CA1063236A - Dish antenna with integral deicer - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A dish antenna having electrical resistance wire in dielectric backing on the reflective metal surface of the dish and an electrical heater cable extending over the length of the antenna feed. The resistance wire and cable are selectively electrically connected to a power source through a thermostat which passes electrical current to the wire and cable only when the ambient temperature is within a prescribed temperature range. This causes heating of the wire and cable and thus of the antenna for preventing the formation of ice on the latter.

Description

~ L-46Z
l 1~63236 1 DISH .~NTENlNA WITH I~TEG~L DEICER -3 BACKGROU~'D OF I~?E.~IO.
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This invention relates to antennas and more particularly to dish antennas with means for preventing the formation of ice thereon.

7 Telephone communication systems commonly use microwave radios 8 for transmitting telephone si~nals over long distances. ~icrowave 9 radios for such an application may employ parabolic antennas wherein the 101 reflector dishes are paraboloids for providing sharply unitirectional il! beams that are transmitted between antennas at different locations.
12¦ Such antennas may be required to operate in all types of weather 1~1 conditions and over extended temperature ranges. It is desirable that 14 ice not be allowed to form or to build up on the antennas since the ice i5 may physically damage the antenna structure or reflect radio signals 16 incident on the antenna. A prior-art technique described in Electrical 17 Engineering, volume 79, page 434, .~ay 1960, employed gas heaters for 18 defrosting microwave relay antennas. Another prior-art technique for 19 deicing parabolic rèflectors was to`rivet a plurality of strip heater elements to the bac'~ of the reflector. Such heater elements had a 21 relatively high failure rate, however, since they were exposed to the 22 elements and were operated over e~tended time intervals. An alternate 23 prior-art technique for preventing the formation of ice on a microwave

2~ dish antenna is to connect a heated radome over the mouth of the reflector.
Such a radome comprises nichrome resistance wires embedded in fiberglas 26 and arranged in a helical pattern. The wire is connected through a 27 ~emperature-sensing thermostat to a source of electrical power. The 28 thermostat is designed to pass electrical current to heat the wires and 29 thus the radome surface when the ambient temperature is in a prescribed te~perature range such as -6 C to +3 C (+22 F to +38 F). At low 3i ~ ,...

~ L-462 1 frequencies such as 2 GHz, the attenuation caused by a radome is minor, 2 being approximately 0.1 dB. At higher microwave frequencies such as i 3l 12.7 GH~, however, the attenuation caused by the radome is significant, 4 in the order of 2 dB. In order to obtain a sufficient increase in antenna gain to compensate for such a 2 dB signal loss in a parabolic ~ antenna having a ~eflector with ~ 6-foot dia~eter mouth, it is necessary r/ to increase the size of the antelma by two steps so that the reflector 8 has ailO-foot diameter mouth opening. This cause~ a consider~ble increase 9 in the price of the parabolic antenna and its support structure. It is lOI desirable to have a parabolic antenna on which ice will not accumulate, ll¦ without having to cover the mouth of the reflector with a rsdome.
12j 13¦ SU~PIARY OF I~E~TION
14l In accordance with this invention, a"fiberglas" reflector of a 15¦ dish antenna is fabricated with electrical resistance wire located within 16~ the refle~tor and generally extending over the whole surface area thereof.
17j Heater cable is also secured to the waveguide feed over the length thereof. ', 18¦ These resis.ance wires and heater cable are connected through a thermostat 9¦ to an external source of electrical power. The thermostat operates over 20l a range of temperatures for passing electrical current which heats the 21 wires and cable, and thus the antenna, to prevent ice from forming 2~l on the latter.

2~ BRIEF DESCRIPTIO~ OF DR~ GS
25 ! This invention will be more fully understood from the following 26j detailed description, together with the drawings in which: ¦
271 FIG. l is a perspective view of a parabolic antenna embodying 28¦ this invention;
291 FIG. 2 is a section view of a heated"fiberglas" parabolic 301 reflector 5 embodying this invention on a mold 7; ¦ j

3~1 - 3 - .
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ll i ¦¦ L-~62 I.l 1063236 l ~I FIG. 3 is a greatly enlarged section view of a portion of the 2 ¦I heated"fiberglas" reflector 5 in FIG. 2 for illustrating the fabrication 3 11 of the reflector;
FIG. ~ is a side view of a heated buttonhook faed assembly 9 5l for the reflector 5 and embodying this invention;
¦ FIG. 5 is a rear-plan view of the reflcctor 5 in FI~. 2 rJ¦ illustrating one arrangement of resistance wire ll within the reflector;
~¦ and 9I FIG. 6 is a rear-plan view of the reflector 5 in FIG. 2 lO¦ illustrating alternate arrangements of resistance wires inside the 1l ¦ reflector.
1~1 13 ! DESCRIPTION OF PREFERRED ~3BODI~E~TS
l4 Referring now to the drawings, a dish antenna embodying this invention in FIG. l includes a heated parabolic~iberglasP reflector 5 lG¦ and a heated waveguide feed assembly 9 which are sho~m in detail in the l71 section and plan views in FIGS. 3 and 4, respectively. In accordance with 181 this invention, a resistance wire ll is an integral part of the '91 reflector 5, the wire 11 being located within the interior of the reflector, as is shown in FIG. 3. Resistance wire preferably extends 21 over substantially the whole surface area of reflector 5 in a manner 22l such as is shown in FIGS. 5 and 6 and is described more fully hereinafter.
23 ~ heater cable 12 also e~tends over opposite sides of the waveguide feed 24 ¦ in FIG. 4. Lead wires 15A, 15B and lSA, 18B from the reflector 5 and 25 ¦ the heated feed assembly 9 are electrically connected together, with 251 common wires l9A, l9B in FIG. 1 being connected through a thermostat 21 27~ and lines 22A and 22B to a source 23 of AC electrical power. In operation, 281 the thermostat 21 closes to connect the wires l9A and l9B to the power 29l source 23 only when the ambiant temperature is between -6 C and +3 C, 3C~ for example, in order to pass an electrical current through the resistance 31i ~ , . L-462 ,i 1063Z36 ~

1 wires in reflector 5 and the heater cable 12 on the feed assembly 9J
2 and to ~ereby heat the antenna to prevent ice from fo~ming thereon.

1~ ..

The reflector 5 is formed over a precision mold 7 having high 1~ contour and surface accuracies. A probe 25 extends above the center of 17 the mold. Initially, a thin layer 26 of mold release is sprayed over 18 the curved surface of mold 7 (see FIG. 3)~ The layer 26 facilitates 19 removal of the finished reflector 5 from the mold. The mold release is water soluble and is removed from the mold and the finished reflector 21¦ by washing these parts with a mild detergent and water. After the mold 2_1 release has dried, an apertured parabolic ring 27 is placed over the 231 central probe 25 on the mold. The aperture 28 in the ring 27 provides 24¦ access for placing the feed assembly 9 in a fabricated reflector from the rear thereof,as is described more fully hereinafter. The periphery of the 261 ring 27 is grooved for providing a surface for securing subsequent layers 27, to the ring.
281 . I
29 ~ In order to provide protection for the molded reflector, a gel-coat 3o ! layer 31 is sprayed over the exposed mold release to a thickness of about 311 _ 5 32 ~
l I
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L-462 1 ~
1063236 ~

1 10 mils. The gel-coat 31 contains selected color 2igment and appropriate 2 additives which absorb ultraviolet r3ys in order to shield subsequent 31 layers from such light. This gel-coat layer 31 is very hard when it dries so as to provide mechanical protection to the reflecting surface 51~ f a me~al layer ;2 which is con~ig~us eherewith.

7 ~ After the ~el-coat layer 31 has thoroughly dried, the metal ~¦ layer 32 is formed on the molded structure. The function of the metal I layer 32 is to provide a smooth and accurate surface that f~ces the 10 ¦ mol~ 7 and which reflects radio frequency electromagnetic waves incident 11¦ on it. The metal layer j? is preferably made of alùminum which is evenly 12¦ flame-sprayed over the gel-coat layer ;1 to a thickness which is greater 131 than the skin depth at the lowest operating frequency of the antenna.
141 By way of e.~ample, the layer 32 on an antenna that operates in a 15~ frequency band having a low frequency end of 1 GHz may be appro~imately 16¦ 8 mils thick. A first layer 3~ of'-fiberglas'~' and polyester resin is 17¦ formed on the back of the metal layer 32 to provide rigid structural 18¦ su?port to the reflector 5. The dielectric layer 33 is preferably a 19¦ mi~ture of polyester resin and chopped"fiberglas'~ that is sprayed onto 20¦ the molded structure to a thic~ness of about S0 mils after the flame-21¦ sprayed metal layer 32 is hardened. This layer 33 also contains an 221 ultraviolet retardan~ to protect it from deteriorating under illumination 231 of such light.

The "fiberglas" dielectric stratum ;3 is manually rolled to 26 bring wet resin up to the e.Yposed surface thereof. While this resin is 271 still tacky,"nichrome"resistance wire 11 is laid out in the resin. The 28¦ wire 11, which may be 20-gauge wire, is preferably arranged in a ~9 prescribed pattern, such as is illustrated in FIG. 5. Care is taken ¦ L-~62 1 to ensure that the turns of the resist~nce wire are spaced apart over 2 the length thereof. The ends 11.~ and 113 of the resistance wire are connected to solid copper bus wires l~A and l~B, respectively, which have a very low electrical resistance. The ends of the bus wires 14A, l~B
preferably e~tend in a direction away from the surface of the mold and are connected to lead wires lS.~ znd l~B when the molded reflector is completed, After the first'`fiberglas" l~ver 33 is cured, a second layer 3~ of chopped"iberglas" and resin is sprayed over the molded structùre to a thickness of approximately 50 mils, which is sufficient to completely cover the turns of wire 11. The stratum 3~ is then cured to 11l dlelectrically insulate the wires 11 from any external structure and from 12¦ each other.

14 ! In order to attach the reflector to a tower, a mechanical mounting 15¦ assembly 3~ is secured to the back of the molded structure~ The mounting 16¦ assembly 37 in FIG. 3 is formed on a separate mold tnot shown) that is 171 shaped like a circular trough having a flat bottom. The trough mold 18 for the mounting assembly 37 is sprayed with layers of mold release 191 and"fiberglas" in a manner similar to that described above in relation 20 ! to the molded reflector structure i~ FIG. 3. A metal plate 38 with 21¦ tapped holes used for mounting is laid onto the"fiberglas" 39 in the 221 trough mold, with the plane surface of the plate 38 perpendicular to the 231 a~is of the mold, and sprayed with another layer of fiberglas . This 24l mounting assembly 37 is then removed from the trough mold, cleaned, and 251 centered on the back of the reflector structure on mold 7. A layer~ 40 26l of"fiberglas" is sprayed over adjacent surfaces of the'fiberglas"
271 layers 34 and 39 and cured to rigidly secure the assembly 37 to the 2B~ molded reflector structure. A final layer 41 of gel-coat which also 29l contains appropriate color pigment and ultraviolet absorber is then 30i sprayed onto the exposed fiberglas strata 34, 39, and 40. After this 3~ , ~2l - 7 -~ , . I

l ¦! gel-coat layer 41 is dried, the fabricated reflector 5 is rc~oved from 2 ! the mold 7 and cleaned.
3 1 ,

4 A buttonhook feed 9 is shown in FI~. 4, togetheT with the 51 ring 27 in the center of the reflector 5 and a lcck ring 42 for purposes of illustration. The feed 9 comprises a length of rectangular waveguide 7 43 which is bent so that the portions 4~ and 45 thereof are aligned.
8 The end IS of the waveguide is flaret to form a horn for illuminating ~ a reflector 5 in which the feed is located. ~ pressure window 46 is lOI placed over the end ~5 of the waveguide to keep foreign objects out of the }l¦ latter. ~ two-piece casting ~7 which fits over the end 49 of the 12¦ waveguide is held together by a strap 50. The casting 47 has a steppet 13¦ circular disc ~1 on the front end thereof. The circular fron~ portion l4l 52 of the disc has a diameter such that it fits snugly into the central lS¦ opening 28 in plate 27. The circular back portion 53 of disc 51 has a 16¦ diameter that is greater than the diameter of the opening 28 in plate 27 171 and less than the diameter of the bolt circle containing threaded holes l~¦ 54 in plate 27. The tiameter of the opening 55 in lock ring 42 is less l9! than the diameter of the disc portioh 53, has a bolt circle that 20l corresponds to that on plate 27, and is employed to hold the feed 9 in 21ll a reflector 5, as is described more fully hereinafter. The waveguide ~¦ feed is heated by a length of heater cable 12 which extends through 23l an opening in the casting 47, over the full length of one side of 241 waveguide 43, across the waveguide adjacent pressure window 46, and 25 ! down the opposite side of waveguide 43 where it is terminated at a 5 2~ point adjacent the casting 47. The cable is secured to the waveguide 43, 27 for example, by"fiberglas' tape 58. The heater cable 12 may, by way 28 of example, be Cuprothal Resistance Alloy # 294 such as is manufactured 2~ b Kanthal Corp., Be~hel, Conn.

32l, - 8 -. !
,, ~ L-462 ! 1063236 i 1 I The antenna is completed by attaching the reflector ; on an adjustable mounting mechanism which is, in turn, attached to a tower 31 tnot shown) or o~her structure. The feed assembly 9 is then turned ¦ at an angle with respect to the aYis of the reflector and passed through the opening ~8 in plate 27 until the disc portion 52 is located in the openin$ ~. A pluralit~ o screws 59 are then threaded through the 7 holes 54 in pl~te ~7 to draw the plates 2/ and 42 together and thereby & to hold the feed 9 in the reflector 7~ The polari2ation of the antenna 9¦ may be changed by loosening the screws 59 and rotating the feed. The lQI loc~tion of the feedhorn 45 may be positioned with respect to the 11~ focal point of the reflector 5 by loosening the screws 59 and rotating 12¦ the feed. The location of the feedhorn 45 may be positioned with respect 13' to the focal point of the reflector S by loosening the strap 50 and 1~ moving the feed axially within the casting 47. The lead wires 18A, 18B

15¦ of the heater cable and lSA, lSB on the reflector are connected together 16¦ and through com~on lead wires l9A, l9B to the thermostat 21 which may 17l be mounted on the flange 3~ ires 22.~, 22B connect thermostat 21 181 to the source 23 of electrical power when the antenna is operating in l9! its intended environment. The thermostat operates to connect the 20~ lead wires l9A, l9B and 22A, 22B together only when the ambient 21 ¦ temperature is in a temperature range of, for example, -6 C to +3 C.

231 Only one'hichrome"resistance wire 11 is shown in FIG. 5 24~ extending over the full surface area of the reflector. In an alternate 251 embodiment of this invention, a plurality of wires 11 and 11' extend 26¦ next to each other over the same area of the reflector. Adjacent ends 271 f the wires 11 and 11' are electrically connected to associated bus 23l wires 14A and 14B. In this arrangement, if one resistance wire develops 2~ an open circuit, the other wire will continue to heat the reflector.

31 1 _ 9 _ 32, L-46~ 1 '. ~
1 ll In another embodiment of this invention, the surface area of 2 !¦ the reflector is divided into a plurality of sections 63 as is shown in 3¦¦ FIG. 6. A nichrome resistance wire 64 e.Ytends over each section in a prescribed manner, several alternative patterns being shown in FIG. 6.
5l The opposite ends 65 and 66 of a resistance wire 64 in each section are 6 ¦ electrically connected to different ones of a pair of low-resistance bus wires 67 and 68 which e~tend around the major portion of the perimeter 81 of the reflector. Care is required in such a structure to electrically 9 ¦ insulate these two bus wires from each other-and the-bus--wire 67 from the 10¦ ends of the resistance wires 6~ at the cross-over points. Reference to 11¦ FIG. 6 reveals that all of the resistance wires are electrically connected 12¦ in parallel. The resistance wires in the various sections 63 are also 13i preferably of the same length so that they will all have substantially 14¦ the same electrical resistance. In this way, the reflector 5 is uniformly 151 heated to substantially the same temperature over the entire surface area 16 thereof. If the resistance wire in one section develops an open circuit, 17 other sections of the reflector are still heated to a sufficient degree 18 to deice the reflector. Alternatively, a plurality of"nichrome"resistance 19 wires 64 may be laid out in each section and connected to the bus w~res.
In such an arrangement, if one of t~e resistance wires in a particular 21¦ section develops an open circuit, other resistance wires will continue 2~l to heat the asso~iated section of the reflector.
23~
241 Although this invention is described in relation to preferred embodiments thereof, various modifications will be obvious to one skilled 2~1 in the art. By way of e~ample, the flame-sprayed metal stratum 32 27¦ may be made of suitable metals other than aluminum. The metal layer 32 23 ¦ may also be formed of aluminum foil or of glamour cloth that is heated 29 ¦ and draped over the gel-coating 31 on the mold. Further, the glass 301 layers 33 and 34 may each comprise a sheet of fiberglas cloth and 3l1 32l - 10 -.,." il l 1063Z36 l I resin that is worked through the glass until it bonds to the bac~ surface 2 ¦ of the adjacent stratum. Also, the reflective metal layer 32 may be a 3 ¦ rigid solid element that is formed on and removed from another type of 4 ¦¦ mold than that shown in FIG. 2. The layers 33 and ;~ of"fiberglas, with

5¦¦ resistance wire ll therein may then be formed on the bac~ of the metal

6 1 dish to pro~ride a mechanism for heatin~ the latter. The scope and breadth 71 f this invention is therefore to be determined from che following claims 8 rather than rom the above detailed descriptions.

~1 l8 20~ '.

2?

25i 30!
31 , ~2 1

Claims (15)

What is claimed is:

1. In a dish antenna for operating in conjunction with a source of electrical power over a temperature range in which icing may occur, the antenna including a reflector and a feed assembly for illuminating the electrically conductive front surface of the reflector with electromagnetic energy for producing a beam of electromagnetic radiation, the improvement comprising:
electrical resistance wire extending over the major portion of the surface area of the reflector, being located behind and electrically insulated from the electrically conductive surface portion of the reflector, and being formed as an integral part of the reflector within the exterior surfaces thereof for electrical connection to the power source for heating said wire and the reflector to prevent ice from forming thereon.

2. The improvement according to claim 1 wherein said resistance wire is located in at least one prescribed pattern in back of the front surface of the reflector.

3. The improvement according to claim 1 including a plurality of resistance wires located in back of the front surface of the reflector, each wire extending over at least a part of the surface area of the reflector, said wires being integral parts of the reflector and being electrically connected in parallel for electrical connection to the power source for heating said resistance wires and the front surface of the reflector to prevent ice from forming on the latter.

4. The improvement according to claim 1 including a thermostat for electrically connecting said resistance wire to the electrical power source for passing electrical current through said resistance wire only when the ambient temperature is within a prescribed temperature range.

5. In the dish antenna according to claim 1 wherein the feed assembly comprises a length of waveguide, the improvement further comprising an electrical heater cable contacting the waveguide and extending over at least part of the length thereof for electrical connection to the power source for heating said cable and the waveguide.

6. A dish antenna for operating in conjunction with a source of electrical power over a temperature range in which icing may occur, comprising:
a reflector element having a front surface that reflects electromagnetic signals incident thereon and having a back surface, electrically nonconductive dielectric material covering at least a major portion of the back surface of said reflector element, and electrical resistance wire embedded in said dielectric material, being electrically insulated from said reflector element, and extending over a major portion of the surface area of said reflector, said resistance wire heating said reflector element when the former is electrically connected to the power source for preventing formation of ice on said reflector element.

7. The antenna according to claim 6 comprising a feed assembly electrically connected to said reflector element for illuminating the front surface thereof with electromagnetic signals.

8. The antenna according to claim 7 wherein said resistance wire is located in said dielectric material in at least one prescribed pattern.

9. The antenna according to claim 7 including a plurality of resistance wires, each extending over at least a part of the surface area of said reflector element.

10. The antenna according to claim 7 wherein said feed assembly comprises a length of waveguide, the antenna including an electrical heater cable contacting the waveguide and extending over at least part of the length thereof for electrical connection to the power source for heating said cable and the waveguide.

11. The antenna according to claim 10 including a thermostat for enabling electrical connection of said resistance wire and said cable to the power source only when the ambient temperature is within a prescribed range.

12. A heated reflector for an antenna where the reflector operates in conjunction with a source of electrical power for preventing the formation of ice on the reflector, comprising:
a reflective element having a front surface for illumination by electromagnetic signals and reflecting electromagnetic signals incident thereon, and having a back surface, an electrically nonconductive dielectric material covering at least a major portion of the back surface of said reflective element, and at least one electrical resistance wire embedded in said dielectric material and being electrically insulated from said reflective element for heating the latter to prevent the formation of ice thereon when said resistance wire is electrically connected to the power source.

13. The reflector according to claim 12 wherein resistance wire extends over a substantial portion of the surface area of said reflective element.

14. The reflector according to claim 13 wherein resistance wire is located in at least one prescribed pattern proximate the back surface of said reflective element.

15. The reflector according to claim 13 including a plurality of resistance wires, each of which extends over at least a part of the surface area of said reflective element.