Note: Descriptions are shown in the official language in which they were submitted.
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1 DISH .~NTENlNA WITH I~TEG~L DEICER -
3 BACKGROU~'D OF I~?E.~IO.
..
This invention relates to antennas and more particularly to
dish antennas with means for preventing the formation of ice thereon.
61
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
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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
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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
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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.
11
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14
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
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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.
24
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
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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.
131
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
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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.
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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.
221
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.
301
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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
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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.
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