Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
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1~ This invention relates to new and useful improvements
in the use of microwaves and in the provision-of a novel micro-
wave horn reflector or applicator.
One of the most severe obstacles that face the con-
struction industry in cold climates is the long cold winter
months. During these months the industry tends to slow down
and concentrate on construction mainly above ground relying
on sites excavated during the summer. The high cost of winter
construction results in unemployment. Thus the design, test-
ing and implementation of new techniques and equipment that
permit all year round operation would naturally be of great
interes~ ~o those involved in ~he construction industry in
Car.ad~ and other similar locations of the world.
I~ is known that microwave power has already been
employed in the construction industry for demolition of concrete
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and highway repair through heating of trapped moisture in the
case of concrete and curing of polymer-concrete patches in the
case of road and pavement repairs. The advantages of the micro- -
wave technique in these applications have been the minimum dis-
ruption of traffic or time to complete the construction due to
the shorter time required relative to conventional techniques
lusing fire or heavy mechanical equipment). Other advantages
are the possible reduction in the cost and noise or pollution
levels. However, one of the principal disadvantages of using
microwave power for these applications is the leakage normally
present which may be dange~ous not only to the operator but to
others standing nearby.
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To date no serious effort has yet been made to aid
the construction industry in the rapid thawing and de-icing of
permafrost and frozen soils, de-icing of street intersections,
sidewalks and airport runways, fast drying of conveyorized wet
sand and other construction materials, noise-free demolition of
concrete pipes, sewers, ~ridges, ~uildings, roads, rocks, de-
icing of windows, windshields, aircraft wings or rapid curing
of concrete and bricks and sensors for simultaneous monitoring
of the curing process~
SUMMARY OF THE INVENTION
This invention comprises a microwave power alternative
to he commonly used mechanical excavation technique for thawing ~`
of severely frozen soils during the winter and illustrates the
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specific advantages in terms of reduction in constxuction time,
cost, noise and environmental pollution which were verified in
conjunction with construction industry personnel. One aspect
of this invention is to provide a horn reflector for use with
microwave energy comprising in combination a horn, a parabolic
reflector operatively connected by one end thereof to said horn
and a corrugated flange assembly connected to the other end of
said parabolic reflector and extending upon each side thereof.
A further aspect of the invention is to provide a de- ~;
vice of the character herewithin described which is simple in
construction, economical in operation and otherwise well suited
to the purpose for which it is designed.
With the foregoing in view and other such advantages
as will become apparent to those skilled in the art to which
this invention relates as this specification proceeds, my inven-
tion consists essentially in the arrangement and construction of
parts all as hereinafter more particularly described, reference
being had to the accompanying drawings in which:
DESCRIPTION OF THE DRAWINGS
Figure 1 is an isometric view of the horn reflector or
applicator.
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Figure 2 is a longitudinal section of Figure 1.
Figure 3 is a cross sectional view along the line
3-3 of Figure 2.
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Figure 4 is a graph showing the power density level
5with and without the corrugated flange .
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Figure 5 is a schematic side elevation of the device
set up for evaluating the performance of the applicator with
clay samples.
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In the drawings like characters of reference indicate
corresponding parts in the different figures.
DETAI~ED DESCRIPTION
The first requirement of a microwave applicator for `
practical use in the construction industry is to possess a
reasonably flat field distribution in the aperture with minimum
15power towards the edges of the radiator. The applicator con- -
fi~uration must therefore allow for maximum and uniform power
in the area of interest as well as minimum VSWR at the frequency
of operation particularly since the applicator aperture is very
closely separated from the ground surface. In order to meet
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this specification, a horn reflector type of radiator is used
which is a combination of a sectoral electromagnetic horn and
a re~lector which is a sector of a paraboloid of revolution.
Since the center of the wave guide is the focal point of the
parabola and since this radiator is essentially an offset para-
boloidal antenna, very little of the energy incident on the re-
flector is reflected back into the feed to produce an impedance
mismatch. Also, due to the shielding effect of the horn, the
far side and back lobes are very small when radiating in free
space. These desirable characteristics, together with high
aperture efficiency, low side and back lobe levels, make the
horn reflector quite attractive for this particular type of
application.
Although the electric field will maintain its charac-
teristic amplitude distribution of the TElo mode as it proceedsalong the H-plane sectoral horn of the applicator, a way must
be found to control the side lobe levels excited by diffraction
effects at the aperture as well as surface waves along the
ground surface. This second requirement of minimum leakage into
the surrounding area can be overcome by incorporating some fea-
tures of the corrugated horn into the design of the applicator. ~;~
This horn has a property of concentrating energy in the main
beam, while maintaining very low back lobe levels, high effi-
iency, and almost monotonic amplitude distribution of the elec-
tric field in the aperture. Its other useful properties include
nearly axially symmetric radiation patterns for a square or con- -
~ical shape, exceptionally low ~SWR and transmission losses.
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In order to combine the attractive properties of both
types of structures (i.e. the horn reflector and the corrugated
flang~ a horn reflector type of applicator coupled to a modified
corrugated ~ is provided. The corrugations are placed on a
180 flange extension of the horn reflector. The corrugations
are essentially metallic strips soldered to the flange which
extended out at 90 to each of the parallel sides of the horn
reflector. The spacing between each two strips is a A/4 slot ,
while the width of each strip is also A /4 in accordance with
well kno~n theory. The material used is preferably copper
sheets of 1/16" thickness while the feed waveguide section was
the standard WR430 waveguide with appropriate co-ax to wave-
guide adapter connection to the magnetron.
Proceeding therefore to describe the application in
detail, reference should first be made to Figure 5 in which
reference character 10 illustrates schematically a magnetron,
connected to a tuning section 11 and having power meters 12
connected thereto by means of cables 13. r
The applicator is collectively designated 14 and is
shown in detail in Figures 1, 2 and 3. A rectangular cross
section waveguide 15 is connected to the tuning section by means
of flange 16 and terminates in the horn 17 which includes the
upper outwardly fiared wall 18 and the lower wall 19 and it
will be observed that the walls 18 and 19 extend at 120 from
the transverse plane of the end of the waveguide as shown in
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Figure 2. However, t ~ angle can be varied depending upon
design parameters.
A parabolic reflector 20 extends from the upper end
21 of the upper wall 18 of the horn and is provided with spaced
and parallel side walls 22 which extend downwardly from the
side edges 23 of the parabolic reflector and enclose same. The
parabolic reflector terminates with a short vertical wall 24
and the wall 19 of the horn also terminates in a shoxt vertical
wall 25 spaced and parallel to wall 24 with the lower edges
lying in the same plane as clearly illustrated.
A corrugated flange assembly collectively designated
26 extends outwardly on each side of the assembly and includes
substantially rectangular planar panels 27 secured to the side
walls 22 of the parabolic reflector at a position spaced above
the lower edges 28 of these side walls.
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A plurality of spaced and parallel longitudinally
extending strips 29 extend downwardly at right angles from the
unaerside of the plates 27 and lie spaced and parallel with
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th~ portions 30 of the side walls below the plates 27. These
strips lie parallel to the longitudinal axis of the
applicator. ~ ,
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Low and high power performance tests have been carried
out with this design as follows:
Low Power Performance Tests
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In order to evaluate the performance of the applicator,
a series of low power tests were carried out. The first of
these tests was a measurement of the VSWR vs. frequency which
was carried out using a network analyzer (Hewlett-Packard type
HP8410S). The results for the frequency range 2410-2480MHz
are given in the following Table 1.
FREQUENCY MHz VSWR
2410 3.95
2420 3.57
2430 3.01
2440 2.32
2450 1.88 `
2460 1.55
2470 1.25
2480 1.24
VSWR vs. frequency for the applicator
Although the applicator performed satisfactorily at ~-
2450 MHz, the best operating frequency was found to be 2480 MHz
which was most probably due to deviations in production tolerances
during rabrication. However, an operating frequency of between
2400 MHz and 2500 MHz is possible.
lU44331
In order to evaluate the radiation characteristics of
the applicator, an x-y plot of the aperture field rather than
the radiation pattern in the far zone was carried out using a suit-
able generator. The detector was an open-ended WR430 waveguide
operating at 2450 MHz and the probing was carried out at a distance
of ~/2 from the aperture where the ground surface is approximately
located in practice. A simple detection technique was adopted
and the power density distribution was recorded in the trans-
verse and longitudinal planes with the maximum power density at
the mid-poi~t of the aperture set at a re~erence level o~ O dB.
The results are given in the following Tables 2 and 3. ;
TABLE 2
x(inch) Power Density x(inch) Power Density
Level Level
O O
~S ~0-5
1.0 -0.4 -1.0 -0.8 ~
1.5 -2.4 -1.5 -2.8 ~ :
2,0 -4.8 -2.0 -7.1
2.5 -10.6 -2.5 -14.0
2a 3.0 -17.0 -3.0 -21
3.5 -23.0 -3.5 -24 . ~ : .
4.0 -27 -4.0 -26
4.5 -29 -4.5 -28
5.0 -35 -5.0 ~30 :
5.5 -40 -5.5 ~33
-6.0 -38 :~
Power density distribution in the transverse plane
measured with respect to the mid-point of the aperture (x = 0).
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TABI,E 3 : .
y(inch) Power Density y(inch) Power Density
Level Level
0 -0.8
1 -0.4 -1 -2
2 -0 -2 -4
3 -0.8 -3 -6
4 -2 -4 -9
-8.8 -5 -12 ;
6 -23 -6 -14
7 -28 -7 -20
-8 -28
Power density distribution in the longitudinal plane
measured with respect to the mid-point of the aperture (y ~ 0). -
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A graphical plot of the power density in the transverse
direction is shown in Figure 2 where a comparison with theapplicator without the corrugated flange is also shown.
Although the corrugated flange introduces a consider-
able improvement in the power density distribution (being
relatively flat near the center and very low near the edges) as
evident from Figure 2, it was found that the maximum power den-
sity location is not at the bisector of the aperture. This is
due to fabrication tolerances in part but~ainly due to the finite
size of the probe aperture and the geometry of the applicator
aperture which reflects different contributions of the edges.
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~igh Power Performance Tests
Since the applicator design appeared to be satisfactory
at low power, various experiments were carried out to evaluate
its performance at high power levels. The first test involved
leakage measurements with the applicator connected to a Philips
magnetron through the tuhin~ section and a transmission/
reflection power meter. The radiator was surrounded by absorbing
material (styrofoam with carbon particles imbedded) and with
the magnetron setting controlled through a variac. The maximum
leakage level readings were recorded by a radiation monitor
(Narda model B86~3) and the results are shown in Table 4.
Power input Maximum leakage -
(Watts) level (mw/cm2)
100 discernable
200 discernable
15 300 0 43
400 0.43
500 0.96 -
600 1.30
700 2.10
20 ~oo 3.40
900 6.30
1 000 ,' 1 0
No load leakage performance of the applicator.
Although the results recorded at no load seem unfavour-
able, the maximum leakage level was found to be 4 mw/cm2 nearthe flange edges when the applicator was loaded with a clay
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sample and properly matched at the input. Since the power at-
tenuation through free space is proportional to the square of
the inverse distance, it seemed that safe experiments could be
performed with this applicator as designed.
The remaining experiments were done on dark clay soil
samples of different sizes which represent soils most commonly
found on construction sites. The temperature of each sample
was assumed to be the ambient outdoor temperature at the time ~-
of the experiment. The samples were all placed directly under ~ -
the applicator at a separation distance of one inch and the
measuring equipment (as illustrated schematically in Figure 3)
was adjusted to affect minimum reflected power as recorded by
the reflection meter. Each sample was irradiated at two minute
intervals and penetration tests were performed using a sharp
pointer (calibrated thin copper rod) at the end of each inter-
val. Leakage power was constantly monitored from the sides of
the applicator using a radiation monitor and the results are
given in Table 5 as follows:
TAsLE 5
TEST #1 Sample size: 14"x14"x4"
Temperature: -10F. -
Power Transmitted: 800 watts
Irradiation Penetration Reflected Max. leakage
Period (min.) Depth (inches) power (Watts) power (mw/cm2)
2 0.25 40 1.3
4 .25-.5 30 1.3
6 .75-1 20 1.3
8 1.25-1.5 20 1.3
1.5-1.75 20 1.3
12 1.75-2 20 1.3
14 2 20 1.3
16 2 20 1.3
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Thawing and leakage performance of the applicator.
TEST #2 Sample size: 14"x14"x2 1/2"
Temperature: -10F.
Power Transmitted: 800 watts -
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Irradiation Penetration Reflected ~ax. leakage2 :
Period (min.) Depth (inches) power (watts) power (mw/cm )
2 0.25 40 1.3
4 .25-.5 30 1.3 ~
6 .75-1 20 1.3 ~- ;
8 1.25-1.5 20n 1.3
1.5-1.75 20 1.3
12 2-2.25 20 1.3
14 2.5 20 1.3 ~ -
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Thawing and leakage performance of the applicator.
TEST #3 Sample size: 8"x14"x5"
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Temperature: -10F.
Power Transmitted: 800 watts
Irradiation Penetration Reflected Max. leakage2
Period (min.) Depth (inches) power (watts) power (mw/cm )
.'~'
2 0.25 40 1.3
4 .25-.5 30 1.3
6 .75-1 20 1.3
8 1.25-1.5 20 1.3
1.75-2 20 1.3
12 2-2.25 20 1.3
14 2.25 20 1.3
16 2.25 20 1.3
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Thawing and leakage performance of the applicator.
TEST #4 Sample size: 14"x14"x4" -
Temperature: 15F.
Power Transmitted: 1000 watts
Irradiation Penetration Reflected ~ax. Leakage2
Period (min.~ Depth (inches) power (watts) power (mw/cm )
2 .5 20 3.4
4 .72-1.25 10 3.4 ~ -
6 1.5-2 10 3.4
8 2.5-3 10 3.4
3.25-3.5 10 3.4
12 4 10 3.4 -
Thawing and leakage performance of the applicator.
TEST #5 Sample size: 14"x7"x5" ~ -
Temperature: 15F.
Power Transmitted: 1000 watts
Irradiation Penetration Reflected Max. Leakage -
Period (min.) Depth (inches) power (watts) power (mw/cm2)
2 .5 20 3.4
4 .75-1.25 10 3.4
6 1.5-2 10 3.4 ~`~
8 2.5-3 10 3.4
3.25-3.5 10 3.4
12 3.75 10 3.4
14 4 10 3.4
16 4.25 10 3.4
18 4.25 10 3.4
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Thawing and leakage performance of the applicator.
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TEST #6 Sample size: 14"x7"x5" - ~
Temperature: 20F. ~ -
Power Transmitted: 1000 watts -
Irradiation Penetration Reflected Max. Leakage2
Period (min.) Depth (inches) power (watts) power (mw/cm ) -~
2 .5 .5 20 3.4 . !
4 1-1.5 10 3.4 i
6 1.75-2.25 10 3.4 ;~
8 2.5-3 10 3.4
3.5-3.75 10 3.4 -
12 4-4.5 10 3.4 ~,
14 5 10 3.4
Thawing and leakage performance of the applicator.
TEST #7 Sample size: same as in Test #5
Temperature: same as in Test #5
Power Input: variable
Power Input Max. Leak~ge Power
(watts) (mw/cm )
outside range of
radiation monitor ~ -
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Leakage performance of the loaded applicator at high
power without the corrugated flange.
It should be noted that the first tests were per-
formed at the same initial sample temperature and, since the
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sample was static, the thawing area was restricted to approx-
imately 5" x 2.5" with the rest of the surface area remaining
frozen. In fact it is evident from test number three that the
effective area of thawing and penetration depth remained the ,;
S same as test numbers one and two indicating that the lateral ~ -
size of the sample beyond the thawing area does not play an
effective part in the static thawing process. The effect of
the sample thickness also does not appear to play any part as
evident from the results of tests number one and two. '~
It was also observed from tests 1 - 3 that there
appears to be an acceleration effect for penetration depth ~' :
ranging between 0.25 and 1.75 inches. This was verified for
thick samples where the penetration appeared to stop after about
2 to 2.5 inches. In all cases the moisture escaping from the -,-,
samples was observed in terms of water droplets which condensed ~
on the face of the applicator. ~,-
The results ~r tests 4 and 5 also indicate that the,
lateral size of the sample does not have any effect on the depth
of penetration and that, due to the higher initial sample tem-
peratures, the thawing area was observed to be about 6 x 3 inches.
Examination of these results indicates that there isa period of accelerating action when microwave power is applied
to frozen clay samples. However, with colder sample temperatures,
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the thawing effect seems to stop at a certain penetration level.
This is most probably due to an equilibrium set up in the heat :
transfer process while most of the energy is used to drive out
the moisture content of the sample. The loss of moisture can
be observed as the water vapour rises above the surface of the
sample under test and water droplets collect on the surface of
the applicator. ~;
The tests also indicate that there is a roughly even
distribution of power density in an area of approximately 5 x
2.5 inches which in fact is equivalent to the physical aperture
of the horn reflector plus the first slot of the corrugated
flange. The microwave power seems to concentrate in this region
leaving the remainder of the sample ~most completely frozen
from the surface down. This demonstrates one feature of the
applicator when used in a static sense but is obviously of
academic interest since applicators of this type would be mov- ~
ing over frozen ground in a practical construction application. ~ ~ -
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Although the flanged applicator presents a considerable
improvement in lowering the leakage level relative to the un- `
flanged applicator, as evident from Table 2, it is believed that
further reduction in the leakage power could be achieved by a
more refined design of the slot dimensions and spacings as well
as the flare angle of the flange.
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It will therefore be appreciated that an improved horn
reflector antenna has been provided which, by incorporating a
corrugated flange, makes the antenna suitable for effective and
safe thawing of frozen soils. Further analysis and experiment-
ation to improve the design of the flange taking the dielectricproperties of the soil and the separation distance from the appli-
cator into account is undoubtedly necessary particularly if micro-
wave power is combined with other forms of energy (e.g. hot air)
and power switching and profiling is employed.
It is believed that this applicator will have various
other applications for outdoor heating with microwave power, For
example, one such application could be the de-icing of street
pavements or airport runways and the like, on which the formation
of ice creates a considerahle hazard to both pedestrians and vehi-
cles and aircraft in severe winter months. Since ice is a low
loss material compared with the concrete pavement, the impinging
microwave power on the ice surface would in this case be mainly
dissipated in the concrete. This would heat up the upper surface
of the concrete resulting in a thin layer of melted water at the
concrete-ice interface. Once this occurred, there would be an
accelerating effect of melting due to the relatively much higher
loss property of water, thus allowlng easy scraping of the ice us-
ing mechanical means. Also of importance is the use of the device
for noise free microwave demolition of concrete pipes, sewers,
bridges, building9, roads, sidewalks and the breaking of rocks.
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The device is well suited for use in microwave de-icing
of windows, windshields and aircraft wings.
Anot~er use of the device enables microwaves to be used
for rapid c~ring of concrete and bricks and sensors for simultane-
ous monitoring of the curing process.
Finally, the data indicates that tne application of micro-
wave power for thawing of soil is favourable and can be used for
further applications in the constr~ction industry, The accelerated ~-
rate of thawing by the present applicator not only proves to be
safe, but also provides a great saving of time particularly when
dealing with frozen soils in foundations and ditches excavated
across major roads for construction and maintenance in the winter -
time, ~
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Since various modifications can be made in my invention
as hereinabove described, and many apparently widely different em-
bodiments of same made within the spirit and scope of the claims `^
without departing from such spirit and scope, it is intended that
all matter contained in the accompanying specification shall be
interpreted as illustrative only and not in a limiting sense.
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