US 3925783 A
The radome heat shield relates to insulating a temperature sensitive antenna from the heat emanating from the inner wall of an airborne radome. The heat shield consists of thin layers of reflecting material that does not interfere with radio frequency transmission therethrough. The heat shield can be manipulated to change the dielectric constant in prescribed incremental areas which provides acceptable boresight error rates through the radome.
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Description (OCR text may contain errors)
United States Patent Bleday et a]. Dec. 9, 1975 RADOME HEAT SHIELD 3,384,895 5/1968 Dorne et al 343/708 3,396,400 8/1968 Kelly et al 343/872  Inventors M'chael Bledayg C(mcordl Fred 3,403,403 9/1968 Howell 343/708 YwngrenfLexmgton, both of 3,747,530 7/l973 Tepper 343/872 Mass.
 Assignee: The United States of America as Primary Examiner-Eli Lieberman represented by the Secretary f the Attorney, Agent, or FirmNathan Edelberg; Robert P. Army, Washington, Gibson; Jack W. Voigt  Flled: Nov. 15, 1974  ABSTRACT  Appl- N04 5247284 The radome heat shield relates to insulating a temperature sensitive antenna from the heat emanating from 52 US. Cl 343/705; 343/872 the inner Wall of an airborne radome- The heat Shield  Int. Cl. H01Q 1/42 consists of thin layers of reflecting material that does  Field 6: Search 343/705, 708, 872 not interfere with radio frequency transmission therethrough. The heat shield can be manipulated to  References Cited change the dielectric constant inprescribed incremen- UNITED STATES PATENTS tal areas which provides acceptable boresight error rates through the radome. 3,075,191 l/1963 Peay t. 343/872 3,128,466 4/1964 Brown et al. 343/705 5 Clalms, 6 Drawmg Flgures SEEKER ELECTRONICS ANTENNA U.S. Patent Dec. 9, 1975 3,925,783
SEEK ANTENNA ELE ONICS RADOME HEAT SHIELD DEDICATORY CLAUSE The invention described herein was made under a contract with the Government and may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalties thereon.
BACKGROUND OF THE INVENTION In radome design an important parameter is the insertion phase delay, which is the phase of a transmitted wave through a panel of the radome relative to the phase of the wave at the same place had the panel not been present. Variations in insertion phase delay across an antenna aperture due to the presence of a radome can result in beam deflection, changes in beam width, and gain loss. Techniques are well established for obtaining the beam deflection errors. Beam deflection away from the original boresighted wavefront is defined as boresight error. A certain amount of boresight error is normally acceptable as tolerance which occurs in tradeoffs between various components of the typical radar system. In addition to contributing to boresight error, radomes used for high speed applications such as supersonic speeds are also characterized by high temperatures and high heating rates. Ceramic radomes afford good thermal shock resistance at these high speeds but high temperatures prevail within the randome during flight.
SUMMARY OF THE INVENTION A radome for use on high speed projectiles are subjected to the relatively high temperatures associated therewith developes high inner wall temperatures that far exceed the nominal allowable temperature of a lightweight antenna within the radome. The temperature sensitive antenna and related components are insulated from the inner wall of the radome by a heat shield comprising thin layers of heat reflecting material which function as an electromagnetic window and therefore does not interfere with radio frequency (rf) transmission. The thin layers of the heat shield can be manipulated to change the dielectric constant therethrough in prescribed incremental areas. The areavarying dielectric constant can be used to assure acceptable boresight error rates are obtained while preventing heat damage to the antenna.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified sectional view of a typical projectile radome with a temperature sensitive antenna protected by a layered heat shield.
FIGS. 2-5 are simplified diagrammatic views of typical heat shield configurations,
FIG. 6 is an enlarged section of FIG. 3 showing the typical structure of the heat shield in detail.
DESCRIPTION OF THE PREFERRED EMBODIMENT In high speed application of projectiles wherein forward components such as antenna are covered by aerodynamically sound, protective radomes, the radome is usually constructed of a ceramic or ceramic-glass structure. At supersonic speeds temperatures rapidly rise from an initial nominal value to a range which may be destructive to temperature sensitive components within the radome. As shown in FIG. I, a typical radome l0'is disposed for housing an antenna 12 and related seeker electronic circuit components 14. Radome 10 is a dielectric material such as ceramic, is transparent to r-f energy, and may have various shapes depending on design and objective parameters. For high speed application the temperature within radome 10 can rapidly build up to several hundred degrees in only a short time of flight.
Antenna 12 is a lightweight antenna of styrofoam faced with copper, which has a nominal maximum allowable termperature of 200F or less. To prevent the high temperatures which emanate from the inner wall of the radome from destroying the temperature sensitive antenna, a heat shield 16 is disposed between antenna 12 and the inner forward wall 18 of the radome. Heat shield 16 may consists of several thin layers of lightweight, heat reflective material which also provides an electromagnetic window for rf transmission. Typically, thin layers 20 of heat-shield 16 may be constructed of a titanium dioxide epoxy filled paper and may be spaced apart by spacers 22.
As shown in FIGS. 2-5, the nominal shape of heat shield 16 can be planar (FIG. 2), hemispherical (FIG. 3), conical (FIG. 4), or hyperbolic (FIG. 5). Any combination thereof suitably formed for structural compliance within the system will also function as the heat shield.
FIG. 6 further discloses an enlarged section of FIG. 3 wherein the individual layers 20 have several arbitrarily selectable shapes allowing the heat shield to withstand the acceleration loads, vibration loads, bending loads,
or other forces which may influence the structure. The number of layers 20 are minimal as determined by the temperature history of the inner wall of the particular type of radome l0 and the limiting temperature of the particular antenna 12. The spacing between layers 20 is Y also dependent on these two temperature factors since the shield must suppress convection currents in addition to being electrically suitable. The spacing between layers 20 need not be constant. The heat shield can be configured for attachments to either the radome or the support structure for antenna 12. v a
The thickness of each layer 20 is essentially nonattenuating to rf microwaves. It may vary as necessary accommodating thicker or thinner patches. The layers need not necessarily be homogeneous, involving density changes, material loading and/or pinholes. In prescribed incremental areas the layers can be manipulated to change their dielectric constant. For example,
I in FIG. 4 a resin filler 30 is disposed around a portion of the inner surface of heat shield 16. This areavarying dielectric constant can be used to obtain acceptablee boresight error rates by changing the beam deflection of rf energy passing therethrough. The dielectric constant of a heat shield layer can be adjusted by applying patches, by varying the density or composition of an incremental area using the resin fillers 30 and by creating pinhole arrays 32 of varying number per square inch in selected layers 20 as shown in the outer layer of FIG. 2B.
The heat shield materials, typically of titanium dioxide epoxy filled paper, is non-metallic and may be treated so as not to be detrimentally absorptive to atmospheric moisture. The heat shield material is also capable of withstanding the prevailing temperatures within the radome so that any temperature induced reactions do not substantially affect rf transmissions. The
material can be treated by pigment additions or by coatings to be reflective to heat radiation.
In operation as a heat shield, the most forward or outer layer absorbs and reflects radiant heat energy. It also absorbs heat energy from convection currents induced by the temperature differences within the radome cavity. The second layer 20, placed parallel or concentric with the outer layer, is located at a discrete distance from the outer layer to suppress convection wholly or to such an extent that heat transfer by convection is minimal. It can also be made reflective such that with both the reflection of heat from the outer layer and the suppression of convection currents, a minimal quantity of heat will be transferred from the radome inner wall to inside of the heat shield. Additional layers within the second layer can be added as considered necessary so that the components within the heat shield are kept below survivable or operable temperatures for the antenna and related temperature sensitive components.
An advantage of the heat shield is that it allows the radome designer a surface onto which dielectric adjustments may be made to compensate for the radome boresight errors and boresight error slopes. Hence, phase tuning and/or fine tuning of the radome acting as an electromagnetic window can be accomplished for the radome/antenna interaction by adjustments in the heat shield. Heat shield 16 also allows the antenna to be made of lightweight plastic foam or the like which is easily gimballed but is restricted for use to enviromental temperatures below 200F. The heat shield allows the use of low temperature components connected to the antenna to be located adjacent the antenna in other packages. These components would ordinarily have to be protected from the radome heat by individual insulation, heat sinking or active cooling with fluids.
The heat shield within the radome can be used where high radome temperatures prevail and thermally sensitive radar and seeker components are used. Typical systems include aircraft and missile, orbiting satallite and station shuttle craft and other higher speed derivative systems.
In established missiles, heat rejection and insulation is accomplished by coating the radar and seeker components with reflectors and insulators. Also, heat sinking is used, i.e., the component is made massive or attached to a larger mass than itself so that the heat can be shared throughout the effective mass rather than confined to only the component. Hence, antenna surfaces were polished to be reflective. Thus, the introduction of lightweight temperature sensitive antennas has reduced the space and weight requirements but has also reduced the thermal damage threshold. The heat shield solves the problem of maintaining the radar operable in a high temperature environment.
Although a particular embodiment and form of this invention has been illustrated, it is apparent that various modification and embodiments of the invention may be made by those skilled in the art without departing from the scope and spirit of the foregoing disclosure. While the inventive antenna heat shield was designed primarily for use on high speed projectiles the heat shield can be used otherwise than has been indicated. For example, in ground radomes subjected to heating from nuclear thermal pulses the heat shield will protect the antenna and related equipment. Accordingly, the scope of the invention should be limited only by the claims appended hereto.
1. In a radar system wherein microwave energy is transmitted from an antenna through a radome, the inprovement of an electromagnetic window comprising: a dielectric heat shield disposed within said radome and spaced apart from the radome inner forward surface for reflecting radiant heat energy and absorbing convection heat energy induced by temperature differences within the radome while allowing microwave energy to pass therethrough, said heat shield having plural concentric layers of spaced apart titanium dioxide epoxy filled paper configured to prevent heat from escaping the aft end of the radome.
2. The electromagnetic window as set forth in claim 1 wherein said paper layers are inhomogeneous, thereby providing a non-uniform dielectric constant for affecting radome boresight error.
3. The electromagnetic window as set forth in claim 2 wherein said paper layers have pinhole arrays discriminately formed therein as a varied quantity per unit area for providing a variable dielectric constant within said shield.
4. The electromagnetic window as set forth in claim 3 and further comprising a resin filler fixedly attached to a portion of at least one of said plural paper layers for varying the density of an incremental area of said shield for changing the dielectric constant.
5. The electromagnetic window as set forth in claim 1 wherein said antenna is a temperature sensitive antenna disposed within said radome, said radome is ceramic, and said dielectric heat shield layers are parallel sheets of variable density paper for providing boresight error adjustment of said radar system, and said titanium dioxide epoxy filled paper being fixed around the circumference thereeof to a portion of the inner aft surface of said radome for isolating said antenna from the forward area of said radome.