Glide slope antenna system

[57] ABSTRACT

A switchless glide slope antenna system maintaining horizontal polarization during capture and track functions. The antenna system comprises a track antenna and a capture antenna arranged in an aircraft landing gear door so that switching from capture to track antenna occurs automatically upon gear extension by cross-polarization of the capture antenna.

Claims, 7 Drawing Figures

<img file="US3662392A_D0001.tif" />

PATENTEDMAY 91972

SHEET 1 OE 3

3,662,392

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INVENTORS,

BRMN P. STAPLETON _DV ROBERT ΛΖ SUTTON BY attorney

PATENTED may 91972

3,662,392

SHEET 2 OF 3

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<img file="US3662392A_D0004.tif" />

Attorney

ΡΑΤΕΝΤΕΟΜΛΥ 91972

3,662,392

SHEET 3 OF 3

<img file="US3662392A_D0005.tif" />

attorney

3,662,392

2

GLIDE SLOPE ANTENNA SYSTEM

This invention relates generally to aircraft antenna systems and particularly to antenna systems for aircraft ILS glide slope landing systems.

Aircraft ILS (instrument landing systems) in the 330 MHZ 5 region are well known. These systems require a horizontally polarized glide slope antenna which is typically located inside the nose radome. However, in order to demonstrate adequate clearance over-threshold it becomes important that the vertical displacement between wheels and glide slope antenna in K these systems is limited to a maximum of about 19 feet.

It is therefore an object of the invention to provide an antenna system for an ILS receiver channel having capture and track antennas for providing horizontally polarized signal components in the respective modes without active switching ¹ i between capture and track antennas.

It is another object of the invention to provide horizontally polarized ILS signal components obtained from an ILS antenna system arranged in a landing gear door.

It is yet another object of the invention to provide an ILS 2C antenna system having reduced ILS antenna system to wheel vertical displacement.

It is a further object of the present invention to provide dual ILS antenna systems, one in each of opposing aircraft nose gear doors. 25

It is still another object of the present invention to provide a slot antenna with the cavity formed by adjacent ribs and outer skin of an aircraft structure.

In accordance with a preferred embodiment of the invention, the dual ILS antenna system includes a capture antenna for each system comprising an array of slots positioned on the outer skin of each aft nose gear door. A further slot antenna is incorporated in the leading edge of each aft nose gear door to provide glide slope track function capability with the nose gear extended. The signal components from the capture and track antennas in each system are combined and then coupled and fed to ILS receivers in each channel.

A full understanding of the invention, and its further objects and advantages, will be had from the following description ₄θ when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of the nose landing gear of an aircraft in extended position with the aft nose gear doors open to show the location of the capture and track antennas according to a preferred embodiment of the present ILS antenna system;

FIG. 2 is a schematic diagram of a dual channel glide slope receiving system in accordance with an embodiment of the present invention;

FIG. 3 is a perspective view of one of the aft nose gear doors showing in more detail the track and capture antennas utilized in one channel of the dual channel system shown in the schematic diagram of FIG. 2;

FIG. 4 is a detailed front view in perspective of a capture an- 55 tenna element;

FIG. 5 is a detailed rear view in perspective of the capture antenna element shown in FIG. 4;

FIG. 6 is a detailed front view in perspective of a track antenna element; 60

FIG. 7 is a detailed rear view in perspective of the track antenna element of FIG. 6.

Present glide slope antennas in the typical inside-the-nose radome location become vertically displaced at increasing distances from the wheel level in the case of larger jumbo jets 65 where the wheels are farther below the nose. The present unique solution utilizes a landing gear door for each glide slope antenna system. Turning now to FIG. 1, the movable antenna system in aft landing gear door 10 will be seen to comprise a track antenna element 12 and a capture antenna com- 70 prising capture antenna elements 14 and 16. When the nose gear 18 is retracted into the wheel well 20, aft nose landing gear door 10 and the opposing aft nose gear door 11 are in a closed position flush with the airplane structure 22. Capture antenna elements 14 and 16 receive horizontally polarized 75 energy and track antenna element 12 is shielded. For purposes of clarity the glide slope antenna system of door 10 only will be described in detail although an identical glide slope antenna system may be installed in the opposing aft nose gear door 11 when a dual channel system is desired for redundancy or other reasons. When the antenna system of door 10 (and door 11 in the case of a dual system) is rotated 90° to the open position as shown in FIG. 1 with gear 18 extended, the capture antenna comprising forward capture antenna element 14 and ¹ rearward capture antenna element 16 becomes vertically polarized and track antenna element 12 receives horizontally polarized energy. A unique feature of the gear door antenna system is that a horizontally polarized antenna pattern is maintained in the horizontal and vertical orientation of the gear door without switching circuits being required between the capture and track antennas of the glide slope antenna system by utilizing a technique of cross polarizing the capture antenna when the door is open to the downward or vertical position as shown in FIG. 1. The horizontal gain of the track antenna comprising element 12 in the forward direction is substantially an order of magnitude larger than the vertical gain of the capture antenna comprising elements 14 and 16.

Turning now briefly to the FIG. 2 schematic diagram of the dual channel glide slope system it will be seen how the signal voltages developed in the antenna systems of each channel are processed, viz., combined and coupled to the receivers in the respective glide slope system channels. The capture antenna of nose gear wheel well door 10 as mentioned previously comprises a dual slot array comprising elements 14 and 16 on the outer skin of door 10. A hybrid 30 coupled to elements 14 and 16 is utilized to provide proper phasing and amplitude balance between the two array elements. Track antenna 12 comprises a slot element on the leading edge of the aft (rear) nose gear door 10 which functions during gear-down conditions. The signals from the capture and track antennas in each channel are combined by coupling to a second hybrid 32 and fed to the receiving means 34. The general manner of signal processing in the one channel from capture and track antennas on door 10 to the receiving means is thus appreciated, the same processing taking place from the capture and track antennas located in door 11 as can be seen along the upper portion of the schematic diagram of FIG. 2.

Turning now to FIG. 3, the location and structure of the capture antennas slots 14 and 16 in door 10 can be seen in more detail. The capture antenna comprises an array of two 5inch slots 14 and 16 in aft nose gear door 10. The fore and aft slots 14 and 16 are spaced in one-half wavelength apart and are fed 180° out of phase. The resultant radiation pattern of the two-slot array is so configured to provide a substantially consistent glide slope capture signal when the nose gear is retracted. Track antenna 12 comprises a 12-inch “1” shaped slot antenna installed on the leading edge of aft nose gear door 10 and provides the glide slope track function capability with the nose gear extended. It can thus be seen that each of slots 14 and 16 of the two element slot array forming the capture antenna is oriented parallel to the longitudinal axis of the aircraft. The longest slot 14 or 16 that was physically realizable in the particular aircraft modified without altering the construction of door ribs 40 was 5 inches, although it should be recognized that when ribs 40 are further spaced apart, a longer slot length will be permitted thus resulting in a more efficient antenna. The present 5-inch slots 14 and 16 formed by adjacent ribs 40 and door skin 40 provided basic impedance data which indicated that each 5-inch slot could be electrically matched within reasonable VSWR limits by employing the combination of a series and a parallel capacitor (as can be seen from the schematic diagram of FIG. 2).

Capture antenna slot element 14 is shown in more detail in front view in FIG. 4 and in rear view in FIG. 5 where it can be seen that the structure utilized comprises a stripline antenna element having a stripline connector 50 and series tuning capacitor 52 which are inside door skin 41. Slot filler element comprises fiberglas cloth material which served as an

3,662,392 aerodynamic filler for the 5-inch slot in door 10. The exposed fiberglas cloth was sanded down in the configuration tested to provide a flush fit after installation of capture antenna slot element 14 in door 10. The exposed fiberglas cloth was then brushed with a thin layer of resin to reseal the fiberglas. TNC stripline connector 50 utilized in the present configuration was an E SCA Model 233/3 right angle stripline launcher manufactured by the Electronics Standard Corp, of America of Port Salerno, Fla. A conductive connection 62 was utilized to provide a conduction path between a first terminal of variable capacitor 52 and the center conductor 107 of the stripline. Variable capacitor 52 was a Model JMC 5141 made by the Johanson Co. of Boonton, N. J., having a capacitance range of 1 to 10 picofarads. Fiberglas board 105 was Ve inch thick with a 2-ounce copper plating forming conductive surface 109 which supports connector 50, the center terminal of connector 50 being connected (as shown by the dotted line) to the second terminal of matching capacitor 52. The aluminum sheet 60 serves as the bottom ground plane and is in contact with the inner surface of door skin 41 in final assembly. Fiberglas board 108 has further ‘/s-inch thick 2-ounce copper plating 107 which forms the center conductor of the stripline slot element 14.1-slot element 12 comprises a fiberglas board 110 having a %-inch thick 2-ounce copper plating 112 which has an “I” shaped slot cut away to show board 110 (see FIG. 7). Inductor 114 is shunted from copper plating 112 to the outer braid or sheath of coax transmission line 117. Capacitor 120 is connected in series with the inner conductor of coaxial transmission line 117 and the conductive material 112 on the opposite side of the I-slot from where inductor bracket 114 is maintained. 1-slot element 12, shown in detail in FIGS. 6 and 7, is then mounted in the leading edge of door 10 (see FIG. 3) in an I-slot of the same dimension with copper surface 112 brought in contact with the metallic surface of the leading edge of door 10 so that I-slot formed in metal coating 112 is superimposed on the I-slot cut in the leading edge of door 10.

The coaxial cable leaving each slot cavity 14 and 16 must be electrically bonded at its outer sheath to the side rib of the respective slot to prevent currents on the coaxial cable outer sheath from flowing outside of the respective cavity.

From the schematic diagram of FIG. 2 it can be seen that each of capture antenna elements 14 and 16 has a series capacitance 52 which is a variable capacitor having a range of one to ten picofarads and a shunt capacitor 70 which can be formed by a metallic surface parallel to the slot. The signals from capture elements 14 and 16 are then coupled to hybrid 30 which functions to provide proper phasing and amplitude balance between the two array elements. Hybrid 30 is a 3 dB, equal amplitude, equal phase, lumped constant type hybrid having three ports. Hybrid 30 comprises a bifilar wound transformer 83 wound on the body of 100 ohm carbon resistor 84, the resistor 84 and bifilar wound transformer 83 being connected between input ports 80 and 81 of hybrid 30. Input port 80 is coupled to capture antenna slot element 16 by means comprising a series capacitor of the same value in the same manner. Transmission line 85 is connected between the two series connected half sections of bifilar wound transformer 83 and output port 82 of hybrid 30. Transmission line 85 has a characteristic impedance of 35 ohms and is one-fourth wavelength long at 332 MHz and was constructed by paralleling two sections of 70 ohm coaxial transmission line. Three port hybrid 30 exhibited an insertion loss at 332 MHz OF 3.20 dB from port 82 to port 80 and a loss of 3.20 dB from port 82 to port 81. With a 50 ohm load on output port 82, the isolation between input ports 80 and 81 at 332 MHz was measured by conventional bolometer techniques to be 22.6 dB. The voltage standing wave ratio (VSWR) of hybrid 30 was measured with 50 ohm loads at input ports 80 and 81 using slotted line techniques to be 1.28:1 at output port 82 at a frequency of 332 MHz. Phase measurements were obtained with the aid of a slotted line by noting the shift in voltage minimum position between ports 82 and 80 and ports 82 and 81 when fed with a constant input signal. At 332 MHz the phase difference between input ports 80 and 81 was found to be 0.96*.

Turning now to four part hybrid 32 utilized to combine signals from track antenna comprising single slot element 12 and the signals from capture antenna elements 14 and 16 arriving from output port 82 of hybrid 30, it will be seen from a comparison with hybrid 30 in the schematic diagram of FIG. 2 that the circuit of hybrid 32 is similar to a pair of hybrid 30 circuits without the section of 35 ohm transmission line 85, and with one of the circuits driving the other. More specifically, four port hybrid 32 is a 6 dB lumped constant type hybrid which is basically two 3 dB hybrids connected back to back. A simplified schematic is shown within block 32. Bifilar transformer windings 94 and 95 are wound respectively on resistors 96 and 97 as winding 83 was done on resistor 84 in the three port hybrid 30. Resistors 96 and 97 are also 100 ohm resistors as resistor 84. The midpoint of bifilar wound transformer 94 is connected to the midpoint of bifilar wound transformer 95. The following measurements confirm that four port hybrid 32 provided satisfactory application in the glide slope system as did hybrid 30 previously described. Insertion loss of hybrid 32 with input ports 90 and 91 and output ports 92 and 93 terminated in 50 ohm loads at 332 MHz was found to be 6.2 dB between ports 90 and 92, 6.3 dB between ports 90 and 93, 6.1 dB between ports 91 and 92 and 6.0 dB between ports 91 and 93. Isolation measurements between two individual ports were taken with the other two ports terminated in 50 ohm loads and found to be 21.2 dB between input ports 90 and 91 and 27.0 dB between output ports 92 and 93 at 332 MHz. The measured VSWR of four port hybrid 32 with three ports terminated in 50 ohm loads at 332 MHz was 1.135:1 at input port 90, 1.145:1 at input port 91, 1.305:1 at output port 92 and 1.22 to 1 at output port D. While a single ILS receiver 101 is shown coupled to one of the output ports, viz., port 93 of four port hybrid 32 in one channel of the dual channel system described, it should be noted that receiving means 34 can comprise a pair of receivers each connected to one of the two output ports of four port hybrid 32 as is shown in the other channel (upper portion of the schematic of FIG. 2) which is processing signals from capture and track antennas on opposite gear door 11.