 Mar. 22, 1938 2,183,123 Mason Dec. 12, 1939 2,226,686 Alford Dec. 31, 1940 2,188,389 Cork Jan. 30, 1940 2,235,003 Arends Mar. 18, 1941 2,267,445 Cork Dec. 23, 194 2,253,974 Dagnall Oct, 14, 1941 2,274,347 Rust et al Feb. 24, 1942 2,275,030 Epstein Mar. 3, 1942 2,311,364 Buschbeck Feb. 16, 1943 2,321,454 Brown June 8, 1943 2,321,521 Salinger June 8, 1943 2,401,344 Espley June 4, 1946 FOREIGN PATENTS Number Country Date 469,245 Great Britain July 21, 1937 524,457 Great Britain Aug. 7, 1940

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