Title:
VOR ANTENNA SYSTEM
United States Patent 3613099


Abstract:
An antenna system for radiating VOR signals wherein metal to metal contact between a central supporting mast and loop radiating elements is used without producing mast excitation, thus providing feasible means for constructing a vertical array of VOR antenna elements.



Inventors:
HOLLINS CLINTON G
Application Number:
04/841023
Publication Date:
10/12/1971
Filing Date:
07/11/1969
Assignee:
SCANWELL LABORATORIES INC.
Primary Class:
Other Classes:
343/739, 343/743, 343/853, 343/884
International Classes:
G01S19/25; H01Q9/26; (IPC1-7): H01Q11/12
Field of Search:
343/741-744,866,739,853,884
View Patent Images:
US Patent References:
3231891Multi-polarized loop antenna array electromagnetically coupled to spaced transmission lineJanuary 1966Stegen
2521550Radio antenna systemSeptember 1950Smith
2490815Loop antennaSeptember 1950Kandoian
2391026Shielded loop antennaDecember 1945McGuigan
2372651AntennaApril 1945Alford et al.
2293136High frequency loop type antennaAugust 1942Hampshire
2283897Antenna systemMay 1942Alford



Primary Examiner:
Lieberman, Eli
Claims:
I claim

1. A VOR antenna comprising four closely spaced antenna arrays, each including four orthogonal elements of hollow construction, said elements being mechanically joined at their extremeties, corresponding elements of said four arrays being substantially parallel, means for mounting the elements of each array in a horizontal plane, each array comprising radial arms of hollow construction extending inwardly from each element junction of said array to a centrally located terminal, a balun gap located at the midpoint of each of said elements, a supporting mast connected in metal-to-metal contact with contiguous element junctions of each of said four antenna element arrays, coaxial transmission means extending through said mast to said centrally located terminal of each array, coaxial feeder lines connected to said coaxial transmission means extending from said centrally located terminal through said hollow arms and said hollow elements to said balun gaps, one conductor of each of said coaxial feeder lines connected to one of said elements on a first side of the respective balun gap and the other conductor extending across the balun gap and connected to said one of said elements on the opposite side of said gap, whereby VOR energy may be applied to the radiating elements without causing the supporting mast to itself become a radiator.

2. A VOR antenna comprised of two or more bays wherein each bay is comprised of a VOR antenna device as described in claim 1.

3. The antenna described in claim 2 wherein VOR energy is transmitted to said radiating elements by said coaxial transmitting means through a network including power divider means and R-F bridge means.

4. The antenna described in claim 3 wherein said network provides four separate output coaxial transmission lines, each feeding corresponding radiating elements at the levels of the vertically arrayed antenna through means including additional power dividing means.

Description:
BACKGROUND OF THE INVENTION

This invention relates to a novel antenna system and a method of radiating energy in a very high frequency omnidirectional radio range air navigation system (VOR).

During the past several years commercial aviation in the United States has made use of the VOR system for air navigation. Basically the system provides direction finding information to an aircraft through the transmission of two signals, one of which is a reference signal bearing north, the second of which varies in time with the azimuth of the receiving aircraft. By comparing the phase difference between the two received signals, the azimuth of the receiver from the transmitting station may be determined.

Practical implementation of the basic system calls for the transmission of a carrier wave modulated by a rotating wave. In order to generate the rotating wave, very early systems used a physically rotating antenna; however, these early systems were quickly replaced by systems which rotate the signal electrically and thus avoid mechanical movement. Electrical rotation is accomplished by supplying two sinusoidally varying sideband frequencies in quadrature to a stationary antenna in order to generate a rotating figure eight pattern. The resultant modulated carrier takes the form of a limacoid rotating in space such that a definite maximum is received by an aircraft once per revolution of the limacoid. When the maximum of the limacoid rotates through north, a reference signal is transmitted to indicate a zero-phase relationship between the maximum of the limacoid and north at that particular instant. As before indicated, phase relationship between the reception of the reference signal and reception of the limacoid maximum by the aircraft determines the azimuth of the aircraft from the transmitting station.

Antenna networks to implement the above-described system have been devised and are in use but have been characterized by certain problems. For one, the antenna system presently in general use (Alford antenna), cannot be directly connected with a central supporting mast because of the problem of vertical polarization and must be insulated from its pedestal since no point on the loop is at ground potential. Consequently, it is impractical from a construction viewpoint and electrically not feasible to build an Alford antenna system in bays. Since arranging antennas in bays is known to be desirable for shaping the radiated pattern, the Alford antenna system is inherently limited in that regard. A second problem involves the need for covering the antenna elements in order to protect the relatively delicate construction of the radiating loops, their impedance matching components and particularly the insulators from the weather. The solution has been to supply a radome over the antenna. A third problem involves the reflection of radiated signals from nearby trees, buildings or hills. This last problem has been attached by leveling large areas of land around the antenna site, usually several acres, and constructing a counterpoise above the ground surface at an appropriate level beneath the antenna.

Unfortunately, the above solutions have generated additional problems of their own in the use of the VOR system. Accumulations of snow and ice on the radome or on the counterpoise have in some instances created sharply distorted patterns due to reflection from the snow and ice. Obviously, the clearance of large sites to provide suitable antenna stations is an impractical solution in highly populated areas, and always a costly solution.

SUMMARY OF THE INVENTION

This invention provides for radiating elements to be arranged around the central supporting mast in such a manner that the mast may provide metal-to-metal connection with the radiating elements without distorting the radiating pattern and without itself becoming a radiator. The invention eliminates the need for radomes over the radiating elements and eliminates the need for a counterpoise when the antenna is arrayed in bays. Arranging the antenna in bays is made practical by the invention.

OBJECTS OF THE INVENTION

The primary object of this invention is to provide method and means for radiating VOR signals whereby weather and site considerations are minimized.

A second and related object of the invention is to provide an antenna system whereby the need for a counterpoise is eliminated.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will best be understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, the description of which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 1a are views in perspective of a prior art antenna system used in a VOR system; the antenna loops are shown in schematic representation.

FIG. 2 shows a tower-mounted array with radiating elements positioned one above the other along a central mast.

FIG. 3 shows a preferred embodiment of an antenna loop and array according to the invention.

FIG. 4 is an electrical diagram showing planes of electrical symmetry.

FIG. 5 is another embodiment of the antenna array.

FIG. 6 shows a VOR distribution system for use in vertically arrayed antenna.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIGS. 1 and 1a show the type of antenna configuration in general use in VOR systems at the present time. Four antenna loops 10, 11, 12 and 13, represented schematically, are shown mounted each upon its own pedestal, 14, 15, 16 and 17 respectively. The pedestals are shown extending downwardly to the wire screen mesh-type counterpoise 26 and, though not shown, they extend further in the downward direction to some suitable support and terminating fixture. Radio frequency VOR energy is fed to the loops by coaxial transmission lines housed within the pedestals 14-17. The particular transmission lines 18 and 19 feeding loop 10 are shown in typical connection. Loop 10, illustrative of each of the four loops, shows condensers 20-23 located at each of the corners of the loop. Electrical operation of the loops is explained in U.S. Pat. Nos. 2,283,897 and 2,372,651 to Alford.

The radome is illustrated at 24 and the counterpoise depicted at 26. The counterpoise usually has a radius of 30 or more feet and is most frequently placed at about a half wavelength below the plane of the radiating loops. The counterpoise must be large enough and must be placed high enough above the ground so that radiation from the loops clears all obstructions in the horizontal plane toward the horizon, and so that reflection from downward directed radiation is kept to a minimum.

Insulators, not shown, are provided atop each pedestal between the pedestal and the loop in order to prevent the pedestals from producing significant amounts of distorting radiation due to simple conduction. Even so, the pedestals do have some amount of R-F energy induced in them from the radiation of the three loops not mounted on that pedestal. As a result, some amount of vertical polarization is present in the radiated signal. It should be noted that induced energy from the associated loop is usually not present on the pedestal because of the symmetrical placement of the loop on the pedestal.

FIG. 2 shows a tower-mounted antenna array in accordance with the present invention wherein five arrays or bays 30-34 are shown mounted on a central mast 35. Mast 35 may be guyed as shown at 36 and 37.

FIG. 3 shows a preferred embodiment of the antenna loop of this invention which makes the construction shown in FIG. 2 practical. FIG. 3 depicts any one of the antenna arrays 30-34 of FIG. 2 since all of the arrays are identical. For purposes of illustration antenna array 30 has been shown in FIG. 3.

In FIG. 3, antenna array 30 is shown to comprise four identical loops 40, 41, 42 and 43. Because of that identity, only loop 40 has been illustrated completely. It is shown to comprise radiating elements 50, 51, 52 and 53, each of which contains a balun gap 54, 55, 56, and 57 at approximately the midpoint of the element. The four elements 50-53 of hollow tubular construction, are joined together at the corners 60, 61, 62 and 63 in an approximately square configuration. Electrically, each of the elements is fed from a four-way junction shown at 65 through feeder lines 71-74 located in radial arms 66, 67, 68 and 69. The hollow tubular radial arms 66-69 extend in the plane of the radiating elements from the junction 65 to the corners, providing mechanical support for the radiating elements as well as acting as a conduit. The coaxial transmission line 70 which carries energy to loop 40 rises through the hollow central mast 35 and connects to the junction 65 through radial arm 68. The coaxial line 70 is split at junction 65 into four separate coaxial feeder lines 71-74 each of which passes through a respective radial arm 66-69 and into a respective radiating element 50-53. Connection is made across the balun gap by connecting the outer conductor to the element on one side of the gap and the inner conductor to the element on the opposite side. For example, in FIG. 3, outer conductor 75 is shown connected to element 51 without crossing balun gap 55 while the inner conductor 76 crosses the gap and connects to element 51 on the opposite side. An impedance compensation network is shown at 77.

An understanding of the electrical operation of the loop 40 can be had by examination of FIG. 4. Here the structural members of the loop are shown schematically with equal excitations at the balun gaps 54-57. The currents on each triangular subloop of the element 40 are represented by I1 -I4. Since a summation of currents in radial member 66 involves the addition of opposing currents I1 and I2, it is apparent that there is no net current flow in radial member 66 should I1 and I2 be equal. Noting that the structure is symmetrical about the lines A--A and B--B, the current functions must also be symmetrical about those lines. It follows that I1 =I2, hence that the radial members are unexcited and do not contribute to radiation.

It is important to note also that the potential between corners 60 and 62 and between corners 61 and 63 must be zero since a choice of paths from 60 to 62 or from 61 to 63 will include two opposite and equal sources, e.g., from 60 to 61 to 62 is equal and opposite to the path from 60 to 63 to 62. In fact, lines drawn from 60 to 62 and from 61 to 63 will lie in zero potential planes normal to the plane of the loop. Since planes passing through the corners of the loop are at zero potential, it follows that a metallic support can be directly connected to the loop at any corner without affecting the radiation characteristics of the antenna, and without itself becoming a radiator. Thus, there is little or no vertical polarization due to R-F energy in mast 35 shown in FIGS. 2 and 3 as a result of the physical metal-to-metal connection of the four loops to the mast. Of course, the balanced circuit presupposes proper orientation of the connections across the balun gaps and a properly constructed distribution system for exciting the antenna. Additionally, it may also be observed that vertical polarization due to induced currents in mast 35 is essentially eliminated by the symmetry of the structure.

Referring again to FIG. 2, there is seen a vertical array of radiating elements arranged in bays. The purpose of such an arrangement is to achieve a shaped radiation pattern which produces very light amounts of radiation below the horizontal plane of the lower bay (30). By so shaping the pattern, reflection from surrounding ground, trees and buildings is kept to an insignificant amount and thus the need for clearing large areas of land and constructing large and unwieldy counterpoises is eliminated. The idea of using vertical arrays is not new but, until now, the vertical arrangement in bays of suitable VOR antennas was impractical. An examination of the prior art as typified in FIG. 1 with its pedestals, insulators, radomes, capacity loading devices, etc., demonstrates how unwieldy and difficult to construct and adjust such an arrangement would be. However, with the array of this invention, a structurally sound vertical antenna array in bays is made possible.

Another important feature of this invention, not heretofore mentioned, is that the antenna loops are designed as broadband antennas and do not need the critical adjustment which is necessary to properly operate the VOR loops of the prior art.

In FIG. 2, an antenna array 31 is positioned on central mast 35 some distance above array 30. Similarly, other bays or arrays 32, 33 and 34 have been stationed at intervals above array 31. All bays are comprised of corner mounted loops identical to that shown in FIG. 3. At this point, it should be noted that the addition of bays 32, 33 and 34 is desirable primarily in order to increase the power of the radiated signal. Two bays are generally sufficient to shape the radiated pattern in such a manner that the need for a counterpoise is eliminated.

DESCRIPTION OF A SECOND EMBODIMENT

FIG. 5 is an illustration of a different, but similar, form of the invention. The radiating elements, 90-93, are shown to comprise a rounded configuration, and the adjacent radial arms of each loop are shown joined together in single radial arms 80-83. FIG. 5 shows a central mast 35 upon which the radial arms 80-83 are mounted. As in FIG. 3, the radial arms preferably extend from the mast to the radiating elements in the plane of the elements. Again, the construction of the radial arms and the radiating elements is tubular or hollow in order to provide for the passage of the coaxial cable to its termination across the respective balun gap. The balun gaps are shown at 94-97.

This alternative form of the invention lends itself to the same type of electrical analysis heretofore discussed in connection with FIG. 4.

Neither carrier nor sideband excitations produce currents in the vertical central mast 35. In the case of carrier excitation, the currents cancel on radial members so that no net current is available to excite the mast. For sideband excitation, the radial currents for each half of a single loop are of opposite sense at the mast 35. Again, the mast is connected to a current null and therefore remains unexcited.

THE DISTRIBUTION SYSTEM

A distribution system for supplying VOR energy to a five-bay antenna is shown in FIG. 6. With the connection scheme shown, the device may be viewed as a four-loop VOR array with each loop comprising a vertical array of five loops. This arrangement has the advantage of requiring but a single pair of R-F bridges or hybrids. The alternative approach of arraying five VOR antennas vertically would reduce the number of power dividers but would necessitate the addition of several more R-F bridges.

In FIG. 6, carrier R-F energy is shown being supplied through input terminal 100 which is connected to power divider 101. Divider 101 in turn is connected with each of two R-F bridges 102 and 103. The first quadrant of the four-loop antenna is shown to be fed from bridge 102 through quarter wave power divider 104. Power divider 104 thus excites loop 40 of array 30 (in FIGS. 2 and 3), and corresponding vertically arrayed loops in each of bays 31, 32, 33 and 34.

Sideband signals are provided at terminals 120 and 121 of the R-F bridges and, as previously stated, must be in electrical quadrature. Thus it is seen that both the carrier and the sideband excitations are provided to the radiating elements.

Added lengths of transmission line are shown at 130-133 in order to match the line lengths of the lower loops with that of the upper loops.

The shaping of the radiation pattern is achieved by properly adjusting or phasing the signals radiated from each of the vertically arrayed loops so that components of radiation downwardly directed toward the ground are virtually eliminated.

Thus it is seen that the instant invention provides a VOR antenna of rugged construction obviating any need for protective radomes, unwieldy counterpoises, and large areas of cleared land.

While the principles of the invention have been set forth in connection with specific apparatus, it is to be clearly understood that this description is made only by way of specific example and not as a limitation to the scope of the invention. Various modifications of the invention may be apparent to the artisan as, for example, by replacing the hollow arms and elements with solid pieces and providing clamps to carry an exposed coaxial cable. All such obvious modifications are, of course, within the scope and spirit of the invention which is defined in the appended claims.