Platform Stabilizer For Pole-mounted Antenna

Abstract

A pole-mounted directional microwave antenna secured to a platform is disclosed. The platform is rotatably mounted atop the pole so that the elevation angle of the beam axis of the antenna may be controlled. A stabilizing outrigger rod is mounted parallel to the pole, and the rod is linked to the platform. Spacers between the pole and the rod maintain a fixed separation, and when the pole is deflected in a plane containing the beam axis the rod is deflected in a parallel manner causing the platform to rotate so that the beam axis remains parallel to its undeflected orientation, thus automatically compensating for wind-induced pole sway in the direction of propagation. Three rods are used to compensate for sway in any direction.

Description

BACKGROUND OF THE INVENTION

This invention relates to antenna mounting structures and, more particularly, to mechanisms for continuously correcting the orientation of a narrow beam antenna to compensate for motion of the mast on which it is mounted.

In order to reduce interference in radio transmission systems omnidirectional antennas have been replaced by highly directive ones. As a consequence, successful transmission from one station to another requires precise alignment of the two antennas and continuous maintenance of this alignment. In the past, steel lattice structures fixed to massive concrete foundations have been constructed to rigidly support the required antennas. These large structures do in fact sway slightly in the wind, but the beam widths of the antennas used are wide enough that even under conditions of sway the radiated beam is intercepted by the next antenna in the transmission path. As narrower beam and higher gain antennas are used, even the minor sway of a 200-foot lattice structure may cause a 0.1.degree. beam to miss the next antenna. For this reason, as well as others, expensive lattice towers may be replaced in the future with simpler and cheaper structures such as single vertical poles.

The pole-mounted antennas would, of course, experience substantial sway and continuous reorientation would be required to maintain alignment between consecutive antennas in the transmission path. An electromagnetic servomechanism could be used to stabilize the antenna orientation but this would add greatly to the complexity, expense and maintenance of the pole assembly. For structural reasons, the pole could not be as high as the older lattice towers and hence many more repeater stations would be required. Thus, expensive stabilizing mechanisms would mitigate the value of the inexpensive pole mounting.

SUMMARY OF THE INVENTION

In accordance with the present invention, an inexpensive stabilizer is provided for a pole-mounted highly directional narrow beam antenna. The orientation of the beam in elevation angle is controlled by the orientation of a surface in the antenna package which is mounted atop the pole. This surface, which may be the supporting platform of the antenna package, is rotatably connected to the pole assembly and when wind loading or other external forces cause the pole to bend or deflect in a plane containing the beam axis a mechanical linkage converts this deflection into linear motion which is translated into a compensatory rotation of the surface.

The mechanical linkage may be provided by an outrigger rod mounted parallel to the pole in a plane containing both the pole and the beam axis. The outrigger is periodically spaced from the pole so that deflection of the pole causes a parallel deflection of the outrigger rod which is rotatably fixed to the platform surface. In this manner, deflection of the rod causes rotation of the platform. An alternative linkage using a belt and pulley arrangement is disclosed in copending application Ser. No. 867,181 filed on an even date herewith by A. A. Penzias and assigned to the assignee hereof.

The bending of the pole will of necessity cause the altitude of the antenna to be reduced but the compensatory rotation of the surface will align the beam along a path parallel to the original beam path for the undeflected condition. Reducing the height of the beam axis even on the order of a foot would not impair coupling at an antenna a mile or more away, whereas the rotation of the elevation of the beam axis by an angle of only 1.degree. would cause a narrow beam to miss the next antenna completely as it would either pass above or below the aperture.

Deflection of the pole in a plane perpendicular to beam axis will not cause any angular change of the beam axis but will merely cause it to be displaced to the right or the left and it will, as in the case of the reduced height, still intercept the next antenna. Therefore, no compensation is required in this direction. However, conventional repeater stations would have a number of antennas facing in two or more directions mounted on the pole and it would be desirable to use three equally spaced outrigger rods to continuously stabilize the platform in response to forces in any direction, rather than provide antenna stabilization for each antenna individually.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a pictorial representation of a conventional pole-mounted antenna illustrating the deflection of the beam axis caused by deflection of the pole. FIG. 1A is a cross-sectional view of conventional antenna package suitable for use with the pole-mounted antenna of FIG. 1.

FIG. 2 is a pictorial representation of a pole-mounted antenna modified in accordance with the invention to correct for deflection in a single plane.

FIG. 3 is a pictorial representation of a pole-mounted antenna modified in accordance with the invention to correct for deflection of the pole in all directions.

FIG. 3A is a top view of the antenna package of FIG. 3.

FIG. 4 is a view of a spacing structure suitable for use in the embodiment of FIG. 3.

FIG. 5 is a pictorial representation of a modified form of the embodiment of FIG. 3.

Elements common to various figures are designated by similar reference numerals.

DETAILED DESCRIPTION

FIG. 1 illustrates antenna package 10 mounted atop pole 11. Package 10 shown in FIG. 1A contains conventional narrow beam antenna apparatus which may comprise, for instance, a parabolic reflector 21 and a plane reflector 22 which is permanently positioned within package 10 to reflect the parallel waves from parabolic reflector 21 so that they pass through a weather-covered aperture 23 in the wall of package 10 to form a beam having an axis 12. The illustrated structure is assumed to be conventional and therefore antenna feeds which would be located at F, the focal point of parabolic reflector 21, and their connection to appropriate electronic circuits, such as a receiver, transmitter or repeater, are not shown. The antenna apparatus may, of course, be of the type without plane reflector 22 in which case parabolic reflector 21 would be positioned to radiate a beam directly through aperture 23. What is essential to the present invention is that the antenna couples with electromagnetic radiation along a narrow beam having an axis 12, whose orientation is determined by the orientation of package 10.

Pole 11 and package 10 are preferably cylindrical in order to prevent a wind-caused twisting moment about the axis of pole 11. However, if pole 11 is rigidly secured to the ground by foundation 13, a force or a component of force may cause the pole to bend and the dotted image in FIG. 1 is an exaggerated illustration of deflection of the pole in the plane containing beam axis 12. As can be seen, a force component from left to right will cause the pole to deflect to position 11' and package 10 will be tilted to position 10'. Beam axis 12 will lie along a line 12' which is displaced from the original undeflected orientation by an angle .phi.. It is evident that if a pole supporting an antenna in a radio relay system is deflected in this manner, the beam may intercept the ground and not the next antenna which is aligned with the original orientation of beam axis 12. A force from right to left will cause the opposite effect and the displaced beam axis would lie above the original orientation of beam 12.

Fig. 2 illustrates an assembly similar to that of FIG. 1 except that it has been modified in accordance with one embodiment of the invention by the addition of outrigger rods 16 and 17. Platform 14, the lower end surface of cylindrical package 10 is secured by a rotatable coupling 15 such as a hinge which supports package 10 and prevents it from twisting about the axis of pole 11 but allows it to rotate about an axis perpendicular to the plane defined by pole 11 and beam axis 12. This plane is, of course, a vertical plane when pole 11 is mounted vertically. Pole 11 is fixed to foundation 13 at 13c. Outrigger rods 16 and 17 are mounted parallel to pole 11 and fixed to foundation 13 at 13a and 13b, respectively and are pinned to platform surface 14 at 18 and 19. Spacers 20 maintain rods 16 and 17 in this parallel position. Under a force or component of force from left to right, for instance, pole 11 deflects to position 11', but if the rods' lengths do not change, spacers 20 force rods 16 and 17 to deflect to parallel positions 16' and 17'. Since rods 16 and 17 are pinned to platform 14, the parallelogram 13c -15-19-13b is deflected to a shape 13c-15'-19'-13b, the opposite sides of which are parallel and hence platform surface 14 is displaced to position 14" and is maintained parallel to its original attitude. It is noted that theoretically only a single outrigger rod such as 17 is needed to accomplish the stabilization of platform 14. Comparison of FIGS. 1 and 2 illustrates that by maintaining platform 14 essentially parallel to its original position under the condition of deflection, beam axis 12 lies along the line 12" essentially parallel to the undeflected orientation of axis 12. The beam will therefore intercept the next antenna in the transmission path whereas it would not if the beam were along path 12' as in FIG. 1.

The size of the outrigger rods is determined by the turning moment at coupling 15, the height of pole 11 and the allowable angle of deflection of the radio beam. This will be discussed later in reference to another embodiment in the invention. FIGS. 3 and 3A illustrate two antenna packages 25 and 26 mounted atop pole 27. In a commercial application it would be expected that repeater stations would have two or more antenna packages and beam axes, such as 28 and 29 of packages 25 and 26 respectively, would be displaced from one another as is depicted in FIG. 3A. Hence, correction for force components parallel to both axes 28 and 29 is required. It is preferable therefore to stabilize combined packages 25 and 26 in response to components of force in any direction and three outrigger rods 31, 32 and 33 are mounted parallel to pole 27, displaced at intervals conveniently 120.degree. apart and spaced by spacers 34. Any force may be resolved into components in the planes of pole 27 and one of the rods 31, 32 or 33. Each rod will cause stabilization of platform 30 in one axis as was discussed above with reference to FIG. 2. Joint 35 is a universal joint which allows such rotation but prevents twisting of platform 30 about the axis of pole 27. Joints 36, 37 and 38 connecting outrigger rods 31, 32 and 33, respectively to platform surface 30 could be ball joints. Pole 27 is rigidly connected to the ground at 40 and each of the rods 31, 32 and 33 are rigidly connected at 40a, 40b and 40c, respectively, by couplings which are preferably adjustable in height so as to permit leveling of platform 30 for purposes of alignment.

The size and characteristics of outrigger rods 31, 32 and 33 are determined by the turning moment M of the combined antenna package about joint 35 and the acceptable deflection of the beam in accordance with the following relationship:

M=Ph (1)

and for three equally spaced rods

where P is the external (wind) force per unit area times the projected area of the package;

h is the effective moment arm of P;

.theta. is the acceptable angle between the deflected and undeflected beam axis positions;

L is the length of the pole from the ground to the joint at the platform surface;

R is the distance of the rods from the center of the pole;

E is the modulus of elasticity of the material forming the rods, and

A is the cross-sectional area of each rod.

FIG. 4 illustrates a preferred arrangement for providing spacing between pole 27 and the outrigger rods 31, 32 and 33 at various heights along pole 27. With reference to rod 31, for example, a pair of spacers 43 and 44 which exhibit essentially no compression or expansion along their axes are pinned to common point 45 on the exterior surface of rod 31. Each of the rods is then pinned to the exterior surface of pole 27 at points 46 and 47 120.degree. separated. Pairs of spacers are similarly provided for each of the other rods 32 and 33.

FIG. 5 illustrates a modification of FIG. 3 in which a hollow skirt 42 identical in size and shape to the combined form of packages 25 and 26 is mounted to platform 30 symmetrically oriented with respect to packages 25 and 26 about universal joint 35. This modification which applies equally to the embodiment of FIG. 2 will balance the wind load P on the antenna package and eliminate the turning moment at joint 35. This reduces the angle .theta. or permits the use of rods with smaller cross sections A.

In all cases it is to be understood that the above-described arrangements are merely illustrative of a small number of the many possible applications of the principles of the invention. Numerous and varied other arrangements in accordance with these principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.

Claims

I claim:

1. An antenna mounting structure comprising,

an antenna package rigidly affixed to a platform,

said package having antenna means for directionally coupling with electromagnetic radiation,

a pole affixed at one end to a foundation,

a universal joint coupling the other end of said pole to said platform,

three stabilizer rods external and parallel to said pole affixed to said foundation surrounding said pole at 120.degree. intervals, said rods having structural properties such that they deflect similarly to said pole,

ball joints coupling each of said rods to said platform, and

a plurality of spacers positioned at various heights along the length of said pole for maintaining a constant separation between said pole and each of said rods for their common lengths, whereby deflection of said pole causes corresponding deflection of said rods and compensating rotation of said platform so that the orientation of said platform is restricted to parallel planes and a fixed direction of coupling of said antenna means is maintained.

2. An antenna mounting structure as claimed in claim 1 wherein said antenna package is cylindrical and said platform forms the lower end of said cylindrical package and wherein a cylindrical skirt geometrically similar to said cylindrical package is attached to said platform and symmetrically displaced with respect to said package about said platform.