US 2935746 A
Description (OCR text may contain errors)
y 1960 A. E. MARSTON ETAL 2,935,746
SPIRAL TROUGHYANTENNAS Filed Oct. 50, 1958 RADIO FREQUENCY DEVICE SPIRAL ELEMENT ROTATOR a RADIO FREQUENCY DEVICE INVENTOR ARTHUR E. MARSTON JULIUS A. KAISER,JR.
ATTORNEY United States Patent SPIRAL TROUGH ANTENNAS Arthur E. Marston, Alexandria, Va., and Julius A. Kaiser, Jr., Kensington, Md.
Application October 30, 1958, Serial No. 770,899 6 Claims. (Cl. 343-761) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
This invention relates to antenna systems in general and in particular to spiral antennas which can be utilized for scanning purposes.
In many applications of radio frequency operative devices it is desirable to have an antenna system of simple structure which is capable of providing scanning of a beam of radiated energy over a sector of space. Conventionally such scanning may be accomplished mechanically or electrically. When it is accomplished electrically it may be provided by a switching of the excitation of various antenna elements, however such is subject to certain limitations which in some instances are undesirable. For example, in the lobe switching type of operation, certain periods of inactivity necessarily occur during portions of the switching cycle, providing non-uniform loading of the radio frequency energy generator and periodic interruption in the operation of the complete locator ssytem. In certain other types of operation, scanning is obtained by the continuous manipulation of complex variable coupling devices between the radio frequency energy generator and the antenna elements rather than the abrupt change from one antenna condition to another. Since the foregoing types of operation are frequently undesirable for one reason or another, it is an object of the present invention to provide an electromechanically scanning antenna systern of non-complex structure.
Another object of the present invention is to provide a scanning antenna system wherein an energy beam may be caused to move substantially continuously over a substantial angle.
Another object of the present invention is to provide a spiral antenna system which will scan a selected sector of s ace.
Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following de tailed description when considered in connection with the accompanying drawings wherein:
Fig. 1 isa schematic showing of an elementary spiral antenna system.
Fig. 2 shows a spiral antenna placed in front of a right angle trough reflector.
Fig. 3 shows a spiral antenna array placed in front of a right angle trough reflector.
In accordance with the basic teachings of the present invention an antenna system is provided wherein a spiral antenna element is disposed in proximity to a reflector in such configuration as to combine the opposite sense circularly polarized signals produced on opposite sides of the spiral antenna element to obtain linearly polarized radiation. As a further extension of the teachings of the invention, a plurality of spiral antenna elements are employed in an array wherein the major directivity of such 2,935,746 Patented May 3, 1960.
radiation may be shifted merely by rotation of individual spiral antenna elements.
A spiral antenna element as typified in Fig. 1 is composed of two electrical conductors disposed in a plane and which originate at a central portion spiralling outward to a selected radius. The two conductors terminate at points which are diametrically opposed to each other. Typically the assembly could be formed with a backing member of insulating material by printed circuit techniques. When such an antenna element is energized by out of phase signals applied to the two conductors at the central points, a broad circularly polarized beam is produced on each side of the flat spiral substantially perpendicular to the plane of Fig. 1. The beam radiated from one side of the spiral exhibits circular polarization of the opposite sense from that radiated from the other side. Aside from this polarization characteristic, the two radiated beams are substantially identical. In many applications it is desirable that such an antenna element radiate to one side of the spiral only. This is accomplished by an appropriate backing of the spiral on one side by a ground plane or a cavity; however in the present instance the beams from both sides of the spiral are both utilized to a particular advantage in a novel combination with a trough reflector.
Although at the present time there is no universally accepted theory for the operation of the spiral antenna element itself, Fig. 1 indicates certain dimensions which in accordance with the following discussion will be of assistance in understanding the design possibilities of this antenna. The two conductor spiral antenna behaves as a two wire transmission line which gradually by virtue of its spiral geometry transforms itself into a radiating struc ture or antenna. It is well known that a two Wire transmission line of relatively narrow spacing yields negligible radiation when excited anti-phase at its terminals. This is due to the fact that the currents in the two wires of the line in any normal cross-section are always electrical degrees out of phase so that radiation from one line is essentially cancelled by radiation from the other. Such is true if the lines are closely spaced in terms of the excitation wavelength and are of equal length. The situation is diiferent when a two conductor transmission line is disposed in a spiral configuration as in Fig. 1 and energized anti-phase by a radio frequency signal; the adjacent conductors of the spiral are not always 180 electrical degrees out of phase. With point 15 of Fig. 1 selected as a point on one of the conductors of the line which is at a selected distance along the conductor from the initial point 16 of that conductor, point 17 of the other conductor will be located a similar distance from its starting point 18. Point 17 is diametrically opposite to point 15 with respect to the center 19 and both points 15 and 17 lie on the circurnference of a circle centered at 19. This implies that the point 15 and its neighboring point 20 on the other conductor directly alongside point 15 lie at such distances from points 16 and 18 respectively that the difference of these distances is precisely the arc length 1720 along the spiral. If the spacing between points 15 and 2!) along a radius from point 19 to point 26 is much less than the radial spacing of points 19 and point 15, the arc length 17-20 is approximately equal to 1r). Assuming that each conductor supports a progressive wave of current and that these current waves are anti-phase at points 16 and 13, where a suitable line feeds the spiral, it is clear that the difference in phase of the two current elements at any point 15 on the two conductor line, measured in radians, is
where x is the current wavelength on the spiral and r is the radial distance from the center 19 to the point 15'.
Thus neighboring current elements are antiphase at the points 16 and 18 and gradually come into phase as one proceeds outwardly along the spiral two wire line. When 1 is A these currents are precisely in phase and radiation is a maximum. When ris 21r the phase change is and the circumference is Where a spiral is taken having a circumference greater than A there will be inner portions where the circumference is a full wavelength for a path length difference of for a band of frequencies rather than a single frequency. This quality makes the spiral antenna an inherently broadband device, the basic requirement being only that the maximum radius be large enough to allow a half wavelength of phase shift between current elements on adjacent conductors at the minimum frequency of the band.
With careful design construction and feed of such a single spiral antenna element, circular symmetry about the spiral axis can be obtained which allows rotation of the spiral about its axis to produce a change in phase of the radiated field everywhere in space without variation in the far field amplitude. With such rotation, one degree of mechanical rotation of the spiral produces a corresponding change of one electrical degree in the phase at any point in the far field. Electrical connection to the conductors may be by leads 21 which go to a receiver or transmitter as desired. These leads typically may be the inner and outer conductors of a coaxial cable or' the two conductors of a balanced line, the latter being preferable, however it is readily understood that the feed could be basically a coaxial cable for simplicity of rotation, with an unbalanced to balanced converter placed between the coaxial line and the spiral conductors. The desirability of such variations and the mechanism for their accomplishment are well known to those skilled in the art.
When a spiral antenna element such as that shown in Fig. 1 is placed in proximity to a special form of reflector, such as a right angle trough 25 shown in Fig. 2, in such position that the plane of the spiral lies in the plane bisecting the angle between the sides of the trough, the field produced at one side of the spiral, say side 26, is reflected by one side (27) of the trough 25 from whence it is emitted to space in a direction substantially parallel to the plane of the spiral antenna element 24 and perpendicular to the trough apex. Such energy is reflected from side 27 with a characteristic direction of circular polarization. Typically this may be right-hand polarization. Similarly, energy from the opposing side (28) of spiral antenna element 24 when reflected from the surface 29 of the trough 25 will travel outward in a direction substantially parallel to the plane of the flat spiral antenna element 24 and also therefore parallel to the energy from side 26 as reflected by plane 27. The energy from plane 29 will however be characterized by polarization of an opposite and therefore typically left-handed sense. When the two fields combine in space, such combination of the right-hand circularly polarized field with the left-hand circularly polarized field will produce a linearly polarized far field wherein the plane of polarization is parallel to the plane of the spiral antenna element 24. This plane of polarization is independent of the angular rotational position of spiral antenna element 2.4; however, the phase of the field at any point is dependent upon the angular orientation of the spiral antenna element 24. In such an antenna arrangement as that of Fig. 2 therefore it is possible to produce a far field having linear polarization and in which the phasing at any point may be controlled merely by the rotation of the spiral antenna element 24 within its plane as shown in Fig. 2. The direction of polarization may be varied by varying the spacing between the spiral antenna element 24 and the trough 25.
When a'plurality of such spiral antennas 30 and 31 are placed in a single trough reflector 32 as shown in Fig. 3 and provided with suitable means 33 for rotation and anti-phase electrical excitation of the two conductors of each spiral, each spiral antenna contributes to the energy in the far field; however when these portions of energy combine in space the net result is a composite field which will be at a maximum in a direction perpendicular to the apex of the trough only when the fields produced by the two spiral antenna elements are in phase in the far field. Where such fields are not in phase, the maximum for the composite field will be achieved in some direction other than that perpendicular to the apex of the trough, theangular displacement being dependent upon the exact phase relationship. The phase change can be brought about by physical rotation of one of the spiral antennas in the plane of the spiral by suitable means 33. In practice antenna arrays have been constructed'which produce scanning angles of the order of 45. In such antenna arrays one element, typically 30, would not rotate whereas each additional element, 31, etc. would be rotated through angles of 6, 20, 30, 40, 50, etc., respectively. Additional elements provide sharper beams however the scan angle is not affected substantially by the addition of other elements.
From the foregoing it is obvious that considerable variation of the specific structure is possible without exceeding the scope of the invention as defined in the appended claims, for example, more than two elements 30, 31 could be incorporated in an array, with the elements being rotated at selected angular relationships and directions as described above to produce particularly desired forms of scanning and beam configuration, all of which may be readily predicted by considering the effect of the rotation upon relative phasing of the energy from the various elements in the far field.
It is likewise understood that considerable variation of the dimensions of the trough and the location of the spiral antennas within the trough reflector is possible. Typical dimensions for the apparatus of Fig. 3 are as follows:
Diameter of each spiral antenna inches 4 Spacing between spiral antenna elements do 4.25 Spacing of spiral antenna element center and apex of trough ..inches 4 Length of trough (along apex) do 36 Width of side of trough (perpendicular to apex) inches 18 Frequency of operation mc.. 1430 Feed system coaxial cable matched to two conductors of each spiral with balanced to unbalanced converter. Scan angle :45
What is claimed is:
1. An antenna system comprising, a two conductor planar spiral antenna element, a right angle trough reflector, and means mounting said element in proximity to the reflector with the plane thereof in the plane bisecting the right angle of the trough reflector.
2. An antenna system comprising a base member composed of insulating material, first and second conductors supported thereby in a planar configuration relatively insulated from each other, each said conductor being in the form of a spiral from a portion having a small radius to a portion having a larger radius, a right angle trough reflector, and means mounting said base member and said first and second conductors in proximity to the reflector with the plane thereof substantially in the plane bisecting the right angle of the trough reflector.
3. An antenna system comprising, a base member composed of insulating material, first and second conductors supported thereby in a planar configuration relatively insulated from each other, said conductors being in the form of a spiral from a portion having a small radius to a portion having a large radius, a right angle trough reflector, means mounting said base member and said first and second conductors in proximity to the reflector with the plane thereof substantially in the plane bisecting the right angle of the trough reflector, and means for rotating the base member about an axis substantially perpendicular to the plane bisecting the right angle of the reflector.
4. An antenna system comprising a plurality of'two conductor planar spiral antenna elements, a right angle trough reflector, and means mounting said elements in proximity to the trough reflector with the plane thereof in the plane bisecting the right angle of the trough reflector. p
5. An antenna system comprising, a plurality of spiral antenna elements, a right angle trough reflector, and means for rotatably supporting said elements in front of the reflector, the elements being disposed substantially in the plane bisecting the trough and rotatable about an axis perpendicular to the bisecting plane.
References Cited in the file of this patent UNITED STATES PATENTS 2,663,869 Adcock et a1 Dec. 22, 1953 2,764,756 Zaleski Sept. 25, 1956 2,850,732 Kondoian et a1 Sept. 2, 1958 2,856,605 Iacobscn Oct. 14, 1958 FOREIGN PATENTS 460,490 Great Britain Jan. 28, 1937 OTHER REFERENCES Pub. I, Ground-to-Air Antenna Uses Helical Array," Electronics, March 1956, pp. 161, 162, 163.
Pub. II, Airborne Spiral Antennas Minimize Drag, Aviation Week, July 14, 1958, pp. 75, 77, 79, 81 and 82.
Pub. III, Antennas, Keans, McGraw-Hill, 1950, page 6. An antenna system comprising, a plurality of base 0 3 8-