Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a horn antenna, applications
therefor and more particularly to a horn antenna having adjustable
direct:ional characteristics.
~lorn antennas are commonly used in microwave applica-
tions because they provide a convenient and simple way in which
to direct and propagate electrical energy at high frequencies.
Common examples may be seen in radar systems, microwave communi-
cation links, and dielectric heating systems to name but a few
applications. In each of these cases, microwave energy may be
transmitted from a horn antenna and propagated through a predeter-
mined medium in a particular pattern that is selected to suit
the application. In radar and communications link applications,
the horn antenna forms part of a greater antenna system and is
often used to direct energy from a focus to the surface of a
parabolic reflecting surface in order that the energy be radiated
substantially as a parallel beam. On the other hand, a horn
antenna may be used in dielectric heating applications to form
a diverging beam which is required to cover a maximum volume,
preferably with a simple energy radiating apparatus.
Fog conditions generally constitute a serious threat
to safe navigation. Various attempts have therefore been made in
the past in an effort to solve the problem of fog dispersal.
These attempts include evaporation by heating the fog with the
exhaust from jet engines, microwave energy and chemical reagents.
Other methods include enlargement of drops for precipitation by
chemical, electrical and acoustic means, mechanical impaction,
mechanical air circulation, e.g. by helicopters, exposure to
lasers and infrared radiation, seeding with hygroscopic agents
and ultrasonic techniques. All of these methods have been applied
to warm (radiation and advection), cold and ice fogs.
The foregoing attemps have met with only limited success
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due to low efficiency, complexity, high cost or excessive time
delay in dispersing the fog to obtain adequate visibility.
In particular, a microwave heating technique described
in U.S. patent no. 3,606,153, which issued on September 20, 1971,
to R.M.G. Boucher, has several drawbacks. Firstly, the technique
applies only to very low density fog, i.e., of the order of 7 gms
of water per 100 cubic m. Also, with regard to passenger ex-
posure to microwave energy and its attendant health hazard, Boucher
indicates that an energy output of 10 mw/cm is acceptable and,
moreover, that the aircraft will act as a shield so that the
passengers will not even be subjected to this radiation level.
According to latest criteria, radiation exposure to humans should
not exceed 1 mw/cm2. Unless the aircraft had no windows to admit
radiation, it is believed that radiation exposure using Boucher's
proposal would exceed the aforementioned currently acceptable
level.
Apart from radiation hazards, it would appear that the
greatest drawback of Boucher's proposal is the very large number
of radiators and reflectors which are required. Not only does
the system become expensive to construct, but it becomes more
technically complicated in view of the extensive transmission
system required to couple the microwave energy from a source to
each radiating element.
A provision of the present invention is an embodiment
thereof and a method of operation which substantially overcomes
the difficulties inherent in known fog clearing systems.
The invention also provides an embodiment having an
adjustable aperture and which exhibits utility in radar surveil-
lance and guidance systems.
The disadvantages and problems of the prior art may be
substantially overcome and the foregoing provisions achieved by
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recourse to the invention which relates to a horn antenna. The
antenna comprises a radiating element having an input end adapted
to be coupled to a source of microwave energy and conductive
sidewalls diverging longitudinally therefrom and terminating in
an output aperture. A parasitic element is provided which has an
apex and conductive sidewalls diverging longitudinally therefrom
and terminating in a base. The parasitic element is coaxially
aligned with the radiating element and is spaced therefrom to
define an annular aperture through which the energy is propagated. ,
The invention also relates to a method for clearing fog
which comprises the step of downwardly circulating the fog over
an area to be cleared so that the lower layers of the fog are
warmed by friction, compression and downwardly moving fog to a
condition where the relative humidity drops below 100 percent and
the fog is sufficiently dissipated.
The invention will now be more particularly described
with reference to embodiments thereof shown, by way of example,
in the accompanying drawings wherein:
Fig. 1 is a diagrammatic perspective view of one embodi-
ment of the invention;
Fig. 2 is a side elevation view of another embodimentof the invention;
Fig. 3 is a sectional view taken along the lines 3-3
of Fig. 2; and
Figs. 4(a), (b) and (c), are graphs indicating energy
radiation patterns of E and H planes in the embodiment illustrated
in Fig. 2.
It is well-known that condensation always occurs in
natural air whose dew point (Td) even slightly exceeds the temper-
ature (T). Two processes act to prevent supersaturation and fogformation. First, in the case of a cold surface, the turbulent
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eddies which transfer heat downward may also transfer water
vapour downward. Second, the hygroscopic nature of the earth's
surfac,e results in a decrease in the dew point below the temper-
ature even at the surface. In spite of these factors, several
processes help to bring about supersaturation and fog formation
such as mixing of saturated air at different temperatures and
vapour densities, radiative flux divergence might cause T to
drop below Td, adiabatic cooling associated with upward motion
or falling pressure, condensation on hygroscopic nuclei below
100% humidities (e.g. smog), etc.
The apparatus and method of the present invention are
most applicable to a radiation fog where the humidity may remain
at 100~ up to a typical height of 1000 feet and beyond that
drops below 100% (i.e. a knee in the curve is at 1000 feet above
the ground). The corresponding values of Td and T may typically
remain at 40F and then drop around the knee to typically 37F
and 38F, respectively (see standard tephigrams). For such type
of fog it would be desirable to create a downward circulation of
the fog above the runway so that the lower layers of the fog are
warmed by friction, compression and downward moving fog to the
point where the humidity drops to say 99~. The method therefore
is to heat the fog in a pattern defining an upwardly diverging
conical surface about the airport so that the upward circulation
of fog along the conical surface results in a downward flow of
fog from the upper layers towards the tip of the cone. For most
radiation fogs, a mere evaporation of 1 gm/m3 of water droplets
along the conical surface would be sufficient to create the
desirable fog circulation. Although the total power required for
this fog density by Boucher's method is 14 megawatts at 2.45 GHz,
it is estimated that 0.5 megawatts with a single antenna would be
adequate by the present method due to the nature of fog circulation
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created. Two embodiments of a horn antenna suitable for the
method of the present invention are shown in Figs. 1 and 2.
The embodiment in Fig. 1 is a horn antenna 10 which is
illustrated in a diagrammatic perspective view in order to show
its pr:incipal structural features as well as a radiation pattern
12 of microwave energy which is important to the operation of the
invention. It will be seen that the antenna 10 comprises a pair
of concentric cones that are coaxially aligned with an inner cone
fitting inside the outer cone. The outer cone comprises a radi-
ating element 13 having an input end that is adapted to be coupled
to a source of microwave energy (not shown) via a rectangular-to-
circ~lar waveguide adapter 14 of which only a portion is shown.
The inner cone is a parasitic element 15 which is coaxially a-
ligned with the element 13 and is spaced therefrom to provide the
configuration shown. Both elements are defined by conductive
sidewalls which may be constructed of either solid sheet stock
or open stock such as wire mesh screens. The angle ~ of the
elements 13 and 15 is the same so that the sidewalls are always
substantially parallel.
The cone angle B of the elements 13and 15 is shown
fixed in the illustration of Fig. 1. It is, however, to be
understood that the angle may be variable, with adjustment being
effected to any desired angle using the umbrella principle. The
cone angle ~ is determined by means of a fog density sensor (not
shown) which is located at the end of a directional coupler in
a feed waveguide (not shown) so as to monitor reflected power
which controls servo equipment (not shown) that is designed to
position the surfaces of the antenna elements in real time. In
all cases, the cone angle ~ increases during the defogging process.
For supersaturated fogs,the parallel conical surfaces
of the elements 13 and 15 may be driven from a source of radio
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frequency energy operating at a suitable frequency depending
on the actual situation. In this case, the length of each
of the elements 13 and 15 must be an appreciable fraction
of the wave length. For example, at 13 MHz this would be
approximately 10 m.
Microwave energy is transmitted from an annular
output aperture 18 of the antenna 10 in the outwardly di-
verging conical radiation pattern 12. The energy heats the
fog laden air dielectrically and causes it to rise upwardly
between the elements 13 and 15 in the direction of the arrows
and into the atmosphere along an extension of their surfaces
which comprises the pattern 12. This results in a downward
axial flow of fog laden air towards the apex of the element 15.
The power output of the antenna is maintained at a
low value in order to minimize the danger of exposure to
humans.
In airport applications, a single antenna of the
type illustrated in Fig. 1 may be located off the centre of
a runway and will be sufficient to dissipate fog dispersed
within the pattern 12.
Since the antenna need only be erected as required,
the hazard of collisions with aircraft is minimized. The
hazard is removed altogether if the antenna 10 is mounted
with its output aperture end flush with the ground. In
this event, the antenna may be raised when required or
maintained flush with the ground by containing the antenna
in a pit and closing the opening of the pit with a cover that
is substantially transparent to the microwave energy. In
this connection, it will be noted that a base 19 of the
element 15 is in a plane defined by the output aperture of
the element 13 which facilitates a flushmounting arrangement.
Another embodiment of the invention is shown as a horn
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antenna 11 in Fig. 2. It will be noted that the structure of
the ant:enna 11 is generally similar to that of the antenna 10,
a principal difference being the manner in which the parasitic
element: 15 is mounted. Unlike the antenna 10 wherein the element
15 is contained wholly within the element 13, a corresponding
parasitic element 16 in Eig. 2 extends outwardly of a radiating
element 17. It will be observed that the element 16 is coaxially
aligned with the element 17. Furthermore, the apex of the ele-
ment 16 extends in the direction of energy propagation and a base
19' thereof is disposed substantially in a plane defined by the
output aperture of the element 17. It should be understood that
whereas the base 19 of the element 15 in Fig. 1 may be either
open or closed, the corresponding base 19' of the element 16 in
Fig. 2 is open.
The spacing arrangement of the elements 16 and 17 is
such that an annular aperture is defined through which the micro-
wave energy is propagated. This is similar for the antenna 10
even though the position of the parasitic element 15 differs
from its counterpart in the antenna 11. A common feature, however,
is to be noted in this regard, that the base of each parasitic
element is disposed substantially in a plane defined by the output
aperture of the radiating element.
The antenna 11 has been operated in the x-band for
experimental purposes (i.e. 8.4-12 GHz) and was excited in the
TEll mode, since it was fed from a rectangular waveguide 20
operating in the TElo mode followed by a rectangular-to-circular
waveguide adapter 21. The waveguide 20 is thus coupled to an
input end of the adapter 21 and is mechanically connected thereto
by means of a flange assembly 22. The output end of the adapter
21 is mechanically connected via a flange assembly 23 to the
input end of the conical horn 17.
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The angle ~ in both elements 16 and 17 may be adjusted
as described for the antenna 10. However, in the embodiment il-
lustrated, the angle is fixed at 30.
Means for mounting the element 16 in the manner illus-
trated is provided by way of a support 24, a cross-sectional
view of which is illustrated in Fig. 3. The support 24 in the
embodiments of Figs. 2 and 3 is fabricated of acrylic plastic,
but any other suitable material may be used provided that it is
substantially transparent to the microwave energy.
An enlarged sectional view of the support 24 is shown
in Fig. 3 in order to more accurately depict its mountir.g features.
For example, it will be noted that the support 24 includes an
annular recess 25 having one tapered sidewall 25' that corresponds
to the angle of the sidewalls of the element 17. The recess 25
thus fits over the peripheral edge of the element 17 which
defines the output aperture and is held in place by mechanical
friction. In a similar fashion, a passage 26 is formed coaxially
with the support 24 and is provided with tapered sidewalls 27
that correspond to the angle of the sidewalls of the element 16.
The aforenoted structure of the support 24 is partic-
ularly convenient since it not only serves to mount the parasitic
element but also acts as a cover to prevent entry of foreign
objects into the antenna 11. Although shown only for the antenna
11, the same principles of construction apply to a corresponding
support for the antenna 10.
In tests conducted using the antenna 11, two different
sizes of the parasitic element 16 were employed. Each parasitic
element was cone shaped as illustrated with an open base 19'
disposed substantially in the plane defined by the output aper-
ture of the element 17. A small size element 16 was used having
a base diameter of 1.25 inches and an overall length of 2.48
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inches. The larger element 16 had a base diameter of 1.75 inches
and an overall length of 3.38 inches. The element 17 was used
with both elementq 16 and had an aperture diameter of 2.75 inches
to which was fitted a separate support 24 for each of the small
and large elements 16.
It was previously described that the antenna 11 was
excited in the TEll mode. Since this mode is not symmetrical in
the azimuth and elevation planes, different radiation patterns
were noted in both E and H planes for different frequencies and
sizes of the parasitic element.
The salient feature of the antenna 11 is that at a
certain frequency within the passband, a main lobe observed along
the axis in the absence of the element 16 (i.e. for a conical
horn antenna) practically disappears or is significantly reduced
in intensity allowing at the same time the first side lobe (one
on each side of the main lobe) to significantly increase in both
the azimuth and elevation planes. This leads to the radiation
pattern 12 as described for the antenna 10 which is desirable in
fog clearing operations.
At other frequencies, an opposite condition occurs,
i.e., the main lobe is revived to even a slightly higher value
than for the conical horn antenna, thus increasing antenna gain,
while the side lobe practically disappears. Since frequency
switching or sweeping is known in the art, the appearance and
disappearance of the main lobe with frequency variations (with
the opposite behaviour for the first side lobe) is a feature of
significant interest in radar surveillance and guidance systems.
The graphs shown in Figs. 4(a), (b) and (c) are plots
of radiation patterns recorded in both E and H planes at differ-
ent frequencies and sizes of the element 16. It will be observed
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that each graph includes four curves. Each one of the curves
characterizes the forward gain of the antenna 11 with respect
to an angular distribution of radiated energy expressed in degrees
of azirnuth angle.
The four curves of each graph are shown in pairs of
two, one pair having an apparent greater amplitude than the other.
The greater amplitude curves represent radiation in the H plane,
whereas the lower amplitude curves represent radiation in the E
plane. It is to be understood that there is no intention to
convey a relationship between the H and E plane curves, the
graphs being depicted in this manner merely as a matter of con-
venience. Accordinsly, the greater peak amplitude value for
each pair of curves represents substantially the same power
output in all cases.
The plots in Fig. 4(a) were taken at a frequency of
9.6 GHz with and without the element 17. The curves 30 and
31 were plotted in the H plane, the curve 31 resulting when the
element 16 was placed in cooperative relationship with the ele-
ment 17 as indicated in Fig. 2. Curves 32 and 33 on the other
hand represent plots in the E plane. Some change in azimuth
angle was noted with and without the element 17. The curve 33
resulted when using the element 17.
In Fig. 4(b), the plots were taken with and without
a large element 17, having dimensions as previously noted, and
at a frequency of 11.51 G~z. Curves 34 and 35 are H plane curves,
curve 35 being obtained when the element 16 was used in cooperative
relationship with the element 17. The curves 36 and 37 are E
plane curves, the curve 37 obtaining with the element 16 in
position as noted.
The plots of Fig. 4(c) were obtained with and without
a large element 16 while operating the antenna 11 at a frequency
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of 8.26 GHz. An improvement in antenna gain is noted in this
figure by the H curves 38 and 39, the curve 38 being obtained
when the element 16 was used. A corresponding increase in gain
may be! seen in the E plane curves 40 and 41, the curve 40
resulting when the element 16 was used.
In view of the plots shown in Fig. 4, it is apparent
that in some applications it would be desirable to vary not only
the frequency but also the size of the parasitic element, but
with the base of the parasitic element always remaining in the
plane of the output aperture of the element 17. With regard to
the latter variable, two possibilities are proposed for its
achievement but are not illustrated.
The first is to use a pre-formed rubber cone coated
with conducting material for the parasitic element 16 and to
vary its size by inflating or deflating it from a gas bottle.
The base 19' of the element 16 is maintained coaxially with the
aperture of the element 17 by means of thin radial rubber lines
under tension which may be employed to join the base to the
peripheral edge defining the output aperture. The second approach
relies on the aforementioned umbrella principle. In this case,
the cone is also made of rubber coated with conducting paint and
is held by spokes which are opened or folded by a crank connected
to mechanical elements similar to those of an umbrella.
Although the antennas 10 and 11 have been described as
generally conical structures, it should be noted that tetrahedral
structures may also be used to obtain similar results.
It willbe apparent to those killed in the art that
the preceding description of the embodiments of the invention
may be substantially varied to meet specialized requirements
without departing from the spirit and scope of the invention
disclosed. The foregoing embodiments are therefore not to be
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taken as limiting but rather as exemplary structures of the
invention which is defined by the claims.
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