US2512078A - Broad-band antenna - Google Patents

Description

June 20, 1950 R. s. wEHNER BROAD-BAND ANTENNA Filed Jan. 22, 1946 ATTORNEY Patented June 20, 1950 UNiTE-o STATES PATENT OFFICE Robert S. Wehner, Port Jefferson, N. Y., assignor to Radio Corporation of America, a'corporation of Delaware Application January 22, 19476,l Serial No. 642,690

` e claims. (c1. o- 33) The present invention relates to broad band antenna matching systems andinore particularly to a system for adjusting the matching network when the antenna parametersar'e not at their optimum values forv broad band response.

An object of the present invention is the provision of an additional series circuit in an impedance matching network for correcting any inadequacies in the parameters'in the antenna for broad band operation.

A further object of the present invention is the provision of a short line section as a part of a broad band impedance matching section.

A further object of the present invention is the provision of an impedance matching system as aforesaid which is electrically and mechanically smooth. v y

A further object of the present` invention is the provision of an impedance matching system as aforesaid which is adapted for the use Ofcommercially available coaxial lcable for the react.- ance elements. f f j,

Still another object of the present invention is the provision of an additional section in an impedance matching vcircuit for adjusting antenna parameters to yield their most favorable characteristics without requiring the antenna to be rebuilt. l

A further object of the present invention is the provision of yan impedance matching element whereby an vantenna ldesigned for u'seyin one particular location may be adapted for use in a different location without rebuilding the antenna.k

Still another object of thepresent invention is the provision of a short line section in 'an im,- pedancematching network to vary the characteristics thereof for increasing the band with. no adverse effects upontheleld pattern of vthe associated antenna.

The foregoing objects and! others which may appear from the detailed description are accomplished by providing inseriesbetween the antenna radiatortitself 4the impedance rmatching network, a short'series section which-has .the

effect of shifting iresonance ofthe antenna-to;- ward higher `frequencies while at the same time having little reffect uponthe `antenna resistance.

This results in. impedance'characteristics looking into the short section terminated by the antenna which areV similar to those of the original antenna except that the' impedance level A'is higher. i I l The present invention-will be more fully'understood by' reference to the following ldetailed i description which is accompanied by a drawing in which: f

Figure 1 illustrates a form of the antenna to which `the present invention is adapted to be applied, while Figure 2illustrates theapplication of the present invention to the antenna of Figure 1;

Figure 3 illustrates the antenna impedance and matching standing wave characteristicsy of the antenna of Figure 1, while Figure 4 illustrates the antenna impedance and matched standard wave characteristics of the antenna as shown in Figure 2.

I have shown in my priorled application, SerialA Number 650,953, filed February 28, 1946, now Patent No. 2,485,177 vissued Oct. 18, 1949, that a series resonant antenna which is shown in Figure 1 including the extending radiator portiony I0 and the sleeve section I4 extending normal to the conductive ground plane GP may be matched to less than a` 2:1 standing Wave ratio on a transmission line TL of characteristics impedance Zu over very wide frequency bands. These frequency bands may be of the order of 2:1 in frequency spread or more. As described in the application referred to above the broad banding is accomplished by means of a simple series transformer matching section including thezinner'conductor I 4 and outer sleeve I2 effectively connected in series between the transmission line TL and the radiator including radiator-"Illiand the outer surface of sleeve I2. Conductor I4 is connected between the lower end of radiator Ill kand conductor I8 of the transmission line-TL while sleeve I2. is connected electrically to ground' plane GP and to the outer shell I 6 oftransmission line TL.l The ratio of the diameter'of conductor-,I 4 tothe inner diameter of sleeve I2 yis so chosen that the characteristic impedance of this matching section is equal or nearly equal to thepro'duct of A and'` Zo where A is the desired maximum'standing wave ratio.' The impedance of the antenna is so adjusted by choosing the feed points,` that is, theposition Iof the junction betweenradiator rod Irand sleeve I2 as to likewise be equal to or nearly equal to the product of A and Zo."v As lI have shown in the previous application, in broad banding an antenna to less than a 2:1 standingwave ratio on a 50 ohm line the best results are obtained .by using a ohm antenna inconjunction with a.100 ohm series im yp eda'n'ce 4matching section. LLI-Iowev'er, it sometimes happens that it is impossible or inconvenient to adjust thek design parameters of;` the antenna so thatjits impedance level is exactly equal to the optimum value of AZO ohms; that is, it may be impossible to so choose the ratio of the diameter of rod I and sleeve I2 or the ratio of the lengths of rod I8 and sleeve I2 to exactly obtain an impedance level of AZO ohms. Some loss of band width then results, this depending upon the sign and magnitude of the departure from the optimum value. If the impedance level is greater than AZO ohms by not more than little harm may be done and the antenna may be matched by transformer I2, I4 having an impedance of AZO ohms over almost as wide a band as could be accomplished under the most favorable circumstances. I-lowever, if the impedance level of the antenna is less than AZ@ ohms by even such a small factor as 5%, a serious loss in band width will result. This is due to the fact that if the resonant impedance of a series resonant antenna is appreciably less than AZO ohms a real value for the electrical length of the AZO transformerV cannot be found such that the antenna will be matched over the entire frequency band in question. The electrical length of the matching section is indicated in Figure l as being of the order of 63 electrical degrees. This matching section, if it is long enough to match the inner resonant region, will be too long or too short to match the antenna to the transmission line at the twoends of the band. Previously, the only way out of this difficulty consisted in using a matching section of a characteristic impedance less than AZO ohms and equal to or less than the impedance level of the antenna. This procedure results in a greater band width than could be obtained with a given low impedance antenna used with an AZo ohm transformer, but the band width thus obtained is always less and sometimes a great deal less than what could be realized with the system if both the antenna and thev impedance matching transformer had the proper impedance.

I have found that-is is possible to remedy the deiiciency of the antenna insofar as the impedance level' is less than', rather than equal to or"greater than VAZo-ohms, by introducing be tween the antenna terminals at the junction place between radiator In 'and sleeve I2, a short length of Zt-ohm transmission line indicated in Figure 2 at 22 Aand 24. The outer sleeve I2 in Figure 2 has an additional inner sleeve 22 and/or there is a surrounding sleeveportion 24 around the inner central conductor I4 whereby the characteristic impedance'of this section yis reduced to 50 ohms; that is, to an impedance equal to the impedance of transmission line TL. The additional section has an electrical length such that the resonance of the antenna is shifted toward higher 'frequencies while at the same time it has relatively little effect upon the an'- tenna resistance.l impedance characteristics result, looking into section 22, 24 from the main impedance matching section I2, I4, which are very similar to those of the original antenna except that :the impedance level ,is higher. The length of this section is so chosen .as to make the system resonant at a frequency higher than the original resonant frequency and nearer the center of the desired band at which frequency the input resistance will be equal to or greater than AZO ohms.

If ZO is the characteristic impedance of this short impedance matching section 22, 24 and 0 its electrical length and if Z=R+iX is the antenna impedance at any frequency higher than the originalresonant frequency of the 4 antenna, then the impedance looking into section 22, 24 is given exactly by the transmission line quotation:

gjZ tan H-f-Z (l "1* 20H2 tan a If 6 is less than roughly one quarter radian (or l5 degrees) the input impedance is approximately given by the following:

From this equation it follows that the input impedance Zin may be made non-reactive; that is,

Xin=0 if 0 is related to R. and X by the following expression:

Equation 8 may be used directly to determine the proper length of the short series 22, 24 since it gives 0 in radians.

An example of the use of a short line section in broad banding antennas will now be given with reference to Figures 3 and 4.

FigureS illustrates in curve 3l the variation in input resistance to the antenna with a variation in frequency while curve 32 gives the variation in input reactance as frequency is varied. Curves 3l and 32 represent the actual measured input impedance of a coaxialraircraft antenna which was to be matched, preferably by means of a simple series transformer to less than a 2:1 standing wave ratio on a 50 ohm line over the rangeV extending from to 420 megacycles.` Thenecessary condition for the matching of an impedance X to less than Azl standing wave ratio Von, a main line of characteristic impedance ZO over aA band of frequencies by means of a simple transformerof impedance Zm is that the impedance Z4 must, `at all frequencies in the band, lie on or within the Am:1 circle of the line Zm where Am is given by the relationship This condition sets a lower limit tothe charf asteristic impedance of the transformer required to inatchal given set of .antenna data over a given range 'sin ce the, entire impedance curve in the complex plane,v must lie within the Am2l standing Wave ratio circle of a line of that characteristic impedance. The area of this circle, that is, the range of impedance which it includes. is directly rproportional to the square of Zm. The minimum characteristic impedance of a transformer capable of matching an impedance R-i-gZA to less than2:,1 standing wave ratio on a 50 ohm line'is 'given by the following equation:

- 25(R2+X2-25R) r-25 The most diflicult point to match in the impedance characteristic of a series resonant antenna is its low frequency limit, since that point is always "characterized by the lowest resistance and the largest, relative to the resistance, re-

actance. The value of Zm required to match the antenna of Figure 1 whose characteristics are shown in Figure 3 over the range of 190 to 420 'megacy'cles may be found by substituting the values of R and X for '190 megacycles in Equation 4. It will be seen that at 190 megacycles 'R and X are equal respectively to 42 and to 76 ohms. Therefore, a transformer having a minimum characteristicimpedance -off'the order of 100 ohms is' requiredg- But, it will be noted that'the resonant resist'- ance of 274 megacycles of the antennal is only antenna-condenser system4v but that thellatter requires much*y higher impedance transformer `than the 100-ohm one used in the example to make a match possible. The low frequency'im- 92 ohms, a value which is appreciably lessthan 5 pedance with the condenser, namely cl2--y111 100, the characteristic impedance of the trans'- ohms, is much harder to match to the stated former. A resistance of 92 ohms lies'within the standard than is the 36-9'68 ohms impedance at 2:1 standing Wave ratio circle of the 50 ohm line the 10W frequency extreme of the antenna-lineby itself, but when a 100 ohm transformer is ap- Section system, the former requiring a 138 ohm plied to the antenna, the region near resonance, (from Equation 4) transformer, which value is Where the antenna is matched initially, becomes not only impractically high from the point of mismatched yif the length ofthe transformer is view of physical realization, but also is so high such as to match the low and high frequency as to defeat the original purpose of using the ends of the band; that is, 92 ohms is suiciently condenser. n smaller than 100 ohms that a real value of the l5 'I'he intermediate line'section of the same charelectrical length of a 100v ohm transformer I2, |-4 acteristic impedance as that of `the main line cannot be found such that the 'antenna is matched TL tends to reduce the necessary length of the over the entire band. The best possible band main series transformer section I2, I4 by an Width obtainable 'with the antenna of Figure 1, amount greater than the length of the-interdetermined by trial and error, is that indicated mediate section 22, 2li. Consequently, the overby the standing wave ratio characteristic plotted all length of the antenna and matching section in curve 33 of Figure 3. This shows the voltage is not any greater than that of the-simple sysstanding wave ratio on a 50 ohm line looking tem shown in Figure 1. In view of the gain in into a 63 degree long 100 ohm transformer terperformance effected by the series section this minated by the antenna III, lI2. While the anfact constitutesv a big advantage of' this method, tenn'a is satisfactorily matchedv at the high and particularly as compared to the bulky series low frequency ends of thepass bands, it is misshunt combination matching section heretofore matched as show'nby the cross hatched portion used. In practical applications, where it is often 35, over some 65 megacycles in the vicinity 'of necessary to build the matching section into the resonance to an extent greaterthan the allowable antenna structure itself in order to obtain a maximum. single self contained radiating unit, space is at This situation may be remediedfby means of a premium.` l the short line section described vabove with ref- At low frequencies, Where it may be convenient erence to Figure 2. It is proposed that resonance to use lengths of commercially available cable for be shifted from 250 megacycles,` where the yresistboth the transformer and the short series v`line ance is only 92 ohms, to midband or 300 megasection, use of the short series line section incycles where the antenna resistance is substanvolves nothing more than the introduction of a tially greater than' 100 ohms-at 300 megacycles. short length of the main line cable -betvveen the The antenna has a resistanceofv 110 ohms with transformer and antenna terminals. At higher a reactance equal to |22 ohmsj If 'these values 40 frequencies, the short section may b-e krealized by are substituted in the Equation 3, it isvfound that slipping an appropriately long piece 1of metal a section of 50 ohm line only .ll'radians long, tubing either WithinA the sleeve I2 or a short or 6.3 degrees, at 300 megacycles has'the desired section of smaller diameter metal tubing around effect. The impedance looking' into this secthe inner conductor I4 nearest the antenna feed tion terminated in the antenna is shovvn by point. curves 4I and 42 in the upper part of'Figure 4. While thek foregoing description has been These impedance characteristics, since the respredicated on the use of coaxial line sections it onant resistance is higher than the' critical should be understood that the short series line value of 100 ohms, are readily matched over the section may equally well be employed with a entire desired band by means of a 100 ohm trans- 50 balanced transmission line and antenna system former. The standing Wave ratio characteristic if desired. for a ohm line terminated in this matched lThe present invention has a further applicaimpedance is plotted in curve 42 in Figure 4l. tion in antennas of such nature that it is diicult It is evident here that the use of the short secto design them to have the proper impedance tion of .main line between the antenna termilevel initially and where, if a mistake in judgnals has resulted in'standingwave ratiov of less ment yshould occur, it is practically impossible than 2:1 over the desired 'frequency' range. to readjust the antenna parameters to yield more For wide band operation the series linel section favorable characteristics Without entirely rebuilde actually reduces the rate of variation ofl reacing the antenna. Typical antennas of this type tance with frequency `vWhile the use of a series are large surface area sleeve antennas of streamcondenser in its placemay increase it. This line shape. In these antennas, because of the eifect is shown for the case of the antenna denecessity for maintaininga special shape and surscribed above inthe following table: i face contour, none 'of the critical dimensions y Slverae ope, .e- System haar sans v cycle Ohms "Ohmsl `Ohm Original Antenna 42 f -76 22 0.89 Antenna plus short linesectioiL 36 -68 K 0 0.62 Antenna plus series condenser-- 42 -111 0 1.01

lIt will be noted that not only is the'` antennaline section combination vmuch flatter than-'the can be changed once the structure is built.` If

' such an *antenna should happen to have fan imagaregol'zs 'I pedance levelf lower than the` optimum value, it is obviously much easier to correct the defect by the simple addition of a short sleeve in the feed line to produce the series matching section than it would be to rebuild the Whole antenna.

Also, the short series line section' may often nd use in the case of matchingv thedriving element in antenna arrays, corner reflectors, and paraboloids. In such systems, eld pattern considerations over a band of frequenciesV mayr be such asv to cause the input impedance level of the driver antenna to be much less than' the optimum value for the desired bandwidth. The short series line section of Figure 2 maybe used to improve impedance characteristics with no adverse effects on the eld pattern.

Furthermore, in aircraft radio, it is sometimes necessary to use an antenna designed for one particular location in either a dilierent location on the same airplane or corresponding location on a larger airplane. The impedance level of series-resonant stub and whip antennas is very sensitive to the radius of curvature of the surface upon which they are mounted, being higher theA smaller the radius relative to the operating wavelength. Some broad band aircraft' antennas depend for their band width upon the high impedance level resulting when these antennas are mounted on the relatively sharply curved fuselage of a small airplane. If such antennas are then moved to flatter surfaces, the impedance level drops and so does the band width. In many cases it is possible to compensate for this loss. of band Width by means of the short line section as described above with the result that itis then possible to use a given broad band antenna in Widely different locations with no design changes other than the insertion of the short additional line section between the antenna terminals and the' matching section.

While I have illustrated av particular embodiment of the present invention, it should be clearly understood that it is not limited thereto since many modifications may be made in the several elements employed and in their arrangement' and ity i's therefore contemplated' by the' appended claims to coverv any such modiiicationsl as fall within" the spirit and scope of' the` invention..

What is claimed is: Y

l. In a broadband antenna system including-"a radiator extending normally to a conductive ground plane', a highA impedance matcl'ii'ng'4 section having an impedance value of AZ@ ohms Where A is the maximum allowable standing wave ratio andf comprising; concentric' conductors co"- axially arranged with respect. to. said radiator, the inner conductor of said matching section being connected to said radiator' an'd the; sleeve-iconductor being connected to said ground plane at a point intermediate itsv length, and' a coaxial transmission line hav-ing an impedance value of Ze ohms and comprising a sheath connected to the sleeve conductor and aJ-cent'er conductor connected to the inner conductor of said matching section, the impedance level of said radiator being less than AZO ohms, a further matching section comprising a length of coaxial transmission line having an impedance value of Z ohms and an electrical length of substantially six degrees interposed between said radiator and said high impedance matching section to transform the impedance level of said radiator to a value at which said high impedance matching section will. be eiectivel over said broad-band. f

2. In a broadband antenna system-including al radiator extending, normally toa conductive ground plane, a high impedance matching sectionv comprising concentricv conductors coaxially arranged with respect to said radiator, the inner conductor of said matching section being con'.- nected to' saidy radiator andthe outer conductor being connected4 to said ground planeA at a point intermediate itsI length, anda coaxial' transmis'- sion line having a sheath connected to the-outer conductor of said matching section and a center conductor connected to the inner conductor of said matching section, the impedance level of said radiator being lower' than optimum for broad band matching with said high impedance matching section, a coaxial transmission line section of the same characteristic impedance asfthat of the first said coaxial transmission line interposed between said radiator and said hi'ghimpedance matching section and having an electrical length substantially one-tenth that of said highv impedance matching section thereby to shift the resonance of said' radiator to' a higher fre'- quency without aie'cting its impedance characteristics.

3. In a broad band antenna system including a radiator extending normally to a conductive ground plane, a high impedance matching section comprising concentric conductors coaxially arranged with respect to said radiator, the inner conductor of said matching section being connected to said radiator and the outer conductor being connected to said ground plane at a point intermediate its length, and a coaxial transmission line having a sheath connectedV to the outer conductor of said matching sectionI and a center conductor connected' to the inner conductor of said matching section, the impedance level of said radiator being lower than optimum for broad band matching with said high impedance matching section, a conductive element surrounding said inner conductor and being connected to one of said conductors adjacent said radiator andl said high impedance matching section to transform the impedance level of said radiator to a value at which saidhighA impedance matching section will be eifective over said broad band.

4. In a broad band antenna system including a radiator extending normally to a conductive ground plane, a high impedance matching section comprising concentric conductors coaxially arranged with respect to said radiator, the inner conductor of said matching section being connected to said radiator and ther outer conductor being connected to said ground plane at a point intermediate its length, and a coaxial transmission line having a sheath connected to the outer conductor of said matching section and a center conductor connected to the inner conductor of said matching section, the impedance level of said radiator being lower than optimum-for broad band matching with said high impedance matching section, a pair of hollow,"cylindrical, conductive elements concentrically arranged within said high impedance matching section at the end adjacent said radiator, one of said conductive elementsv having thel inner surface thereof coincident with said inner conductor and the outer surfacey of the other being coincidentV with the inner surface of said outer conductor to constitute a further impedance matching section to transform. the impedance level: of said radiator to a value at whichsaid high impedance matching section will be elective over said broad band.

5. In a broad band antenna system'including a radiator of given diameter extending normally to a conductive ground plane, a high impedance matching section having an impedance value of AZO ohms where A is the maximum allowable standing wave ratio and comprising concentric conductors coaxially arranged with respect to said radiator, there being an overlap between said radiator and the sleeve conductor of said matching section, the inner conductor of said matching section having a diameter smaller than said given diameter and being connected to said radiator, said sleeve conductor being connected to said ground plane at a point intermediate its length, and a coaxial transmission line having an impedance value of Zo ohms and comprising a sheath connected to the sleeve conductor and a center conductor connected to the inner conductor of said matching section, the impedance level of said radiator being less than AZO ohms,

the relative diameters of said radiator and said y sleeve conductor having values providing an impedance value of Zo ohms interposed between said radiator and said high impedance matching section to shift the resonance of said radiator to a higher frequency without affecting its impedance characteristics.

6. In a broad band antenna system including a radiator extending normally to a conductive ground plane, a high impedance matching section having an impedance value of AZo ohms where A is the maximum allowable standing wave ratio and comprising concentric conductors coaxially arranged with respect to said radiator, the inner conductor of said matching section being connected to said radiator and the sleeve conductor being connected to said ground plane at a point intermediate its length, and a coaxial transmission line having an impedance value of Z ohms and comprising a sheath connected to the sleeve conductor and a center conductor connected to the inner conductor of said matching section, the impedance level of said radiator being less than AZo ohms, means to alter the spacing between said conductors of said high impedance matching section for an electrically short distance at the end thereof adjacent said radiator to render the impedance value thereat substantially equal to Zo ohms to transform the impedance level of said radiator to a value at which said high impedance matching section will be effective over said broad band.

7. In a broad band antenna system including a radiator extending normally to a conductive ground plane the outermost extremity of said radiator being a quarter wavelength from said ground plane, a high impedance matching section having an impedance value of AZO ohms where A is the maximum allowable standing 10 wave ratio and comprising concentric conductors coaxially arranged with respect to said radiator, the inner conductor of said matching section being connected to said radiator and the sleeve conductor being connected to said ground plane at a point intermediate its length, and a coaxial transmission line having an impedance value of Zo ohms and comprising a sheath connected to the sleeve conductor and a center conductor connected to the inner conductor of said matching section, the impedance level of said radiator being less than AZo ohms, a further matching section having an impedance value of Z0 ohms interposed between said radiator and haid high impedance matching section to transform the impedance level of said radiator to a Value at which said high impedance matching section will be eiective over said broad band, said high impedance matching section having a length of substantially 63 electrical degrees and the length of said further matching section being substantially one-tenth of that value.

8. A broad band antenna structure including a radiator extending normally to a conductive ground plane, a sleeve coaxially arranged in endto-end relationship with said radiator, there being an overlap between said radiator and said sleeve, said sleeve being connected to said ground plane at a point intermediate its length, a conductor within said sleeve connected to said radiator, said conductor and said sleeve constituting a high impedance matching section, the spacing between said sleeve and said radiator at said overlap being less than the spacing between said sleeve and said conductor therewithin, a coaxial transmission line comprising a center conductor and a sheath, said sheath being connected to said sleeve and said center conductor being connected to said conductor within said sleeve,

f whereby the impedance level of said radiator is REFERENCES CETED The ioilov/'ing references are of record in the le of this patent:

UNTED STATES PATENTS Number Name Date 2,163,860 Berndt Aug. 8, 1939 2,184,729 Bailey Dec. 26, 1939 2,184,771 Roosenstein Dec. 26, 1939 2,239,724 Lindenblad Apr. 29, 1941 2,241,582 Buschbeck May 13, 1941

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