US 2844818 A
Description (OCR text may contain errors)
'July 22, 1958- A. ALFORD 2,844,818
ANTENNA Filed July 22. 1953 2 Sheets-Sheet 1 B a A T15. l Q) 'July 22, 1958 I ALFQRD. R 2,844,818
ANTENNA Filed July 22. 1953 2 Sheets-Sheet 2 United States ANTENNA Andrew Alford, Cambridge, Mass. Application July 22, 1953,.Serial No. 369,613
17 Claims. (Cl. 343-767) Thepresent invention relates to directional. wide band antennas in ranges running approximately. from 100 to 2500 megacycles. For many years past such directional antennas were used with many types of transmitters in connection with. which attempts were made to establish as wide a frequency band as possible, so as to take in the desired frequencies at which the signals were to be transmitted. The frequency range of'such. antennas had ratios of approximately two to one, but efforts to increase this range were not veryisuccessful. For such purposes sleeve antennas with'refiectors having 135 corners were used, but results were however, not promising for increased band range because" the following limitations were'encountered.
(a) The frequency range of the sleeve antenna appeared to be limited to approximately 2.3 to' l Without the reflector and 2.2 to 1 or less with'theva'rious types of'reflectors which were tried. V
(b) Theradiation pattern split into two'majorlobes at about 2.3 times the lowestjfrequency at which" the antenna impedance could be compensated.
' The limitationunder (ti) has" to'd'o'wi'th the proper ties of the conventional sleeve antenna. under (1)) is 'associated wi'th phenomena of more fundamental-character. Adipole antenna when placed infront of a reflector results inrwo signalsbeing sent out in the direction of'the desired: beam,"nam'ely thedirect signal and the signal sent back to the reflector. When the. space between the antenna and the reflecting surface is' not too different from; A Wave length of the signal, thentwosignals 'are in the same phase. with each other and therefore: add'. When the.frequency 'is changed so that this space. becomes: onehalf wave length, the two. signals-tend: to cancel oneanother and; therefore, produce. atnull in the direction normal. to the:reflector. At the lowest fre'quency 'within'i the: operating band, the spacing between/the, antenna and a.fiat reflector cannot bemade -much; less; than. about .22. wave length. because even atthosespacings therezis an: adverse effect: oni the antennainput impedance. Thisefiecttisr difficulttozc'ompensa-teifon At azfrequency about; 213- timesl the". lowest frequency,1the-spacing1 between theiantenna and the re- Hector. becomeswequal-i to: one-half. wave length: and-the reflectedrsignal tends-to cancel; the: direct signal. from the antenna. Since-thebeaml width.iseofdhe ordenof. 35 .to 60. ,.,the. reflector used;isqtoossmallato make it pose sible for the. reflected signal to overpower the direct A 2,844,818 Patented July .22, 71958 in obtaining this broad band frequency range, first of which comprises proper antenna grouping, and secondly,
the use of a so-calle'd flared slot antenna which'at the upper frequencies in the desired range. has some directivity in andof' itself. These features and other princir ples upon which the present. invention is based will be more fully discussed in the rest of the specification set forth below when taken in connection with the drawings illustrating an embodiment of the same, in which:
Figure 1 shows a perspective view of an antenna" group according to the present invention employing a reflector;
Figure 2 shows a single antenna unit with a portion removed to show a flat slot on one side.
Figure 3 shows a pair of antennas with connections partly in diagrammatical representation.
Figure 4 shows the development of the array of antennas of the, present invention illustrating. the principle upon which the present invention is based.
Figure 5' shows typical patterns of an array. of the kind shown in Figures 1 and 4'.
Figure-6' shows a modification of the. arrangement of Figures I to 4 inclusive with a parabolic reflector and a internal conducting members whichwill be described later.
The limitation signal.. two signals--are:"=then-abont equal. s.o':that.a
Inoverco-ming the diflicul'es gaboveset forth, therapplicant. hasrin. generalq used-twmfeatures-by means. of which .aifrequency rangein the-ratio oliZ 3.3 ton1. orcbetter has been obtained. Two elements particularlystandiiout The caps are separated" from'the central part by flat slots and 6 respectively; These flat slots are" comparatively narrow att-he; point'of feed and widest at'th'e side diagrammatically opposite the feed. The feeder for the caps-- or the feed across the slots 5 and 6 are the co: axial feed cablesT and 8 which have outer conductors 9 =and 10- respectively and inner conduetors 13 and" 14 respectively which cross the gap or slot at its-narrowest point-and" which areconductivel y connected to the other side of the: slot through conductive blocks 15 and I6 respectively. 7 v r 'I-hecoaxialbranch feeders 7' and 8f extend through the central armportion 1 of-the antenna and through the stem 4' as indicated by the sections 17' and 18 in" the stem-. of'the antenna. The antenna, therefore, is in the form. of: a T' withslotsor air gaps on each side of the armbranches of the- T. While it is preferable to construct the antenna of cylindrical conductors, it is not necessaryto confine the crosssection of the antenna to this shape and 'as hasbeen statedabove', other equivalent shapesf'providing the same results however may be used forithe T forming the antenna. e
As indicatedmore clearly; in Figures Z and 3,.the coaxial feedin-g sections 17 and 18 in the stem 4 are extended throughithe sides of the stem as at'19 and 20',- Figure 1, into ajunction hox 22 where they connect to thebalun or coupler" 23, 24*with-in the shielded conductor 25. As will be seen; each coaxial branch 19,- only one of which is showniiniFigure's 1 and 3, has its inner conductor 2:6 coninctlreisame manner as the first antenna which has been 3 described above. The power to the balun 23, 24 is fed through the coaxial cable comprising the inner conductor 27 and the outer conductor 28. The antennas of the pair, similar units of which are designated as 'A and B respectively, are mounted on a plate 30, which comprises part of the reflector, the rest of which is formed as a conductive frame having rods 31, 41, etc., tied together at the end by cross rods 32, 32, etc. Any other form of grill or full plate is satisfactory,-providing the spacings between the conductors are small compared to a wave length so that no energy will be transmitted through the reflector but will be reflected back. Dimensions and spacings of the units from one another and from the reflector and also therelation and shape of the slot will be more fully explained in portions of the specification which follows below.
Before discussing this, it is however, well to discuss the principles underlying the structures described above. When a broad band antenna, such as for example, the antenna described in Figure 2 is placed near and parallel to the metal sheet, the field reflected signal from the sheet produces a change in the impedance of the antenna. An identical effect could be produced in the absence of the sheet by an image antenna in which the current is in opposite phase to the current in the primary antenna. These two equivalent arrangements are illustrated in views A and B, Figure 4. Consider now two identical primary antennas such as A and B placed in front of a metal sheet as illustrated in view C, Figure 4. This arrangement is equivalent to that shown in view D in Figure 4, in which the sheet has been replaced by images A and B. When the current in antenna B is equal to and in the same phase of the current in antenna A, the effect of the image A 011- the impedance of the antenna A can be exactly neutralized by the opposite effect of antenna B, provided the distance S between A and B is made equal to 2D the distance between the antennas and their images. Under these conditions, the impedance of the antenna A is subject only to the efiect of image B which is S /2 or 1.41 times as far away from the antenna A as its own image A. By the use of this expedient, the two antennas can be placed closer to a reflector. In fact, the distance S between the two antennas can be made .707 of the distance between a single antenna and its image before the effects of the reflected signal on antenna impedance is equalized by the position of the adjacent antenna.
As may be expected, the distance S need not be equal to 2D; but may be made considerably less this value before the beneficial effects of the antenna B on the impedance of antenna A and vice versa is lost. This possibility of varying distance S within the limits allows one to make this distance equal to one half of wave length at a frequency in the upper portion of the antenna range. The advantage of this arrangement is to decrease the radiation at right angles to the direction of the beam, thus simplifying the task to be performed by the reflector.
The following is given as an example of the dimensions of the antenna system in a range from approximately 300 megacycles to approximately 1080 megacycles which is a ratio of about 3.6 to 1. For such a construction, the overall length of such antenna was 13 /8 inches; the spacing between the center lines of the two antennas was 9"; the spacing between the center line of each antenna and the center line of the reflector taken normally to the antennas was 6 inches, the diameter of each antenna was 2% inches. With these dimensions, the distance between the antennas aud the reflector becomes A wave length at approximately 440 megacycles and one half wave length at approximately 880 megacycles. Under such conditions, with ordinary dipoles of the prior art, the beam would split with frequency approaching /27\ at 880 megacycles. This difliculty can be avoided by making use of the slot dipole antenna of Figure 3, in which the coaxial feeders apply the energizing voltage notcentrally but atone side of the slot. When a slot is ener 4 gized in this manner, the dipole itself can be made to have a directional pattern with the maximum directed away from the feeder connecting across the slot.
These experiments show that when the circumference of the dipole as measured around the slot is of the order of or less, the radiated pattern in the plane at right angles to the dipole is more or less omnidirectional but when the circumference becomes /2)\ this pattern becomes directional with the maximum directed away from the point of feed.
In the example given above, the flared slot becomes one half wave length at 685 megacycles. It is quite directional at about 600 megacycles and remains directional at frequencies above 600 megacycles. At the lowest frequency in the operating range, the distance between the reflector and the antenna is equal to .17 and the space between antennas is .228)\. At the highest frequency of the band, the distance between the reflector and the center line of each antenna is .628)\ the spacing between the antennas is .855). and the circumference of the slot becomes .78)\. Overall dimensions of the reflector may be 30 x 17". If'desired a larger reflector could be used. The flaring of the slot increases the low impedance at the point of feed. The impedance increases to a more useful value at frequencies at which the circumference of the slot becomes half wave length. This follows because the voltage distribution along the slot increases in the flared slot further away from the point of feed and therebyfmaintains theimpedance at a higher value than it would. normally have. over the frequency band with a slot of uniform dimensions. The flaring of the slot may be in .the general relation as indicated by the drawing of about 3 m4 to 1. It will be noted from Figure 2 and Figure 3, that the feed across the slot is placed on that side of the antenna nearest to the reflector so that when-the radiation of the antenna becomes more directive in itself, as will be with higher frequencies, the beam will be directive away from the position of the reflector. It has been possible in the design herein described, to maintain a constant voltage standing wave ratio which for the most part, except at the very ends of the band, is in the vicinity of 1.5 or thereabouts; while at the ends of the bands, this voltage standing wave ratio may rise slightly higher, but can be held down to below 2 with a proper series section in the balun disturbing the characteristics of the antenna at otherfrequencies.
An example has been given for an antenna operating within the range from 300 to 1050 megacycles. Antennas have been designed which will operate over other frequency bands as an example from 600 me. to 2140 me. and it is possible using the features herein described, to design wide band antennas in other frequency ranges.
Distances fromthe antennas to the reflector have been measured from the center of the cylinder in which the air gap lies perpendicular to the reflector surface. This may be considered substantially the effective distance of the antenna from the reflector where the radiation of the antenna is uniform in the plane in which the air gap lies. Where the radiation pattern changes with the changein frequency, the radiation becomes directive away from the reflector. However in the present case, the effective distance of the antenna from the reflector is still considered the distance D.
It was found that a flat reflector of 35). long and .43A is satisfactory where A corresponds to the lowest operating frequency. Larger reflections may be used.
Typical curves of radiation pattern for 300,700 and 1000 me. in a horizontal plane when the reflector is perpendicular areshown in Figure 5. The radiation is directive through the axis and perpendicular to the plane of thereflector. a r I In Figure 6 there istshown'aparabolic reflector 40, in the focus of which is positioned a dipole 41"with slots 42, 42,- symmetrically positioned on eachside of the stem 43. of the T-shaped structure; The slots 42, 42 are flared outward towards the reflector and fed across the narrow points of the slot farthest away; from the reflector. In' this construction a very narrow beam patternmay, be obtained since the maximum radiation from the antenna is directed into the reflector. @At the lowerfrequencies in the operating band where the antenna itself is non-directional, the reflector spacing is such that the direct and the reflected radiation add. At the higher frequencies in the operating band, the'spacing becomes such as to result' in potential can cellation of the reflected signal by the direct signal. The latter signal however is only a fraction of the former, andtherefore can be overridden by it.
Having now described my invention, I claim: 1. Abroad band high frequency directional antenna system comprising a pair of parallelly positioned T- shaped antennas, means forming air gaps in the cross bars of the T-shaped antennas symmetrically placed with respect to the stern of the T, said air gaps being in planes substantially perpendicular to the cross bars'of the Ts and extending all around their peripheries, a conductive reflector having a surface spaced at a distance D, from said cross bars and said cross bars spaced at a distance S from each other, said distance S being not greater than 2D. and not lessthan D /i where thedi'stance D is equal wherek; is a wave. length corresponding to a frequency within the transmitting band. I
2. A broad band high frequency directional antenna system comprising a pair of circularly slotted antennas, a conductive reflecting surface positioned on one side of the antennas, means supporting said antennas in 'a fixed position with regard to said reflecting surface, the effective distance of the antennas from said surface being D, said antennas having an effective spacing from one another equal to S where S is not greater than 2D and substantially not less than D /2 with the distance D being equal to and substantially not less than DVE with the distance D being equal to where A is a wave length corresponding to a frequency within the transmitting band.
4. A broad band high frequency directional antenna system comprising, a conductive reflecting surface, a-
pair of circularly flared slotted antennas having the plane of the slot substantially perpendicular to said conductive reflecting surface, means providing said conductive reflecting surface at a fixed distance from said slots, said conductive reflecting surface having an effective distance from said slots of D and said slots being spaced from "'6 oneanother' at a distance'S" where Sis notgreater than 2D and substantially not less than D /2 with the dis tance D being equal to greater than 2D and substantiallytnot" lessthan Dx/i with the distance D being equal to where is a wave length corresponding. to a frequency within the transmitting band.
6. A broad band high frequency directional antenna system comprising a conductive reflecting surface, a pair of circularly flared slotted antennas having the plane of the slot substantially perpendicular to said conductive reflective surface with the narrowest part of the slot nearest to the reflective surface, means for feeding the slot across' its narrowest point, means providing said conductive reflective surface at a fixed distance from said slots, said'conductivereflecting surface having an efl'ective distance from said slots of D and said slots being spaced from one another at a distance S where S is not greater than 2D and substantially not less than D /i with the distance D being equal to where is a wave length corresponding to a frequency within the transmitting band.
7. A broad band high frequency directional antenna system comprising a pair of parallelly positioned T-shaped cylindrical tube antennas each having a cylindrical air gap on each side of the cross bar of the T symmetrically positioned in substantially parallel planes transverse to the axis of the cross bar, means for feeding said antennas through the stem of the T across the cylindrical gap, a conductive reflector for said antennas, said conductive reflector being substantially perpendicular to the plane of said air gap and spaced at a distance D therefrom, said antennas being effectively spaced at a distance S from one another where S is not greater than 2D and substantially not less than Dx/i with the distance D being equal to where A is a wave length corresponding to a frequency within the transmitting band.
8. A system as in claim 7 wherein said feed across the air gap is at a point nearest the reflecting surface.
9. A broad high frequency directional antenna system comprising a reflecting surface, a pair of circularly flared slotted antennas having the planes of the slot substantially perpendicular to said reflecting surface, means for feeding said slot across its narrowest point said narrowest point being positioned nearest to said reflecting surface and flaring out to a widest point at a diametrically opposite position across the slot, the effective distance of the antennas from said surface being D, said antennas having an effective spacing from one another equal to S "7 where S is not greater than 2D and substantially not less than D /2 with the distance D being equal to y is not more than 2D and substantially not less than D /2 where D is equal to where is 'a wave length corresponding to a frequency within the transmitting band.
11. A broad band cylindrical conductive antenna having a slot substantially transverse to its axis, said slot extending all around the periphery of the cylinder, means feeding the slot across one point of its periphery, said slot having peripheral length not less than .4)\ Where A corresponds to a frequency within the operating band whereby the radiation from said slot is substantially directional away from the point of feed in the plane of the slot.
12. A broad band antenna structure as in claim 11, wherein said slot has a peripheral length between .41 and .8).
13. A broad band directive antenna comprising a conductive reflector and a dipole radiator having a pair of flared slots in substantially parallel planes normal to the reflector and means for feeding said slots at a point where the slot is narrowest.
14. A broad band directiveantenna comprising a conductive reflector and a dipole radiator having a pair of flared slots in substantially parallel planes normal to the reflector and means for feeding said slots at a point where the slot is narrowest, said slots being flared in the directio'nof the reflector.
15. A broad band directive antenna comprising a conductive reflector and a dipole radiator having a pair of flared slots in substantially parallel planes normal to the reflector and means for feeding said slots at a point where the slot is narrowest, said slots being flared in a direction away from the reflector.
16. A broad band directive antenna comprising a conductive reflector and a dipole radiator having a pair of flared slots in substantially parallel planes normal to the reflector and means for feeding said slots at a point where the slot is narrowest, said slots being flared in the direction of the reflector, said slots being spaced from the reflector substantially one-quarter of a wave length in the lower frequency band range whereby the direct and reflected signal add, and greater in the higher frequencies of the band range whereby the maximum energy is directed towards the reflector and reflected.
' 17. A device as in claim 16, wherein the reflector is substantially a parabola.
References Cited in the file of this patent UNITED STATES PATENTS 2,414,266 Lindenblad Ian. 14, 1947 2,416,252 Fisher Feb. 18, 1947 2,635,187 Dorne Apr. 14, 1953 FOREIGN PATENTS 590,413 Great Britain i July 17, 1947 869,089 Germany Mar. 2, 1953