US2487622A

US2487622A - Three-phase slot antenna system

Info

Publication number: US2487622A
Application number: US650952A
Authority: US
Inventors: Robert S Wehner
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1946-02-28
Filing date: 1946-02-28
Publication date: 1949-11-08
Anticipated expiration: 1966-11-08
Also published as: GB648277A

Description

Nov., S, @949 R. a WEHNER 294%?,522

THREE-PHASE SLOT ANTENNA SYSTEM Filed Feb. '291,A 3.94@ 4 Smets-Sheet l 20 210 zwwswfm" 2702600 50 40 INVENTOR ATTORNEY Nom 1949 R. s. wal-mm THREE-PHASE SLOT ANTENNA SYSTEM 4 Sheets-Sheet 2 Filed Feb. 28, 1946 500 ,9417 .930 1020 .M60 1100 jf40 mm1/Mfr w Me WM L L www w j M w H En Tl W-llw- ,MCM7 Mi- VQMM m M 0W M N zal? MFA/cy Innunmm l .lum M lllm... y lll-i ATTORNEY Nw@ @y w49 R. s. WEHNER www THREE-PHASE SLOT ANTENNA SYSTEM Filed Feb. '28, 194e 4 Shenyang@ a i ,2 il F g 17@ 26:22, 2C-5f@ fffa INVENTOR ATTO RN EY @www Nom 89 1949 R. s. WEHNER THREE-PHASE SLOT ANTENNA SYSTEM 4 Sheets-Sheet 4 Filed Feb. 2S, 3.946

ATTORNEY Patented Nov. 8, 1949 2,487,622 THREE-PHASE SLOT ANTENNA SYSTEM Robert S. Wehner,

of Delaware Application February 28,

Port Jelerson, N. Y., assignor to Radio Corporation of America,

a corporation 1946, Serial N0. .650,952

12 Claims. (Cl. Z50-33) The present invention relates to short wave antennas and, more particularly, to such antennas for use on modern high speed airplanes.

An object of the present invention is the provision oi an antenna for high speed airplanes which introduces no aerodynamic drag.

A further object of the present invention is the provision of an antenna for high speed airplanes which is entirely faired into the body of the fuselage or wing of the airplane.

A further object of the present invention is the provision of an ultra high frequency antenna having the following electrical characteristics:

l. A field pattern for horizontal polarization that is substantially uniform in the horizontal plane and in the hemisphere below the horizon.

2. Impedance characteristics such that the antenna may be matched With a less than 2:1 standing wave ratio to a 50 ohm line over a frequency band at least 10% wide.

The foregoing objects and others which appear from the following detailed description are attained in accordance with the principles of the present invention by providing a triangular arrangement of three thin rectangular slots cut in the conductive skin ci the ship in either the fuselage or wing and backed by resonatlng cavities. The cavities are fed in a three-phase arrangement such that horizontally polarized energy is radiated in a substantially uniform field pattern and such that relatively flat input impedance characteristics are attained.

The present invention will be more fully understood by reference to the following detailed description which is accompanied by a drawing in Which:

Figure l illustrates in a plan view a simple single rectangular slot and the radiating pattern in planes parallel to the mounting surface of said antenna, while Figure 2 is a transverse cross sectional View of the antenna of Figure l;

Figure 3 is a graph illustrating the variation in anti-resonant resistance of the antenna of Figure 1 with a variation in the placement in the feed point to the antenna.

Figure 4 is a eld strength pattern showing the distribution of horizontally polarized energy in the plane of the antenna of the present invention;

Figure 5 is a family of curves illustrating the resistance-reactance characteristics of a single slot and a three-phase slot antenna over a band of frequencies, while Figure 6 is a curve 4of the antennaof Figures 7 and 8 illustrating the band width the present invention;

illustrate in schematic form two representative ways in which the antenna of the present invention may be energized, while Figure 9 is a curve illustrating the relationship between the standing wave ratio on the branch lines of Figure 8 with reference to the input standing wave ratio on the main transmission line;

Figure l0 is a sectional view of the antenna of the present invention, while Figure 11 is a transverse section taken along lines II, II of Figure l0.

A simple faired-in slot antenna, suitable for use on aircraft, is shown in Figure 1. This includes a thin rectangular slot I2 cut in the conductive skin of the ship, The skin of the ship acts as a conductive ground plane and is denoted by reference letters GP. The minimum length L of the slot must be of the order of onehalf wavelength at the operating frequency while the width W of the slot may be quite narrow, only 1/20 or 1/30 of a wavelength or less, so it is mechanically practical for use at frequencies in ultra high frequency ranges. Such a slot may be fed in a variety of ways, but one of the easiest methods involves backing the slot with a rectangular resonating cavity I4 (Figure 2) of the same cross section and a quarter wave in depth. This cavity may be fed from the coaxial cable TL, the outer conductor I6 of which is connected to one lengthwise wall of the cavity at some point on its vertical center line and the inner conductor I8 of which extends through cavity I4 to tie on to the opposite side wall.

Since the electric lines of force are directed across the cavity, parallel to the short sides, the radiating surface of the antenna may be regarded as the displacement current sheet across the slot aperture. The displacement current sheet is likewise polarized across the slot aperture. Consequently, the field pattern in planes parallel to that in which the slot is cut is linearly polarized in a directon transverse to the slot, The resultant radiation pattern, denoted by line 20 in Figure l, is roughly a figure-eight with maxima in directions parallel to the slot and with minima in directions transverse to the slot. The degree of directivity is about the same as that existing in a plane containing a thin half wave dipole, the difference being that in the case of a dipole antenna, the maxima occur in directions transverse to the length of the antenna. It is apparent that a half wave slot such as shown in Figure 1 is as unsuitable for uniform horizontally polarized field patterns as a simple horizontal half wave dipole would be.

Aside from the radiational limitation mentioned above, the simple slot antenna of Figure l is marked by input impedance characteristics same manby the expression Z1Zg=(Zo/2)2 are the input impedance of the the dipole antenna respectively and Z is the lmpedance of free space.

theoretical basis for twoy outof the slot antenna input impedance characteristics; pedance level and the fact that it is anti-reso`l nant rather than resonant. Since Zo, the imabove resonance, the reverse must be true of a The input impedance character-` istics of a slot 25 centimeters long, 1 centimeter wide, backed by a cavity of the same cross secdeep, were measured for several positions of the feed transmission line TL over a range of frequencies extending from 900 to 1150 megacycles. One set of the resultant impedance data is graphically shown in Figure 5 wherein curve 25 illustrates the variation in resistance with a Variation in the frequency while curve 26 illustrates the variation in reactance. These curves reveal the single slot antenna to be sharply anti-resonant with a high impedance level. Data run at other positions of the feed point, that is, at the other values of the ratio d/D, where d is the distance of the cn- The chief effect of feed posif tion upon impedance characteristics is in terms the region over which measurements were actu- This or other standard low impedance lines over only relatively narrow frequency bands. For example,

mission line by means of a conventional quarterwave geometric mean transformer yields only 31.4% band width with less than a 2:1 standing wave ratio on a 50 ohm line. This is indicated by curve 30 of Figure 6. It might be expected thatl the simple slot antenna reactive relative to the impedance level; that is', it has a much higher value of QV and there'- fore a smaller intrinsic bandr width than would be obtained by matching a high impedance slot down to a low level. Greater band widths might possibly be realized by use of'complicated matching sections but it is believed that any of the type shown in ratio on a standard 50 ohm cable.

Now consider a delta shaped array of antenna slots such as those indicated in the diagram at the center of Figure 4 wherein the slots are identied by reference characters 4l, 42 and 43. This arrangement, with the displacement-cur'- rent sheets across the slot apertures represented by suitably averaged quarter wave long, the iield in the plane of the array may be shown to be given by the following expression:

where 1 @wat [1 -l-cos2 0-2 cos 0 sin (90 cos 6)] (2) and nre. 5 slot antennas, may be matched to 50v Ohm Tv muthal angle a, and

where RPE2 signifies Real Part of E2 IPEZ sianiiies Imaginary Part of E2, etc.

Equation 1 gives the relative iield strength in the plane of the array as a function of the. azlis plotted in Figure 4 as. curve 45. It will be noted that curve 45 is very nearly circular, exhibiting an even greater degree of circular symmetry than is yielded by a turnstile array; that is, by two crossed dipoles fed in phase quadrature. Such arrays of linear radiators have been commonly used heretofore in applications calling for uniform distribution of horizontally polarized radiation, but the application of this principle to slots is believed to be new. It will be noted that in the plane of the slots the polarization is linear but rotates with the carrier frequency. In the direction normal to the plane the polarization is circular, while in intermediate directions the polarization is elliptical.

The neld pattern shown in Figure 4 is not affected by moderate deviations from resonant frequency. The three-phase antenna yields substantially the same pattern over the entire range over which its input impedance characteristics are such that it may be matched to a standard transmission line, that is, input impedance rather than pattern characteristics will limit the useful bandwidth of the antenna.

It is readily evident that if n antennas of equal input impedance Za are to be fed with currents of equal magnitude an 'ri-phase relationship from a matched main transmission line of characteristic impedance Zo, the individual input impedances Za must equal nZo and the individual antennas must be fed by the equivalent of branch lines of characteristic impedance nZo, each successive branch line differing in length from the preceding by degrees. This result may be attained in two slightly different mechanical arrangements. The two arrangements are schematically illustrated in Figures 7 and 8.

In Figure 7, for example, the main transmission line TL, having characteristic impedance Z0, is directly connected across the first antenna identified by reference numeral 50, having an input impedance of 3Zo. In shunt across this antenna and across transmission line TL is connected a second section of transmission line TL1 having an electrical length of 120 and having a characteristic impedance equal to 1.520. This section of transmission line TL1 leads to the second antenna likewise having an input impedance of 32o and to a second section of transmission line TLz, likewise having an electrical length of 120 and having a characteristic impedance of 3Z0. Transmission line TL2 is terminated by its connection to the third antenna 52 having an input impedance of 3Zo.

The alternative arrangement shown in Figure 8 employs three physically separate and distinct transmission line sections TLi, "ILz and TLs. These three sections of transmission lines are all connected in parallel at one end and at the other ends are directly connected to antennas 50, 5l and 52. As before, the antennas each have an impedance 3Zo. Each of the transmission lines has a characteristic impedance equal to 32.0 and they differ in length by 120 electrical degrees. In the event that the individual antennas 5f), 5l and 52 are not perfectly matched to the individual branch lines; which will be the case if the system is to operate over an appreciable range of frequencies; perfect n-phase feeding will no longer exist and this may be expected to have at least some adverse effect both on the field pattern and on the input impedance of the array.

. possibility of interaction However, this effect is much smaller than might be expected. In the case of three identical antennas fed in three-phase relationship, it can be shown that if 7c is the magnitude of the reflection coefficient due to mismatch on the individual branch lines, and if K is the magnitude of the reflection coefficient due to the mismatch on the main feed line, then these two quantities are related by the expression K==7c3. Since the magnitude of the reflection coefficient is by definition less than unity it is evident that three-phase feeding has a pronounced broad banding effect. In terms of the standing wave ratio, more commonly used as a measure of mismatch, the aboveexpression is equivalent to s (S3-P38) (BSZ-t1) where S is the standing wave ratio on the main line and s is the standing wave ratio on the branch lines. This expression is plotted in Fig-Y ure 9 and indicates that the standing wave ratioon the branch lines, plotted as abscissae, can become quite high before the standing wave ratio on the main line, plotted as ordinates, becomes appreciably greater than unity.

While the main line reflection coefficient decreases as the cube of that existing on the branch lines only at or near the resonant frequency, that is, at that frequency at which the length of the branch lines are proper, the three-phase system has compensating tendencies over an appreciable range of frequencies to either side of resonance. That such compensating tendencies are appreciable even with a system involving individual antennas as inherently narrow band as the simple slot antenna of figure (Z) is demonstrated by curves 54 and 55 of Figure 5. These curves were calculated from measured input impedance data for a single ohm slot antenna input impedance, combined in a three-phase arrangement. It is evident by comparing curves 54 and 55 with 25 and 26 which relate to a single slot antenna that the input impedance of the threephase array is much flatter than that of the component antennas. The band width of a threephase array fed directly without an external matching section is 15.2% with less than a 2:1 standing wave ratio on a 50 ohm line as is lshown by curve 56 of Figure 6, as compared with the 3.4% for a single slot and matching section as shown by curve 30.

In the foregoing example of the broad banding effect of three-phase feed on slot antennas, the between the individual slots has been ignored. This'is not a serious objection to the arrangement, however, since it is evident, from considerations of symmetry and the fact that all three slots are identical, that the mutual impedances between any pair of slots in the array is the same as that existing between' any other pair of slots. Furthermore, the input impedance of any one slot in the array is equal to the difference between the self-impedance of that slot, that is, its input impedance when mounted by itself in a large ground plane, and the mutual impedance between any pair of slots. So regardless of the magnitude of the mutual impedances between the slots their input impedanoes are identical.

Of course, it is not sufficient that the individual input impedances of the component antennas be identical. rIhey must also be equal to three times the characteristic impedance of the main transmission line. Howeven this makesv little difference as far as the practical use of the antenna is concerned si-nce any input impedance impedance between the slots may be compensated for merely the position Furtherplane being parallel to the of the feeds of the individual slots. The individual slots are backed up by cavities 50, 62 and 64 having the same transverse dimensions as the slots and approximately a quarter wave in depth. The cavity 64 carries on one side wall an input cable connector TLC having a threaded outer acteristicl impedance of 70 ohms. The said inner conductor eventually passes through cavity 52 having an outer shell 'l5 and an inner conductor T6. An adjustableshorting plug T8 is provided constructed of a low loss dielectric such as polyethylene or polystyrene. At the point Where conductor 10 passes into slot 62 an inner conductor 12 branches off and being surrounded by outer shell `|3 forms a feed line for cavity 50. Said feed line like that formed by conductors 'I0 and 7| has an electrical length of 120 degrees. The diameters of conductors 12, I3 are so proportioned as to provide a characteristic impedancel of 140 ohms. Conductor '|2, after passing through cavity Si), is terminated in an end stub 82, the conductor |2 lbeing connected electrically to theI end of the outer shell 82 of stub 82.

As shown in Figure 11 the feed plane of the antenna is located far enough above the base of cavities 60, '62 and 64 array that thev input impedances of the separate slots, allowing for the eiect of interaction are each equal to 140 ohms, although any gure between and |65 would be satisfactory for a.- delta slot system fed from. a I ohm coaxial line. The system is thus approxithree feed positions may be mately equivalent to three` 140 ohm. impedancesl the overall input impedance is approximately one third of 140 or 47 ohms, close ing radiation eld is set up.

If desired, the input connector TLC can `be located on the inner wall of cavity 64 rather than on the outer' wall as shown in Figure 10. The outer wall is preferable, from the standpointl of convenience in making connections to the transmitter. Since cavity 64 is thus fed in a different manner from cavities 60 and 62 some slight discontinuity effect will be noted.

system shown in Figure 10, the restrictionis notl severe. If air-dielectric cavities and lines are used, the maximumy slot length is approximately 2/3 of the operating mid-band wavelength. Sincey deviate from resonance by at least 24` to either side before the system becomes inoperative dueto insufficient slot length. A potential band Width of at least 48% is considerably larger than that over which the impedance characteristics willbe satisfactory.

The cavities 69 and 52 are preferably closed by thin conductive window (Figure 11) of polystyrene or other low loss' dielectric material'.v The particular manner of' securing the closure where-l by the cavities are rendered The anges may easily be screwed or cemented on to the side walls of the cavity.

While I have illustrated a particular embodiment of the present invention, it

ing that a rotating eld is radiated.

2. An antenna system including a numberl of elongated narrow slots in a conductive sheet, said in a delta formation, a conof high frequency energy and a multi-branch transmission line connecting said source to each of said cavities, the lengths of the connections from. said sourcev to said cavities so differing that a rotating eld is radiated.

3. A n antenna system including a number of elongated narrow slots in a conductive. sheet.

said slots `being arranged in a delta formation, a conductive walled cavity back of each slot, a source of high frequency energy and a multibranch transmission line connecting said source to each of said cavities, the lengths of the connections from said source to said cavities so differing that a rotating field is radiated, the height of the point of entry of said connections into said cav ities being so chosen that the impedance presented at said connections bysaid cavities is equal to the impedance of said connections.

4. An antenna system including a number N of elongated narrow slots in a conductive sheet, said slots being arranged in a regular geometric pattern, a conductive walled cavity back of each slot, a source of high frequency energy and a multibranch transmission line having a primary section of Zu ohms characteristic impedance and branch sections connecting said source to each of said cavities, the lengths of the connections from said source to successively energized cavities differing by is N degrees whereby a rotating field is radiated, the height of the point of entry of said connections into said cavities being so chosen that the impedance presented at said connections by each of said cavities is N times the characteristic impedance of the said primary section of transmission line, or equal to the characteristic impedances NZo ohms of said branch sections.

5. An antenna system including a number of elongated narrow slots in a conductive sheet, said slots beingarranged in delta formation, a conductive walled cavity back of each slot, a coaxial transmission line from high frequency energy transducer means, the inner conductor of said line being coupled to each of said cavities in turn to energize the same, the length of line between successively energized cavities being such that a rotating field is radiated.

6. An antenna system including a number of elongated narrow slots in a conductive sheet, said slots being arranged in delta formation, a conductive walled cavity back of each slot, a coaxial transmission line from high frequency energy transducer means, the inner conductor of said line being coupled to each of said cavities in turn to energize the same, the length of line between successively energized cavities being such that a rotating field is radiated and the characteristic impedance of said transmission line so differing between successively energized cavities that the impedance of the transmission line at any point is equal to the resultant parallel impedance of the first succeeding cavity and the succeeding portions of the system remote from said transducer means.

'7. An antenna system including a number N of elongated narrow slots in a conductive sheet, said slots being arranged in a regular geometric pattern, a conductive walled cavity back of each slot, a coaxial transmission line from a high frequency energy transducer means, the inner conductor oi said line being coupled to each of said cavities in turn to energize the same, the length of the line between successively energized cavities being in N electrical degrees and the characteristic impedance of said transmission line so differing between successively energized cavities that the impedance 10 the resultant parallel impedance of the first suc-i ceeding cavity and the succeeding portions of said system remote from said transducer means.

8. An antenna system including a number N of elongated narrow slots in a conductive sheet, said slots being arranged in a regular geometric pattern, a conductive walled cavity back of each slot, a coaxial transmission line from a high frequency energy transducer means, the inner con ductor of said line being coupled to each of said cavities in turn to energize the same, the height of said line in said cavities being such as to present to said line an impedance N times that of said line, the length of the line between successively energized cavities being electrical degrees and the characteristic impedance of said transmission line so differing between successively energized cavities that the impedance of the transmission line at any point is equal to the resultant parallel impedance of the first succeeding cavity and the succeeding portions of said system remote from said transducer means.

9. An antenna system including a number N of elongated narrow slots in a conductive sheet, said slots being arranged in a delta formation, a conductive walled cavity back of each slot, a source of high frequency energy and a multi-branch transmission line connecting said source to each of said cavities to energize the same, the lengths of the connections from said source to successively energized cavities differing by in N electrical degrees whereby a rotating field is radiated.

10. An antenna system including a number N of elongated narrow slots in a conductive sheet, said slots being arranged in delta formation, a conductive walled cavity back of each slot, a coaxial transmission line from high frequency energy transducer means, the inner conductor of said line being coupled to each of said cavities in turn to energize the same, the length of line between successively energized cavities being such that a rotating field is radiated, said transmission line being terminated in an adjustable shortcircuited tuning stub whereby the feed to said cavities may be equalized.

11, An antenna system including three elongated narrow slots in a conductive sheet, said slots being arranged in delta formation, a conductive walled cavity back of each slot, a coaxial transmission line from high frequency energy transducer means to energize said cavities, the inner conductor of said line passing through the first and last of said cavities and having a branch connection passing through the second of said cavities, the length of line between successively energized cavities being such that a rotating field is radiated and the characteristic impedance of said transmission line so differing between successively energized cavities that the impedance of the transmission line at any point is eoual to the resultant parallel impedance of the first succeeding cavity and the succeeding portions of the system in the direction of said last cavity, said second and last cavities being provided with short-circuited tuning stubs connected to said line whereby the feed to said cavities may be equalized.

l2. An antenna system including a number of elongated narrow slots in a conductive sheet, said O the trarlSmSSlOr! lille at any l 011t iS equal t0 75 slots being arranged in a polygonal formation in 'said's'he'et a oondiictiVewalledv cavit'ybck ofeavcliV Y slot, a transmission linev from high frequency toV energize thev same, thev lengthv of line between successively energized cavitiesk being such that a rotating eld is radiated'.

ROBERT S. WEHNER;

REFERENCES CITED' The following references are of record in the lef of thisy patent:

112 UNITED STATES' PATENTS Number