Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
ANTI-ICING AND D~-ICING SYSTEM FOR
REFLECTOR-TYPE MICROW~VE ANTENN~S 2 0 n 6 4 5 o
FIELD OF THE INVENTION
The present invention relates generally to reflector-type
microwave antennas and, more particularly, to a unique anti~
icing and de-icing system fox such antennas.
DESCRIPTION OF RELATED ~R~
Previous anti-icing or de-icing systems for microwave
antennas have used either direct electrical heating or forced
hot air heating. In the direct electrical heating systems,
electrical power is supplied to insulated flexible heating
elements in the form of strips, panels or mats attached ;~
directly to the rear surface of the reflector. Heat generated
by the heating elements is transferred directly to the
reflector, and then throughout the reflector, by conduction.
Such heating systems are relatively expensive and are -~
extremely difficult to install in the field. The interface ;
between the heating elements and the reflector is sensitive to
irregularities in the reflector surface, and any imperfection
in the adhesive bond between the heating element and the -~
reflector allows water to penetrate into the interface. Such ;
water penetration reduces the effective heat transfer to the
reflector, deqrades the adhesive bond, and eventually leads to
delamination of the heating element from the reflector. ;
. ~ . .
2 ~ ` :`
'~
~3G~SO
In the forced air systems, heated air is blown into
and/or circulated around a plenum formed by an enclosure
attached to the rear side of the reflector. The air is heated
by electrical resistance heaters, or by combustion of a fuel
such as oil or gas. The warm air heats the reflector by
convection and conduction. These hot air systems are
relatively expensive, require ducting for the heated air (and
the exhaust fumes if the air is heated by fuel combustion),
and require a blower to force the heated air into and/or
circulate the warm air around the plenum. : -
SUMMARY OF THE INYENTION
It is a primary object of the present invention to
provide an improved anti-icing and de-icing system for
reflector-type microwave antennas, which can be fabricated at
a substantially lower cost than other anti-icing and de-icing
systems for such antennas.
It is an important object of this invention to provide
such an improved anti-icing and de-icing system which can be
easily installed either during manufacture of the antenna or -
in the field.
It is a further object of this invention to provide such
an improved anti-icing and de-icing system which does not
require any fuel combustion nor exhaust ducts nor blowers and,
therefore, is extremely quiet.
Yet another object of this invention is to provide such ~ -~
an anti-icing and de-icing system which does not require any
critical or sensitive attachments to the reflector skin.
~.
3 ~
...
,. . ,!,~
2(~ 50 ~ ~
A further object of this invention is to provide such an
anti-icing and de-icing system which is highly efficient in
its consumption of energy and highly sensitive in its
distribution of energy for heating the antenna reflector.
Still another object of this invention is to provide such
an improved anti-icing and de-icing system which requires
little maintenance and service and has a long operating life.
Other objects and advantages of the invention will be
apparent from the following detailed description and the
accompanying drawings.
In accordance with the present invention, the foregoing ~ -
objectives are realized by providing an improved anti-icing
and de-icing system for reflector-type microwave antennas
having a paraboloidal reflector and an associated feed horn
for launching microwave signals onto the reflector and
receiving microwave signals from the reflector, the system
comprising a non-conductive, insulated enclosure forming an
enclosed cavity adjacent the rear side of said reflector, and :
radiant heating means within said enclosure for heating the ~,~
rear side of said reflector with radiant energy, whereby the
air in said cavity is in turn heated by heat transferred to
said air from the rear side of said reflector. `
In its preferred form the radiant heating means comprises ;;- ~-
at least one infra-red heating source, and includes a highly
reflective mirror coating disposed on the inside surface of
the insulated enclosure behind the heating source to direct -
the radiant energy emanating from the back of the heating ;
source to all regions of the paraboloidal reflector. Smaller
';
~(~1364SO
: additional areas of highly reflective mirror coating are
placed on the rear surface of the paraboloidal reflector
itself immediately in front of the heating source to divert
excess radiant energy emanating from the front of the heating
source and to disperse it to all regions of the paraboloidal
reflector.
BRIEF DESCRIPTION OF THE DRAWTNGS
FIG. l is a side elevation of a reflector-type microwave
antenna having an anti-icing and de-icing system embodying the
invention;
FIG. 2 is a vertical section taken generally along line -~
2-2 in FIG. 1 to provide a rear elevation view of the major .
portion of the antenna structure; ~
FIG. 3A and FIG. 3B are vertical sections as in FIG. 2 :
illustrating additional structural details;
FIG. 4A and FIG. 4B are enlarged sections taken generally
along line 4-4 in FIG. 3A; -;
¦ FIG. 5A and FIG. 5B are enlarged sections taken generally
along line 5-5 in FIG. 2;
FIG. 6 is an enlarged cross section of an exemplary
insulated sandwich-type sheathing for use with the antenna of
FIGS. 1-5; and
FIG. 7 is a detailed view of a vertical section taken
generally along line 6-6 in FIG. 1.
'. ' .
. .~ ' .
: '
X(~64~i0
DESCRIPTION OF THE PREFERRED EMBODIMENT
- While the invention is susceptible to various
modifications and alternative forms, certain preferred
embodiments thereof have been shown by way of example in the
drawings and will be described in detail. It should be
understood, however, that it is not intended to limit the
invention to the particular forms described, but, on the ;~
contrary, the intention is to cover all modifications,
equivalents, and alternatives falling within the spirit and
scope of the invention as deEined by the appended claims.
Turning now to the drawings and referring first to
, .:
FIG. 1, the illustrative antenna includes a paraboloidal
reflector 10 for reflecting both transmitted and received -~
microwave signals between a remote station and a feed horn 11.
The reflector 10 is preferably biaxially stretch formed,
stamped or hydro-formed from an aluminum disc or sheet, with - ;
the periphery of the disc being bent rearwardly and then
.. ...
outwardly to stiffen the reflector. The feed horn 11 is
located at the focal point F of the paraboloid which defines
the concave surface of the reflector 10. The horn ll is ~ ;
supported by an L-shaped bracket 22 disposed on the end of a
boom 20. The boom 20 is cantilevered from the bottom of a
vertical beam 13 and connected to the beam by a pair of ;~
gussets 21 bolted to the beam and the boom. As can be seen in -`
FIG. 1, the illustrative antenna is of the "offset" type
wherein the focal point F of the paraboloidal surface is
offset from the center line CL of the antenna aperture.
On the rear side of the reflector, the antenna is mounted
, : . :; ,.. ~
- 6 -`~:
;~ .,
~1 ' :.-.
X(~ 50
on a vertical post 12 by a framework which includes the curved
vertical beam 13 and a pair of side arms 14 extending
laterally from opposite sides of the beam 13. The two side
arms 14, which are preferably aluminum castings, are bolted
rigidly to opposite sides of the vertical beam 13, which is
suitably formed from rectangular aluminum tubing. The outer
ends of the two side arms 14 and the upper end of the vertical
beam 13 are fastened to the rear side of the reflector 10.
The side arms 14 also include rearwardly extending
flanges 15 for pivotally securing the antenna to a mating
mount casting 16 fastened to the top of the post 12. This -
pivotal mounting facilitates aiming of the antenna by
permitting the antenna to be readily adjusted in elevation by
means of an adjustment strut 17. When the antenna has been
adjusted to the desired elevation, the flanges 15 are locked
rigidly to the mount casting 16 by tightening a nut on a bolt ;~
which is passed through the flanges and the mount casting.
The outer ends of the two side arms 14 and the upper end
of the vertical beam 13 are fastened to the rear side of the
reflector at three spaced mounting locations, and the
fastening means at each of these three locations includes
swivel means for permitting relative tilting movement between
the frame members and the reflector surface before the
fastening means is tightened. Thus, the outer ends of the
side arms 14 and the upper end of the vertical beam 13 are - ~ -
fastened to support members 18 on the rear side of the
reflector. The details of this mounting and support structure ~ ;
are described in U.S. Patent No. 4,819,007, issued April 4,
2(~G~50 ~
1989, and assigned to the assignee of the present invention.
In accordance with one important aspect of the present
invention, the illustrative antenna includes an anti-icing and
de-icing system comprising a non-conductive insulated ;~
, . ~
enclosure forming an enclosed cavity adjacent the rear side of ~` -
the reflector, and a radiant heat source within the enclosure
for heating the rear side of the reflector with radiant
energy. The radiant heat source does not directly heat the ~ ~
air in the cavity, but rather heats the rear surface of the ~ `;
reflector. Heat is then transferred through the reflector to ~ ;~
its front surface, and throughout the reflector, by
conduction. Heat is also transferred from the rear surface of
the heated reflector into the air in the enclosed cavity by
conduction and free convection. The non-conductive enclosure
minimizes heat losses from the enclosed cavity so that the ~
warm air in the cavity provides a stable, uniform temperature ~;
over the entire area of the reflector.
In the illustrative embodiment, the non-conductive
enclosure is formed by two insulating panels 30 and 31
attached to the periphery of the reflector 10 and to each
other. The panels 30 and 31 are relatively rigid and are ;
preferably made by molding a polymeric material such as ABS
(acrylonitrile-butadiene-styrene) or a fiberglass-reinforced
polymer. As shown in FIGS. 1 and 2, each panel 30 and 31 is
of generally semi-circular shape with a contour generally
parallel to that of the rear surface of the reflector. The
outer periphery of each panel 30 and 31 terminates in an outer
flange 30a or 31a which fits flat against the outer lip of the
, ~: .
8 ``
' '
.
. Z~6~SO
~ reflector 10. To fasten the flanges 30a and 31a to the
-- reflector lip, a plurality of U-shaped clips 32 are inserted
over the outer edges of the two adjoining members and fastened
thereto by clamping screws 33, as shown in FIG. 4A.
Alternatively, a plurality of U-shaped spring clips requiring
no clamping screws can be used, or a single long flexible U-
shaped spring strip can be clamped over the entire outer
flange (see FIG. 4A).
In order to retard the loss of heat from within the
cavity to the atmosphere outside the enclosure, the inside,
surface (i.e., the concave surface) of each of the panels 30
and 31 is lined with heat insulating material 50 (see FIG.
5A). The insulating material 50 may be any suitable non-
conductive heat retardant material, such as fiberglass batts,
polystyrene foam sheets or polyurethane foam sheets.
Alternatively, the panels 30 and 31 may themselves be made of
material which is sufficiently insulating so that no
additional insulating material lining is needed.
The insulating enclosure is formed in two parts (i.e., by ;
the two panels 30 and 31) to enable it to be installed over
the supporting framework for the reflector 10. Thus, each of
the panels 30 and 31 has a slot 30b or 31b extending outwardly
from the inner edge of the panel to enable the panel to fit
over the flanges 15 which connect the side arms 14 to the
mount casting 16 (see FIG. 2). After the panels are in place,
those portions of the slots 30b and 31b not occupied by the
flanges are covered with access cover plates 36a and 36b-which
are fastened to the panels 30 and 31 by a plurality of screws
. g
``` x~ o ~:
~ 37. ~ ~
-As shown in FIG. 3A, the adjoining inner edges of the ;
panels 30 and 31 are attached to the curved vertical beam 13 ~ -
at their adjacent edges 30c and 31c which overlap each other.
A plurality of screws 38 are used to fasten the two panel
edges 30c and 31c to the curved vertical beam 13.
To provide a radiant heat source inside the cavity formed
by the insulating enclosure, the two panels 30 and 31 are
provided with infra-red heating units 42 and 43. These
heating units are mounted on the inside surfaces of the panels
30 and 31, and each unit contains at least one electrically -
powered infra-red heating lamp or metal element 44 which
extends into the cavity between the panels and the reflector
for a short distance in front of the insulation 50 (see FIG. 2
and FIG. 5A). The infra-red lamps or metal elements 44 are
thus spaced some distance away from the rear surface of the ~ ~
paraboloidal reflector, so that when the lamps or metal ~-
elements 44 are energized, they emit infra-red energy which
illuminates a broad area of the region of the rear surface of
3 20 the reflector 10 opposite the lamps.
The use of infra-red heating lamps or metal elements 44
is advantageous in that these lamps or elements emit radiant
energy over a range of directions. ~eating lamps concentrate
most of their emitted radiant energy in front of the lamp,
whereas metal elements spread their emitted radiant energy -
more evenly in all directions. In either case, but especially
in the case of the heating lamps, the relative proximity of
the rear surface of the paraboloidal reflector to the heating
.
: ~
2~6~
lamp or the metal eleme~t would result in too much radiant
energy being directed to the relatively small region of the
paraboloidal reflector surface immediately opposed to them
(see FIG. 5A). Consequently, these small regions of the
paraboloidal reflector would get too hot, and the rest of the
paraboloidal reflector would remain too cool, resulting in
inefficient use of the radiant energy supplied by the radiant
heating units 42 and 43. This would cause slower melting of
the snow and ice on the front of the reflector surface than
could be obtained optimally, and possible distortion of the
shape of the paraboloidal reflector surface due to non-uniform
temperature effects.
In accordance with a feature of this invention, these
problems are avoided, and more even distribution of the
radiant energy is ensured over all of the rear surface of the
paraboloidal reflector, by the provision of means for ;-
directing the radiating energy emanating from the heat source ;~
to all regions of the paraboloidal reflector. More
specifically, the small regions of the rear surface of the
paraboloidal reflector 10 immediately opposed to the radiant
heating units 42 and 43 are coated with a highly reflective
mirror surface material, as shown in FIG 5B and FIG. 6.
Preferably, the coating is formed of aluminum foil tape or
glossy silver paint, or other like material which is highly
reflective to infra-red radiation. The small regions of
reflective mirror surface material 51 immediately opposed to
the radiant heating units 42 and 43 reflect away most of the ~ -~
radiant energy impinging on these small regions, scattering
11 ~ .
' ~:
200G~iO
this energy to broad regions of the insulation material 50
covering the lnside surface of the enclosure panels 30 and 31.
This prevents the small regions of the paraboloidal reflector
from over-heating.
In order to further enhance the efficient distribution of
radiant energy over the entire rear surface of the
paraboloidal reflector 10, the surface of the insulation
material 50 covering the inside surfaces of the enclosure
panels 30 and 31 that face the inside of the enclosure is also
coated with highly reflective mirror surface material 52 (see ~
FIG. 5A). If the panels 30 and 31 are themselves made of -
sufficiently insulating material, so that no additional
insulating material lining 50 is required, the inside surfaces
of the panels 30 and 31 themselves are coated with highly
reflective mirror surface material 52. This can be ;
accomplished using aluminum foil tape, glossy silver paint, or
large sheets of aluminum foil which are either self-adhesive
or attached with cement.
The insulating material 50 and the highly reflective
mirror surface material 52 may also be combined into a single
item, insulated sheathing 53, which, as shown in FIG. 6, is a -
sheet type sandwich material consisting of an insulating -
material core with highly reflective mirror surface material
on one or both of its faces. One such type of combined
material which is commercially available is Celotex Tuff-R~
insulating sheathing, which consists of a semi-rigid
polyisocyanurate foam board insulation 54 with a reinforced
aluminum foil facer 55 on the printed side and a solid
12
6~
aluminum foil facer 56 on the other side. Other similar types
of such insulated sheathing 53 are also available and may be
used just as conveniently. Typically, the foam board core 54 ~ -
comprises the insulation material and the solid aluminum foil
facer side 56 comprises the highly reflective mirror surface
material. The reinforced aluminum foil facer side 55 is
placed against the inside surface of the panels 30 and 31, and
the insulated sheathing 53 is attached thereto using cement,
rivets, screws, nuts and bolts, or any other suitable fixing ~-
device, material, or combination thereof.
With the above-described arrangement, the radiant energy
emitted by the infra-red heating lamps or metal elements 44 in
directions other than towards the rear surface of the
paraboloidal reflection 10, and the radiant energy reflected
away from the paraboloidal reflector regions immediately
opposed to the infra-red heating units by the highly
reflective mirror surface material 51 coating these small
regions, impinges on the highly reflective mirror surface
material 52 coated on the inside surfaces of the insulated
panels 30, 31 which form the entire back of the enclosure.
Most of this incident radiant energy is reflected away and
scattered over the entire rear surface of the paraboloidal
reflector 10, where it is primarily absorbed, thereby
efficiently heating the paraboloidal reflector. ~ ;
The air inside the cavity, which is normal atmospheric
air, remains essentially unheated by the radiant energy
because the air is virtually transparent to the short-
wavelength infra-red radiant energy emitted by the radiant
13
............................................................................ ,',~
heating units 42 and 43. The opaque rear surface of the
paraboloidal reflector 10, however, does absorb a substantial
portion of the radiant energy incident upon it and is thereby
heated in accordance with the Stefan-Boltzman law. That
portion of the incident radiant energy which is not absorbed
by the paraboloidal reflector 10 is reflected therefrom and
impinges on the other metal components of the antenna within
the enclosure formed by the panels 30 and 31, and also upon
the highly reflective mirror surface material 52 coating the -~
inside of the panels or the insulation 50. Since all of these
surfaces are also opaque, the radiant energy is again
partially absorbed and partially reflected at these surfaces.
Virtually all of this radiant energy is reflected by the
highly reflective mirror surface material 52. ~ ~ ~
This process of absorbing and reflecting the incident ~ -
radiant energy is repeated at each successive impingement with
a surface. Since the cavity is essentially totally enclosed,
virtually no portion of the radiant energy can escape.
Accordingly, the process continues until all of the radiant
energy is absorbed by the interior surfaces of the enclosure,
mainly the rear surface of the paraboloidal reflector 10 and
the other metal components of the antenna within the
enclosure.
As the paraboloidal reflector and the other metal
components of the antenna within the enclosure become heated,
some of the radiant energy absorbed by these surfaces is re-
emitted into the cavity as longer wavelength infra-red
radiation; this radiation does heat the air in the enclosure
` ~.
14 ~-
. . ~
' ''
3~,
r!
2~6~5(~ -
to some extent. Also, the heated rear surface of the
paraboloidal reflector and the heated surfaces of the other
metal components of the antenna within the enclosure warm the
air in the cavity by natural conduction and convection, since
these surfaces tend to be at a higher temperature than the air
in the cavity. The air thus warmed circulates within the
enclosure by natural un-forced convection, and acts to further
insulate the rear surface of the paraboloidal reflector lO and
to stabilize the temperature thereof to a uniform level.
It should be noted that the use of radiation as the
:: .
primary heat transfer means for heating the paraboloidal
reflector provides a substantial advantage over the prior art
conduction and/or convection means of doing so, particularly
for antennas intended for outdoor use. In radiant heat
transfer, the rate of heat transfer between two objects
depends directly on the fourth power of the temperature ;~
difference between them, as shown by the Stefan-Boltzman law.
Thus, the radiation-based system of the type disclosed herein
is very sensitive to small temperature differences throughout
:: .'::::
the paraboloidal reflector surface. Colder areas of the
reflector surface will therefore absorb much more radiant
energy and so become heated faster than will warmer areas of
the paraboloidal reflector 10.
Since ice or snow 57 on the front surface of the
paraboloidal reflector 10 (see FIG. 5B) remains at a constant
temperature (32 F or 0 C at standard atmospheric pressure) as
it melts, absorbing its heat of fusion, the immediately ~-~
underlying paraboloidal reflector surface is also maintained
~ ~
: ` -,: -
.: '.''',~
~' ''''`'''"`.
6~SO
at that temperature until melting is complete. Areas of thefront surface of the paraboloidal reflector that are free of
ice or snow 57 become heated faster, since the air 58 in -
contact with the surface is much less heat conductive than ice
or snow and thus cannot carry away the heat as quickly.
Therefore, the colder areas of the paraboloidal reflector
immediately underlying any unmelted ice or snow will
preferentiallv absorb more radiant energy and thus more heat;
as a result, the heat is directed exactly where it is needed
without the use of any additional energy control and
distribution system. This uniform heating of the paraboloidal
reflector minimizes thermal distortion of the reflector
surface.
With the prior art conduction and/or convection type of
heating systems, such as electrical resistance heater or hot-
air heater systems, the amount of heat input is independent of
the amount of heat needed by any particular area of the
paraboloidal reflector surface. In conduction and convection
systems, the rate of heat transfer depends directly on the
first power of the temperature difference between the heat
source and the object to be heated. Therefore, these types of
systems are much less sensitive to the temperature differences
between the areas of the paraboloidal reflector still covered
by ice or snow and the areas free of ice or snow, and hence
are much less responsive to the variations in heat input
needed by the various areas of the paraboloidal reflector
surface.
,
'.
.-
16
, . . ''.
2~ 5~
These systems supply heat energy much more
indiscriminately to all areas of the paraboloidal reflector
surface, with the result that unless a complicated and
expensive control and distribution system is used, much more ;~
of the energy supplied by the heat source is wasted into the
atmosphere, rather than being used to melt ice or snow on the
front surface of the paraboloidal reflector. Consequently,
such systems are less efficient in energy consumption than the
radiant heating system invention disclosed herein. Further,
such prior-art systems must operate longer before completely ~;
melting the ice or snow on the paraboloidal reflectori this
results in higher energy usage and leads to greater risk or
longer periods of degraded antenna performance due to presence
of ice or snow on the antenna's reflecting surface. In
addition, greater thermal distortion of the paraboloidal
reflector surface is caused due to less even heating. All the
above problems are solved by using the above-described
radiation-based arrangement, in accordance with the system of
this invention.
In order to control the supply of power to the infra-red
lamps or metal elements 44, at least one electrical power and
control box 45 (see FIG. 3B) is mounted on the outside of the
panel 30 and/or 31. Alternatively, the control box may be
mounted on the outside of access cover plates 36a and/or 36b.
Within the control box, a conventional thermostat control
senses the ambient temperature and energizes the radiant
heating units 42 and 43 whenever the ambient temperature is
within a selected "icing" range, e.g., 22F to 38 F. When the
~ ' ~
17 ~ ~`
.'" : ~:
.".., ..'.
'~ 0
;o
-- ambient temperature is outside the selected "icing" range, the
thermostat control de-energizes the heating units.
Suitable radiant heating units for use with a 1.8-meter
antenna are tubular quartz heat lamps, or metal element
radiant heaters, having a total wattage of approximately 1700
watts. These heating units have an average service life of
5000 hours in normal operation for the quartz heat lamps or at
least 10,000 hours in normal operation for the metal element
radiant heaters. If desired, the heating unit life can be
extended by using a moisture sensor to supply power to the
heating units only when the humidity is above a selected level
in conjunction with an ambient temperature within the "icing"
range.
It should be noted that the anti-icing and de-icing
system of this invention has a narrow profile, which means
that it adds little to the wind load of the antenna.
The anti-icing and de-icing system of this invention may
be used on subreflectors as well as the main reflector of
microwave antennas. Subreflectors may have either concave or
convex reflecting surfaces, and main reflectors may be either
one-piece or made up of several pieces. In each of these -
cases, the panels 30 and 31, the associated insulation
material 50, the highly reflective mirror surface material 52
or insulated sheathing 53, and the highly reflective mirror
surface 51 can be molded or otherwise shaped to conform to the
shape of the particular subreflector, the one-piece main
reflector, or the main reflector pieces to be used. -
' .
. -.
18
. . `
~7V~~ S~`''-"' '~ ~ ~ ' ~ : .
~.'-',; '. ' :: . ' ' ' ' '
r ~', ~ , ,~" ~ . '~ `' ' ~ "'~''," ~' ~' ', ~'.'~. i ~
':, ,',.;.,".,... " "`,
