US 3868689 A
An ILS marker beacon antenna of the type wherein a pair of parallel conductors form a balanced transmission line and support a plurality of lateral radiators of graded lengths. An unbalanced RF transmission line extends through one of the conductors to the apex feed point of the antenna with one conductor of the unbalanced line connected to one of the pair of conductors and the other conductor of the unbalanced line connected to the other of the pair of conductors for flow of RF current from the feed point along the balanced transmission line to the radiating elements. An electrically small dipole monitor antenna mounted on the parallel conductors terminates the balanced transmission line by short circuit in a region near the back of the log periodic antenna to prevent back radiation and monitor antenna operation.
Description (OCR text may contain errors)
lUited @ttes atent 1191 Lin et a1.
[4 1 Feb.25,1975
1 1 LOG lPERllQDIQ POLE MUUNTED MARKER BEACON ANTENNA  Assignee: Texas-Hrustruments Incorporated,
 Filed: July 30, 1973 211 App]. No.: 383,736
343/792.5, 343/821  int. 1. ..G01s1/68  Field of Search 343/101, 703, 792.5, 821,
Primary Examiner 'l". H. Tubbesing Assistant Examiner-Richard E. Berger Attorney, Agent, or FirmHarold Levinc; Rene E. Grossman; Alva H. Bandy  ABST1RAT An ILS marker beacon antenna of the type wherein a pair of parallel conductors form a balanced transmission line and support a plurality of lateral radiators of graded lengths. An unbalanced RF transmission line extends through one of the conductors to the apex feed point of the antenna with one conductor of the unbalanced line connected to one of the pair of con ductors and the other conductor of the unbalanced line connected to the other of the pair of conductors for flow of RF current from the feed point along the balanced transmission line to the radiating elements. An electrically small dipole monitor antenna mounted on the parallel conductors terminates the balanced transmission line by short circuit in a region near the back of the log periodic antenna to prevent back radiation and monitor antenna operation.
13 Claims, 11 Drawing Figures SHED .3 U? 3 LOG PERIQDIC POLE MOUNTED MARKER BEACON ANTENNA This invention relates to an antenna for an ILS marker beacon. In a more specific aspect, the invention relates to an antenna-monitor system in which the monitor modifies the radiation pattern of the antenna to provide a particular and desirable back radiation pattern.
Also provided is a continuous indication of the operation of the antenna system in order to provide a warning when inoperable.
The present invention is particularly suitable for use as an ILS marker beacon antenna used during aircraft approach to the end of an airport runway. Generally well known is the use of an outer marker and an inner marker. Such markers comprise a properly shaped electromagnetic field directed vertically over a glide path. Measurement of the intensity of the field provides a pilot with an indication of height providing a check during an approach to touchdown. Such checks are available at what are known as the outer marker and the inner marker. Sometimes intermediate marker beacons are installed between the outer and inner markers.
In the past, marker beacon antennae have comprised arrayed dipoles over a counterpulse. Such installations are expensive and occupy relatively large areas. Limited are the places that they can be installed. For example, a counterpoise comprising a horizontally disposed mesh wire net of about 25 feet by 18 feet and supported above the ground is necessary. Dipoles in a particular configuration are then supported at a given distance above the counterpoise. The radiation from such units is then transmitted at a controlled level such that a signal level dependent light will be triggered in the aircraft as it passes over the marker beacon at the proper height thus confirming accuracy of the approach.
Because of the large size of such systems and the limitations on locations for installation, it has been found desirable to provide a marker beacon antenna which more efficiently utilizes space. At the same time the beacon must be reliable in its operation and provide field patterns compatible with aircraft operations. Operation of marker beacons is at a single frequency, 75 megahertz. Heretofore dipoles or other narrow band antenna have been employed. However, the pattern of log periodic antennae approaches the pattern desired for marker beacon applications. The present invention employs a log periodic antenna as its basis with a modified back radiation pattern so that it may be suitable for operation as a marker beacon antenna.
The present invention is directed to a log periodic antenna which can be simply pole mounted and whose back radiation pattern is modified by a monitor antenna such that a unique and highly desirable radiation pattern is produced.
More particularly in accordance with the present invention, there is provided a log periodic antenna which is adapted to be pole mounted in a vertical plane with the plane parallel to the glide path of an aircraft. The antenna comprises a balanced transmission line to which radiating elements are attached. The transmission line is formed by two parallel rods spaced apart in a direction perpendicular to the radiating rods. The transmission line is fed at the apex by coaxial feed line extending upward from the ground through one of the tubes to the feed point where at that point the outer shield of the cable is connected to the tube through which the coaxial feed line passes and the inner conductor is connected to the other of the tubes. The RF currents flow upward through the unbalanced coaxial cable to the apex at the point of symmetry and from their flow back down through a balanced transmission line formed by the two tubes. As the current flows down the balanced transmission line to a region where the elements are resonant, the current flows out onto the elements and is radiated.
Log periodic antennae have significant back radiation. In accordance with the present invention, a monitor antenna is installed in the log periodic antenna array at a location to provide a termination of the balanced transmission line and modify the back radiation pattern so that there is no back radiation. At the same time, the monitor provides for a continual indication of the proper operation of the composite antenna system.
For a more complete understanding of the present invention and for further objects and advantages thereof,- reference may now be had to the following description taken in conjunction with the accompanying drawings in which:
FIG. 1 schematically illustrates an instrument landing path with outer and inner markers;
FIG. 2 illustrates a log periodic antenna;
FIG. 3 illustrates log periodic antenna modified and installed in accordance with the present invention;
FIG. 4 illustrates generally applicable radiation patterns for log periodic antennae as shown in FIG. 2;
FIG. 5 illustrates the radiation patterns for the antenna of FIG. 3; v
FIG. 6 illustrates a pole top of the antenna of the present invention;
FIG. 7 illustrates the monitor dipole;
FIGS. 8 and 9 illustrate two forms of structure for transition from unbalanced to balanced transmission line feed for the antenna of the present invention;
' FIG. 10 illustrates a monitor dip-ole structure wherein the dipole elements are formed in a printed circuit board construction; and 7 FIG. 11 illustrates the transformer connection to the monitor dipole.
A pole mounted beacon has several salient features which allow it to solve problems inherent in conventional marker beacon antennae. It has been found that pole mounted log periodic dipole antennae may be so constructed as to satisfy pattern and gain requirements for the marker beacon application. They are relatively small, of the order of 50 inches long, inches wide and inexpensive to build. Pole mounting helps to solve the problem of vandalism. It occupies a minimum area so that it lends itself particularly to a greater variety of airport siting installations than the large conventional marker beacon antenna using large ground screens. For example, some airports have runways extending out over the ocean. Although a marker beacon is needed, it is not practical to install a conventional ground screen marker beacon over a salt spray area. A simple pole mounted marker beacon on the other hand in sh allow water near the shore will function appropriately. Further where the center line extension of a runway falls in the center of an expressway of an urban area, the installation of conventional marker beacon antennae would be impossible. However, the small pole mounted antenna would be acceptable.
The problems of siting and installation will be apparent by reference to FIG. 1 which illustrates a portion of a Federal Aviation Authority instrument landing system wherein the airport runway is preceded by a glide path 11 typically 4 miles in length. An outer marker beacon near the beginning of the glide slope typically produces a properly shaped electromagnetic field 12 directed upwardly from the ground and through which the landing aircraft will pass. The strength of field 12 is so regulated that when the field is passed through by an aircraft during approach, an outer marker light will be energized on the aircraft instrument panel if the aircraft is at the desired altitude. Lateral information also is provided by the localizer field.
FIG. 2 illustrates a log periodic antenna in which two parallel rigid conductors and 21 are spaced apart and are fed at the apex terminals 22 and 23. The conductor 20 supports radiators -36 with radiators 30, 32, 34 and 36 extending in one direction and radiators 31, 33 and extending in the opposite direction but are in the same plane as elements 30, 32, 34 and 36.
Similarly, conductor, 21 supports radiators 40-46 with radiators 40, 42,44 and 46 extending in the direction opposite radiators 30, 32, 34 and 36 and opposite radiators 41, 43 and 45. Radiators 40-46 lie in the same plane.
' Antennae of the general construction shown in FIG. 2 are known as log periodic antennae. The present invention involves the use of such an antenna. The field is uniquely modified by integration ofa monitor antenna to the log periodic antenna. The array is then mounted on a pole which occupies very little space at the marker beacon site.
In FIG. 3, antenna 19 of the present invention is mounted on a pole 50. Brackets 51 and 52 secure the two conductors 20 and 21 to maintain them parallel while insulating them one from the other. The planes common to the radiators on conductors 20 and 21 are vertical and are parallel to the glide path with which the installation is associated. An excitation source 53 is mounted on pole 50 and is connected by way ofa coaxial cable 54 to antenna 19. Preferably the conductors 20 and 21 are hollow metallic tubes of materials such as aluminum. The coaxial cable 54 is an unbalanced transmission line. It conveys RF energy from the source 53 to antenna 19. Cable 54 is threaded upward through the center of conductor 21. At the apex of the antenna 19 the shield ofconductor 54 is connected to conductor 21 and the center conductor of coaxial cable 54 is connected to conductor 20. By this means, the unbalanced transmission line comprising the cable 54 is connected to properly feed a balanced transmission line comprising the conductors 20 and 21.
In use as a marker beacon antenna, it is essential that the back radiation, i.e., the radiation down toward the ground, be reduced to zero or deeply nulled. In accordance with the present invention, a small monitor dipole 60 is mounted between the two bottom pair of radiators, i.e., above radiators 30 and 40 and below radiators 31 and 41. The antenna 60 is mounted to provide continuous electrical monitoring ofthe log periodic antenna. It is so designed as to cause a null of the back radiated signal. A short circuit termination shorting conductors 20 and 21 at the monitor antenna forces both conductors 20 and 21 to DC. ground potential thus avoiding static electrical charge to accumulate on the antenna. A cable (not shown) leads from the dipole 60 to monitoring equipment associated with the operation of the marker beacon antenna 19.
The effect of the presence of the dipole 60 is illustrated by comparison of FIGS. 4 and 5. In FIG. 4, two radiation patterns are shown one pattern is for the E field, curve 62, and one is for the H field, curve 64. Curves 62 and 64 are for the antenna of FIG. 3 without the monitor dipole 60. Note the presence of significant back radiation. The E field has a lobe 62a and the H field has high level back radiation.
Contrast the patterns of FIG. 4 with the patterns of FIG. 5 for the antenna of FIG. 3 in the presence of the monitor antenna. The E plane radiation characteristic is shown by curve 66 with the minimized lobes 66a and 66b present and with a null in the downward direction. In the H field, the curve 68 was measured. The differences represented by FIGS. 4 and 5 indicate the utility of the invention as a marker beacon antenna. 1
In FIG. 6, antenna 19 is illustrated mounted on top of a pole 70. A suitable insulating bracket 71 mounted on the pole top supports the conductors 20 and 21.
FIG. 7 illustrates dipole 60. Dipole 60 comprises a pair of rods 60a and 60b. In one embodiment where the antenna 19 had radiators of lengths shown in Table I, the dipole of FIG. 7 had a length of 20.5 inches. A shunt element 606 is 13.4 inches long spaced 2.0 inches from the radiators 60a and 60b. Radiators 60a and 60b were fabricated from 0.3125 inch diameter aluminum rod. Shunt element 606 was fabricated from 0.1875 inch diameter aluminum rod.
Radiators 60a and 60b are connected through insulating bushings and 81 mounted on the wall of the metallic shielding box 82 to the terminals of a variable capacitor 83. Inductances 84 and 85 connect the terminals of capacitor 83 to a 1:1 toroidal balun transformer 86. An output winding of transformer 86 is connected by way of a coaxial connector 87 and a suitable cable to a monitoring device 88 located at a station where the operation of the antenna systems conveniently may be monitored.
Monitor antenna 60 occupies a position in the field of antenna 19 such that it produces an output signal from the transformer 86 representative of the strength of the field from antenna 19. A continuous indication that antenna 19 is energized is thus provided. At the same time, the monitor antenna 60 modifies the field pattern as shown in FIG. 5.
One of the points for concern in making and operating the antenna is to provide a suitable transition from an unbalanced transmission line formed by coaxial cable 50, to a balanced transmission line formed by the conductors 20 and 21. A suitable feed arrangement is shown in FIG. 8.
In FIG. 8 the coaxial cable 54 passes upward through tube 20 and thence to a suitable coaxial right angle 90.
A central conductor 91 extends laterally above the end of tube 20 toward the axis of tube 21. The rod 91 is secured as by a screw 92 into the end of a metallic angle member 93. Member 93 is secured by set screw 94 to tube 20. Set screw 95 secures angle member 90 to tube 21. The shield of the coaxial cable 54 is connected to member 90. The central conductor is'connected by rod 91 and member 93 to the tube 20. By this means the antenna is fed symmetrically at its apex. A gap 96 between members 90 and 93 is to be filled with insulating material for operation in inclement weather.
FIG. 9 illustrates a modified form of the feed. A small diameter semirigid coaxial cable is employed. In this embodiment, tubes 20 and 21 are about 1.0 inch in diameter. The upper end of the tube 20 is closed by a metallic spool 100. An insulating plate 101 is secured on top of spool 100 by a metallic threaded insert 102. The plate 101 is similarly secured to tube 21 by a threaded insert 103 cooperating with a metallic spool 104. The coaxial cable 105 extends upward through spool 100. The right half of the upper surface of plate 101 is provided with a metallic strip 1101a. The left half is similarly provided with a metallic strip 1011b with a gap 101C between layers 101a and 10112. The shield on the coaxial cable 105 is connected to layer 101a. The central conductor is connected to layer 1011b. By this means the antenna is symmetrically fed at the point of symmetry 101C. in this embodiment, the cable preferably is one having a diameter of 0. 141 inch and is of the type made by Precision Tube Company of North Wales, Pa. 19454.
In one embodiment of the invention, capacitor 83 was a piston type adjustable capacitor manufactured and sold by J. F. D. Electronics, Brooklyn, NY. and identified as part No. PC5111 providing from 0.8 to 11.0 picofarads capacity. Inductors 84 and 85 each consisted of TI part No. 055340-10 (3.74.0 uh) variable inductors with all but 11 turns of wire removed from each.
Transformer 86 comprised 22 turns on both primary and secondary, the core being of the type manufactured by lndiana General Corporation of Keasbey, NJ. J. and identified as ferrite toroid part No. F624-l9. FlG. 11 illustrates the manner in which the balun transformer windings are wound on core 86.
In FiG. 10 the monitor antenna has been illustrated in the form of a printed circuit board construction. An insulating panel 110 has conductive paths 111, 112 and 113 plated thereon leading to terminals 114 and 115. Spaced plated terminals on an extension 116 serve for mounting the components of the monitor antenna. The capacitor 83 of FIG. 7, the inductors 84 and 85 and the toroidal transformer 86 are shown diagrammatically in their respective positions with other interconnections shown dotted. The extension 116 may be suitably housed or potted following installation of the discrete components thereon. The housing or potting structure preferably is provided with a metallic coating to make contact with the adjacent members (20,21) of the log periodic antenna to short the same and provide the desired termination.
Having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art and it is intended to cover such modifications as fall within the scope of the appended claims.
What is claimed is: 1. An lLS marker beacon antenna which comprises:
a. an antenna having a pair of spaced parallel constructure is supported between said parallel conductors to monitor a field of log periodic antenna, modify its back radiation pattern and short circuit said conductors.
3. The combination of claim 1 wherein a pole supports said antenna with said parallel conductors vertical and said feed point at the top thereof.
4. An ILS marker beacon antenna which comprises:
a. a log periodic antenna having a pair of parallel conductors forming a balanced transmission line with a plurality of radiators of graded lengths extending generally in a plane from said pair of parallel conductors,
b. an unbalanced RF transmission line extending upward through one of said conductors to an apex feed point of said antenna,
c. means for connecting one conductor of said line to one of said pair of conductors and the other conductor of said line to the other of said pair of conductors for flow of RF current from said feed point downward along said transmission line to said radiators, and
d. a dipole monitor structure mounted parallel to said radiators and having support structure connected electrically to terminate said antenna-by a short circuit near the bottom of said log periodic antenna.
5. An ILS marker beacon antenna which comprises:
a. a log periodic antenna having a pair of spaced parallel conductors forming a balanced transmission line with a plurality of parallel radiators of graded lengths extending from said pair of spaced parallel conductors,
b. means for supporting said antenna above the earth along an aircraft glide path with the apex vertical and with the radiators extending parallel to said glide path,
c. a coaxial RF transmission cable passing upward through one of said conductors,
d. means for connecting the inner conductor of said cable to one of said pair of conductors and for connecting the outer conductor of said cable to the other of said pair of conductors for flow of RF current from said feed point downward along said balanced transmission line to said radiating elements, and
e. a monitor dipole mounted parallel to said radiators to sense the antenna field and having support structure connected electrically to terminate said antenna by short circuit near the bottom of said log periodic antenna.
6. Means to provide an electromagnetic field as a marker beacon along an airport glide path which comprises:
a log periodic antenna pole mounted above the earth below said path with the apex up and radiating elements thereof parallel to said path,
source means to excite said antenna at beacon frequency at said apex,
means to short circuit said antenna near the back thereof, and
means including a monitor antenna housing mounted in the region of said short circuit to provide a monitor signaldependent upon the existence'of an elec-W tromagnetic field from said log periodic antenna andto null the back radiation from said log periodic antenna.
7. The combination set forth in claim 6 in which said monitor antenna is supported by conductive means, which conductive means contacts said antenna to establish said short circuit.
8. The combination set forth in claim 6 in which coupling structure connects said source means to said antenna for balanced transmission line operation thereof.
9. The combination set forth in claim 7 in which an unbalanced transmission line feeds excitation current to said apex through one of two transmission line elements of said antenna and a balanced feed connecting structure connects said unbalanced line to said antenna apex.
10. The combination set forth in claim 9 in which said transmission line is a coaxial cable insulated from one element and wherein coupling means connects the shield of said coaxial cable to said one element and the center conductor of said cable to the other of said two elements at said apex.
11. The combination set forth in claim 9 in which: said two elements are electrically short circuited by said housing near the back of said log periodic antenna, an insulator couples said elements in spaced apart relation at said apex, two conductors on said insulator are each connected to oneflof said two elements, and a a a V 7 an excitation source including a coaxial cable extends through one of said elements and terminates at said two conductors for balanced excitation of said log periodic antenna from the apex thereof. 12. The method of establishing an electromagnetic field as a marker beacon along an airport glide path which comprises:
exciting a pole mounted log periodic antenna with radiating elements parallel to said path with the apex of said antenna directed upward, and shorting said antenna near the back thereof with monitor antenna structure to null the back radiation while establishing a monitor signal. 13. The method of claim 12 in which excitation current is fed to said apex through an element of said antenna for balanced feed of said antenna at said apex.