US3388401A - Parabolic antenna with low-loss flexible radome - Google Patents

Description

June 11, 1968 W 3,388,401

PARABOLIC ANTENNA WITH LOW-LOSS FLEXIBLE RADOME Filed June 30, 1965 m/vewroe. WALTER F. WE If? United States Patent "ice BfSSAtll PARAEGLTC ANTENl -IA Wi'lfl LQWJLOSS FLEXEBLE Walter F. Weir, Whitby, ilntario, Canada, assignor to Andrew Antenna @ompany Limited, Whitby, Ontario, Canada, a corporation of Canada Filed June 30, 19 65, Ser. No. 463,346 lltl tllniins. (til. 343-S72 This invention relates to parabolic antennas protected by radome covers, and more particularly to a construction employing a thin flexible low-loss radome or front cover which is capable of satisfactory operation in regions of high wind velocity.

Where parabolic antennas are exposed to severe weather conditions, it is conventional to provide radome covers, normally mounted on the rim of the parabolic reflector. Such a radome is normally constructed of a suitable plastic, designed to produce maximum mechanical strength with minimum electrical transmission loss. Such radomes are normally rigid structures, bowed outwardly for structural strength against wind pressure. in general, the strength and thickness of the radome are determined in accordance with the weather conditions, pria iy wind velocities, to be encountered, and radome constructions for use in high winds, such as one hundred or more miles per hour, are normally of substantial thickness and weight.

An additional factor in the design of radomes for use under unfavorable weather conditions is the problem of icing and snow accumulation, which of course not only introduce large transmission losses, but additionally scriously distort the radiation pattern of the directive antenna. This problem is normally dealt with by incorporation in the radome of suitable heating elements, again introducing inevitable transmission loss, in addition to requiring highly precise design and construction in order to avoid distortion of the radiation pattern.

Prior to the present invention, the design of radomeequipped antennas for operation under adverse Weather conditions accordingly involved not only the introduction of substantial transmission loss, detracting from antenna e ficiency, but also the large added expense of the radome structure, in addition to the difficulties of installing and removing the bulky radome, particularly with dishes of diameter of the order of ten or twelve feet. Further, where heating is required, the heating power may well be the greatest power requirement of an entire station or link, this requirement often being virtually prohibitive, particularly in unmanned installations at remote locations.

It is the purpose of the present invention to provide a radome equipped antenna structure which avoids these problems. In general, the present invention stems from approaching the problem of withstanding high-velocity winds in a substantially different manner than the approach represented by the constructions discussed above, which provide additional mechanical strength for the radome more or less in proportion to the maximum expected wind pressure. In the present construction, no attempt is made to rigidize the radome to provide mechanical strength. On the contrary, the radome is deliberately made flexible and yielding. advantage is taken of the fact that a flexible structure can readily withstand, without damage, forces, impact or steady, substantially exceeding those which would cause se ere damage to a rigid flat or convex structure of the same relatively small thickness and electrical loss. The radome employed in the present invention is preferably a simple thin waterproof fabric of high tensile strength and low electrical loss characteristics, resiliently deforming in response to the forces of differential pressure between its surfaces. This rezilient flexibility is preferably provided in both the mounting of the radome cover at its edges and in the employment of a resilient or rubberlike material.

3,388,401 Patented June 11, 1968 The simple tarpaulin-like sheet or cover just described requires further special provision in order to make it practical for use in high winds. If such a simple cover is placed over the front of a reflector dish without other special construction, the inward excursions of the radome would rapidly destroy the feed at the focus of the parabolic reflector, in addition to tearing the radome by the repeated contact. in the present invention, these excursions are prevented by building up within the cavity formed by the reflector and the radome cover an average or static pressure largely balancing the average pressure exerted on the outer face of the radome. This is done by connecting the interior through a suitable conduit to an open mouth or air scoop facing in the direction to have the same wind exposure as the outer face of the radome. This air scoop or mouth is located at a point radially spaced from the perimeter, so that it is not in the low-pressure region immediately surrounding the edge of the forward surface of the radome cover produced by the high flow velocity along that surface. As will hereinafter be more fully discussed, the best operation requires that the leakage through the reflector dish to the lee side of the structure not be suflicient to prevent the build-up of the internal equalizing pressure, and the conduit should be designed to provide low flow resistance, both for this purpose and also to admit air at a sufficient speed to reach equilibrium within a reasonably short time upon changes in average wind pressure on the outer face. However, satisfactory operation does not require that mere transient gusts be followed.

The construction as just described produces fully satisfactory operation under a large variety of adverse weather conditions with a minimum of impairment of transmission, and with a radome structure of cost, portability, and convenience of installation and removal heretofore unknown for use under comparable weather conditions. Further, the motion of the radome cover under the action of the wind, while insufiicient to produce damage to the feed structure, is nevertheless readily sufficient to prevent the accumulation of ice or snow, thus making unnecessary the provisions for heating heretofore common.

For fuller understanding of the invention as generally described above, together with certain of the further features of construction and advantage afforded by the invention, reference is made to the embodiment of the invention illustrated in the annexed drawing and described below.

In the drawing:

FiGURE 1 is a front view in elevation of a radomeprotected parabolic antenna constructed in accordance with the invention; and

FIGURE 2 is a side v'ew, partially in elevation, and partially in section taken along the line 2-2 of FIG- URE 1.

The antenna it incorporates a buttonhook waveguide feed it with its terminal end 12 at the focus of, and directed toward, the conductive surface 13 of a parabolic reflector 14, in conventional fashion. The radome member or cover 17 is a relatively thin sheet or membrane of resilient flexible material providing low microwave loss, preferably a tough waterproof sheeting such as a waterproofed fabric of a suitable fiber such as nylon. One highly suitable material is the high-tensile-strength composite waterproof fabric marketed under the trademark i-lypalon. The radome 17 is more or less tightly stretched over the forward end of a tubular extension or pattern-shaping hood 18 having its rearward end secured about the peripheral edge of the reflector T4. The radome 17 is held in place by a cord 19 which is threaded through suitable eyelets closely spaced about the periphcry of the radome sheet, with loops of the cord 19 being engaged by hooks 2t) spring-biased rearwardly by springs 21 bearing against bushing mounts 22 engaging the rearwardly extending rim of the hood 18. The structure thus far described is supported by means of a conventional framework including vertical members 23, horizontal members 24, and braces 25, the manner of mounting this assembly on a tower or similar support also being conventional and being omitted from the drawing.

An air scoop or conduit assembly generally designated at 26 faces in the forward direction to have the same wind exposure as the radome 17 and communicates with the enclosed space 27 behind the radome, converting the wind velocity to a substantially static pressure within the enclosure acting on the inner surface of the radome to produce approximate balance with the wind pressure on the outer surface.

The air scoop 26 comprises a tube or conduit 28, preferably as shown, having a 90 curve, the inner end of the conduit being fitted into an opening in the hood 18, and the outer end being generally coplanar with the radome member 17 and radially spaced therefrom. A protective mesh screen 30 is preferably installed on the mouth of the scope 26 to prevent the entrance of birds.

It is impontant to observe that the pressure-balancing passage (or passages, if more than one is employed) to the interior should meet certain requirements for optimum performance. The illustrated location of the forward end or mouth places it in a region where the air pressure is at least approximately equal to the pressure on the front surface of the flexible radome. As is well known, large pressure gradients due to velocity and turbulence exist over the surface of a body of such a configuration (virtually the antithesis of conditions for streamline flow) and, in addition, the pressure pattern varies greatly with the direction of the wind. It is desirable to minimize the likelihood that the pressure acting at the mouth of the pressure-balancing passage will be unduly low. Accordingly, to assure that the mouth pressure varies primarily with wind pressure on the radome, rather than with mere differences in wind direction, the forward end is desirably placed so as to produce this result, i.e., to avoid likelihood of appearance at the mouth of pressures much lower than those on the radome, due, for example, to Venturi effects in the pressure pattern. A mouth location radially outward of the forward portion of the enclosure and facing forwardly, as illustrated in the drawing, is in general suitable for this purpose, but even better independence of wind direction may be obtained by further forward and radially outward extension if so desired.

It is also required that the shape, size and length of the passage between the mouth, serving as the air scoop, and the interior, be dimensioned to have an air-flow resistance or dynamic pressure-drop much smaller than any flow-paths, leakage or otherwise, from the interior to low-pressure regions such as the rear of the reflector. For high wind velocities, the ratio should be less than 1 to 3, and preferably less than 1 to 4. Otherwise, the pressure within the enclosure cannot rise in the desired fashion, since the pressure drop through the passage is excessive. With substantially no leakage from the dish reflector, this criterion is of course easily met. With substantial leakage flow-paths from the interior to lowpressure regions, achievement of best results requires that the flow passage be efliciently designed to produce free flow of air into the interior.

The flow resistance of the passage, it will be seen, as related to the volume of the interior, is also of considerable importance in determining the speed of pressure equalization between the front and the interior, particularly in the case of increase of average wind velocity, where the response time is also slowed by the leakage. Obviously, there is necessarily some time lag in following wind changes, and the effects of rapid gusts are in no event fully eliminated by pressure equalization. However, it is found that gusts are normaly transients occurring about, or superimposed upon, relatively steady average wind pressures; where the slow-changing components of wind pressure are equalized adequately, gusts which would produce repeated impacts on the feed if superimposed on a large inward deflection of the flexible radome are rendered relatively harmless when superimposed on the equalized-pressure condition. Accordingly, the flow resistance may produce substantial time-lag in following changes of wind pressure without depriving the system of its benefits. Also, it is found in practice that pressure patterns on the face of the drum-like structure due to flow patterns frequently produce, over a substantial range of frontal wind angles, higher steady-state internal pressure than the pressure acting on the front surface, so that the radome is bowed outwardly by the connection of the interior to an external point receiving the full wind pressure. Transients occurring about such a steady-state condition are of course completely harmless.

In most practical constructions, the maximum flow resistance is largely limited by the first factor mentioned above, i.e., the leakage, since this is normally appreciable and flow resistance meeting this requirement will normally provide sufliciently fast response time to prevent damage to the feed. For unusual conditions, a stronger cover, with greater tension on its edges, may be employed if found necessary.

The application of the teachings of the invention to produce structures substantially different in appearance from that illustrated in the drawing will be obvious to those skilled in the art. Accordingly, the scope of the protection to be afforded the invention should not be determined merely from the single embodiment illustrated and described, but should extend to all utilization of the teachings of the invention as defined in the appended claims, and equivalents thereof.

What is claimed is:

1. An antenna comprising:

(a) a parabolic reflector, a feed at the focus thereof, and a radiation-transparent radome covering the aperture of the reflector and forming a protective enclosure,

(b) and at least one air conduit having its outer end facing in the direction to have the same wind exposure as the outer face of the radome and its inner end within the enclosure to substantially equalize the average pressure on the inside and outside of the radome,

(c) the radome consisting essentially of a thin weatherproof flexible elastic sheet.

2. An antenna comprising:

(a) a concave conductive reflector structure of transverse dimensions substantially greater than its depth forming a relatively shallow forwardly open cavity and a feed in the transversely central region of the cavity,

(b) a thin flexible dielectric diaphragm radome over the open end of the cavity,

(0) and at least one conduit having an outer end on the exterior of the cavity facing forwardly and having substantially the same wind pressure exposure as the diaphragm and an inner end within the cavity,

(d) the conduit substantially equalizing the average pressures on the opposite faces of the flexible radome to minimize its deflection in response to wind pressures.

3. The antenna of claim 2 having the outer end of the conduit radially outward from the radome.

4. The antenna of claim 2 having the conduit of substantially lesser air-flow resistance than the aggregate of any air-leakage apertures in the reflector.

5. The antenna of claim 2 wherein the radome is a composite of a dielectric fabric and a rubber-like coating.

6. The antenna of claim 2 wherein the reflector structure comprises a circular parabolic surface and a conducting forwardly extending hood on the front edge of the parabolic surface.

7. The antenna of claim 2 having the conduit extending radially from the interior and bending forwardly to terminate substantially in the plane of the radome.

3. The antenna of claim 6 having the raclome extending across the front edge of the hood and having springs securing the periphery thereof and maintaining rearward tension.

9. The antenna of ciairn having a single such con extending downwardly from the interior and bending orwardly to a point or" termination substantially as far forward as the plane of the radorne, the conduit having less air-flow resistance any other flow-paths from the interior to the rear of the reflector.

The antenna of claim 2 wherein the ratio of the 0 air-flow resistance of the conduit resistance of any direct leakage tcrior to the rear of he reflector flow-path to the air-flow flow-paths from the inis less than 1 to 3.

Referenpes Cited UNETED STATES PATENTS 2 959,785 11/1960 Leatherman et al. 343872 3,351,947 i1/1967 dart 343872 XR

Claims (1)

1. AN ANTENNA COMPRISING: (A) A PARABOLIC REFLECTOR, A FEED AT THE FOCUS THEREOF, AND A RADIATION-TRANSPARENT RADOME COVERING THE APERTURE OF THE REFLECTOR AND FORMING A PROTECTIVE ENCLOSURE, (B) AND AT LEAST ONE AIR CONDUIT HAVING ITS OUTER END FACING IN THE DIRECTION TO HAVE THE SAME WIND EXPOSURE AS THE OUTER FACE OF THE RADOME AND ITS INNER END WITHIN THE ENCLOSURE TO SUBSTANTIALLY EQUALIZE THE AVERAGE PRESSURE ON THE INSIDE AND OUTSIDE OF THE RADOME,

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