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Description  |
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FIELD OF INVENTION
This invention relates to antennas and more particularly to helical
antennas.
DESCRIPTION OF THE PRIOR ART
One aspect of the operation of a helical antenna is the directivity of the
antenna. The antenna either radiates electromagnetic waves to its
surrounding medium or receives the waves therefrom with an angular
directivity that may be represented by what is known as a far field
pattern.
The antenna operates in an axial radiation mode when it has a far field
pattern with a main lobe representative of a main beam that is directed
coaxially with the axis of the antenna. The operation in the axial mode
occurs when the antenna guides the wave along the axis with a phase
velocity equal to the phase velocity of the wave in the surrounding
medium. It should be appreciated that the antenna may be made to operate
in a broadside radiation mode.
Another aspect of the operation of the antenna is the polarization of a far
field, associated with the wave, that propagates from the antenna. The
polarization may be measured by a linear test probe, such as a dipole,
that is disposed at a selected distance from the antenna in a plane
orthogonal to the direction of the propagation. When the measured field is
constant with a rotation of the probe, the far field is referred to as
being "circularly polarized."
Usually, a maximum field and a minimum field are measured during the
rotation of the probe. When the maximum is measured with the probe at a
given orientation, the minimum is usually measured with the probe
orthogonal to the given orientation. The ratio of the maximum to the
minimum is called an "axial ratio." The axial ratio is an indication of
the difference of the polarization of the far field from circular
polarization.
Although the prior art is replete with helical antennas, usually the main
beam is not axially symmetrical, the axial ratio deviates substantially
from unity and the far field pattern has undesirably large side lobes.
Additionally, the side lobes in one azimuthal plane are usually different
from the side lobes in another azimuthal plane. Moreover, the main beam is
usually not directed coaxially with the axis of the antenna.
In an antenna system of a communication satellite, for example, the antenna
has to operate over a wide frequency range. Moreover, during the operation
of the antenna, either a plurality of transmitters or a plurality of
receivers are connected to the antenna. The outputs of the transmitters
may be connected via a multiplexer network comprised of a plurality of
filters. However, the multiplexer is bulky, heavy and lossy.
Alternatively, the transmitters may be respectively connected to a
plurality of helical antennas that are in close proximity to each other.
Although the plurality of antennas obviates the bulk, weight and losses of
the multiplexer, the proximity of the antennas causes a coupling
therebetween which results in loss of directivity of the antennas.
A maximization of directivity and a minimization of bulk, weight, and loss
are critically important in the communication satellite. Therefore, it is
desirable that the antenna system include decoupled helical antennas in
close proximity to each other.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, first and second coaxial
helical antennas are comprised of resonating elements tuned to first and
second frequency pass bands, respectively.
According to another aspect of the present invention, a composite antenna
is comprised of a plurality of untuned coaxial helical antennas with a
known angular displacement therebetween. Excitation of the antennas with
respective signals that have a phase relationship corresponding to the
angular displacement causes an additive combining of electromagnetic waves
radiated by the antennas, whereby the transmitted power of the antennas is
additively combined.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a plan view of a printed circuit assembly in accordance with a
first form of the embodiment of the present invention;
FIGS. 2A and 2B are plan views of dipoles in the assembly of FIG. 1;
FIG. 3 is a perspective view of a hollow cylinder upon which the printed
circuit of FIG. 1 is wound;
FIG. 4 is a perspective view of the first embodiment of the present
invention;
FIG. 5 is a plan view of an assembly which may be used as an alternative to
the assembly of FIG. 1;
FIG. 6 is a fragmentary section of FIG. 5 taken along the line 6--6;
FIG. 7 is a side view, partly in section, of an antenna assembly wherein a
dielectric rod is maintained;
FIG. 8 is a side view of a second form of the embodiment of the present
invention;
FIG. 9 is a side view of a helical winding used for side lobe suppression
of antennas in the second form of the embodiment; and
FIG. 10 is a side view of a third form of the embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In a first form of the embodiment of the present invention, a plurality of
tuned helical antennas are coaxially wound upon a hollow cylinder, whereby
the antennas are colocated. When a helical antenna is tuned, it is
suitable for either radiating or receiving an electromagnetic wave within
a pass band of frequencies. Therefore, the plurality of tuned helical
antennas may, for example, be used to provide frequency diplexing.
As shown in FIGS. 1-5, a printed circuit assembly 10 (FIG. 1) includes a
helical antenna 12 made from a plurality of similar thin metal dipoles
(12A, FIG. 2A) of the type that are used in a microwave strip line. The
dipoles (12A) of antenna 12 are resonating elements that are coupled to
each other in a manner similar to end-fire elements of a microstrip
filter.
Antenna 12 is disposed upon a surface of a pliable, electrically insulating
substrate 14 that has the shape of a rectangular sheet. Antenna 12 may be
disposed by the use of any of the techniques well known in the printed
circuit art or in any other suitable manner. The dipoles of antenna 12
define a portion of a first spiral of Archemides on substrate 14, whereby
antenna 12 defines a helix when subassembly 10 is bent over a cylindrical
surface suitably aligned with printed circuit 10. Because the dipoles of
antenna 12 define the portion of the first spiral of Archemides, the pitch
of the defined helix is a linear function of displacement along the axis
of the defined helix. Accordingly, antenna 12 has a low pitch end 12L,
where the pitch of antenna 12 is least, and a high pitch end 12H where the
pitch of antenna 12 is greatest.
As known to those familiar with microstrip lines, antenna 12 has a first
pass band that is determined by the length and the spacing between the
dipoles of antenna 12. An exemplary dipole 12A (FIG. 2A) of antenna 12 has
an end-to-end length 16 that is slightly longer than an ideal end-to-end
dipole length of one half of a first wavelength associated with the center
frequency of the first pass band. Length 16 is slightly longer than the
ideal dipole length to compensate for mutual coupling and finite width of
the dipoles of antenna 12.
In this form of the embodiment, printed circuit 10 is wound around a hollow
cylinder 18 (FIG. 3) made from an insulating material. Moreover, the axis
of cylinder 18 is in the direction of an arrow 20 (FIG. 1) that is
perpendicular to an edge 22 of substrate 14.
FIG. 4 is an illustration of the first form of the embodiment wherein
cylinder 18 has an outer circumference approximately equal to the first
wavelength. Additionally, substrate 14 has a width 23 approximately equal
to six circumferences of cylinder 18. Accordingly, when substrate 14 is
wound around cylinder 18, layers 24-29 form an antenna assembly 30 wherein
antenna 12 defines a first helix with six turns. Moreover, when
subassembly 30 is formed, corners 15 and 17 of substrate 14 (FIG. 1) are
on layers 24 and 29, respectively.
Printed circuit 10 (FIG. 1) additionally includes an antenna 32 made from
thin metal dipoles that are disposed upon substrate 14 to define thereon a
portion of a second spiral of Archemides. An exemplary dipole 32A (FIG.
2B) of antenna 32 has an end-to-end length 33 (analogous to length 16)
that is slightly longer than an ideal end-to-end dipole length of one half
of a second wavelength associated with the center frequency of the second
pass band.
When substrate 14 is wound around cylinder 18 (FIG. 4), antenna 32 defines
a second helix with six turns, where the pitch of the second helix is a
linear function of displacement along the axis thereof. Accordingly,
antenna 32 has a low pitch end 32L, where the pitch of antenna 32 is
least, and a high pitch end 32H where the pitch of antenna 32 is greatest.
It should be understood that the distance from the outer circumference of
cylinder 18 to layer 29 is less than one tenth of the outer circumference
of cylinder 18 whereby the first and second helixes are of substantially
constant diameter.
As known in the art, the gain of a helical antenna varies approximately as
the square root of its axial length. It has been discovered that when the
axial lengths of the first and second helixes are 3.04 times the first and
second wavelengths, respectively and the diameter of cylinder 18 is
approximately 0.33 times either the first or the second wavelengths,
antennas 12 and 32 each have a gain of approximately 13.5 db.
It should be understood that because antenna 12 has the first pass band and
antenna 32 has the second pass band, antennas 12 and 32 can either radiate
or receive electromagnetic waves only within the first and second pass
bands, respectively. Since antenna 12 can neither radiate nor receive the
waves within the second pass band and antenna 32 can neither radiate nor
receive the waves within the first pass band, antennas 12 and 32 are
electromagnetically decoupled from each other.
Assembly 30 (FIG. 4) has an end 34 that abuts a grounded metal plate 36
which has the shape of a flat disc. Ends 12L and 32L are connected to
respective coaxial feed lines (not shown) that pass through plate 36. A
power transfer either to or from antennas 12 and 32 is provided via the
feed lines. As well known to those skilled in the art, the transfer of
power is maximum when antennas 12 and 32 provide an impedance match
between the feed lines and free space. Typically, the feed lines and free
space have impedances of 50 ohms and 377 ohms, respectively.
As known in the art, the impedance of a helical antenna determines the
phase velocity of an electromagnetic wave that passes therethrough.
Moreover, the impedance of the helical antenna is determined, in part, by
the pitch of the helical antenna. Since antennas 12 and 32 have a pitch
that is a linear function of axial displacement, when ends 12L and 32L are
connected to the feed lines, antennas 12 and 32 have impedances of
approximately 50 ohms proximal to the feed lines and approximately 377
ohms distal therefrom. In other words, antennas 12 and 32 are conceptually
similar to transformers.
As known to those skilled in the art, in the absence of ground plate 36,
antennas 12 and 32 have far field patterns with substantial side lobes due
to radiation caused by currents on the outer conductor of the coaxial feed
lines. In an alternative embodiment, ground plate 36 may have a curved
surface that focuses the waves that are radiated by antennas 12 and 32.
As shown in FIGS. 5 and 6, as an alternative to the dipoles (12A, 32A of
FIGS. 2A and 2B), antennas 40 and 42 are comprised of thin rectangular
metal strips (41, 43, etc.) with rounded ends. The strips are disposed
lengthwise upon substrate 14 to define the portions of the spirals of
Archemides described in connection with antennas 12 and 32. Moreover, the
strips are disposed upon both surfaces of substrate 14 with the strips
(41) on one surface partially overlapping the strips (43) on the other
surface.
The lengths of the strips comprising antennas 40 and 42 are equal to one
half of the first and second wavelengths, respectively. The length and
spacing of the strips and the overlap cause antennas 40 and 42 to have the
first and second pass bands, respectively.
It is well known that the electromagnetic wave has a phase velocity through
a medium in proportion to the dielectric constant of the medium. As
explained hereinafter, the dielectric constant of the medium is altered to
provide an impedance match between a feed line and free space.
As shown in FIG. 7, exemplary antennas 12E and 32E are included in an
assembly 30E having an end 34E that abuts ground plate 36, assembly 30E
being constructed in a manner similar to assembly 30 described
hereinbefore. Moreover, assembly 30E is wound around cylinder 18 wherein a
rod 44 is fixedly maintained near end 34E. Rod 44 may either be solid or
hollow.
Rod 44 is formed of two portions; a cylindrical portion 46 that is made
from a material that has a first dielectric constant and a tapered
cylindrical portion 48 that has a second dielectric constant which is less
than the first dielectric constant. One end of portion 46 is connected to
the end of portion 48 that has the larger diameter. The first and second
dielectric constants and the tapering of portion 48 causes the interior of
cylinder 18 to have its highest dielectric constant near end 34E.
In this form of the embodiment, antennas 12E and 32E have the same pitch as
antennas 12 and 32, respectively. For reasons explained hereinbefore, rod
44 causes the impedance match between a feed line and free space when the
axial lengths of antennas 12E and 32E is less than the axial lengths of
antennas 12 and 32.
As shown in FIG. 8, a second form of the embodiment of the present
invention includes a first helical antenna 60 and a second helical antenna
62 that are comprised of solid conductors. Therefore, antennas 60 and 62
are untuned. Antennas 60 and 62 are included in an assembly that has an
end which abuts ground plate 36 in a manner similar to assembly 30
described hereinbefore.
Antennas 60 and 62 are coaxially wound around cylinder 18 with a 180 degree
angular displacement therebetween. Antennas 60 and 62 may be disposed upon
a pliable substrate, as described in connection with the first form of the
embodiment (FIG. 4) or constructed in any other suitable manner.
The circumference of antennas 60 and 62 approximately equals a midband
wavelength associated with a midfrequency of an operational range of
frequencies at which antennas 60 and 62 either transmit or receive
electromagnetic waves. Preferably, antennas 60 and 62 have a pitch (P)
that is a linear function of axial displacement along antennas 60 and 62
for reasons given in connection with the first form of the embodiment.
Since the circumference of antennas 60 and 62 is approximately equal to the
midband wavelength, there is an approximate phase change of 360 degrees in
a signal that passes through one turn of either antenna 60 or antenna 62.
Because of the 360 degrees phase change and the 180 degrees angular
displacement, when antennas 60 and 62 are excited with first and second
signals, respectively, that have a phase difference of 180 degrees, waves
that are transmitted by antennas 60 and 62 are additive. Therefore,
antennas 60 and 62 are a composite antenna that combines power from two
sources which provide signals that have a phase difference of 180 degrees.
Correspondingly, when a circularly polarized electromagnetic wave is
received by antennas 60 and 62, feedlines connected thereto are provided
to the signals that have the 180 degree phase difference.
In a similar manner, a composite antenna for combining power may be
constructed from three or more coaxially wound helical antennas that have
an angular displacement therebetween substantially defined by a
relationship which is given as:
.theta. = 360/N (1)
where
.theta. is the angular displacement between the helical antennas; and
N equals the number of helical antennas.
Usually, current through a helical antenna is circumferentially
asymmetrical because of standing waves along the antenna. The
circumferential asymmetry is an indication that the antenna does not match
its feed line to free space. It has been learned experimentally that the
greater the number of helical antennas in a composite antenna, the more
the current is circumferentially symmetrical. Moreover, by increasing the
number of the helical antennas, the gain of the composite antenna is
increased and the side lobe levels of the far field pattern of the
composite antenna is reduced. However, little increase of the gain or
reduction of side lobe levels is achieved by including more than four
helical antennas in the composite antenna.
It should be understood that one alternative embodiment may include a
plurality of tuned antennas, similar to antenna 12, that are coaxially
wound with an angular separation in accordance with the displacement
relationship (1). Another alternative embodiment may include coaxial first
and second groups of tuned antennas, similar to antennas 12 and 32,
respectively. The antennas of each of the groups are wound with the
angular displacement in accordance with the relationship (1). The antennas
of the alternative embodiments provide high gain, axial symmetry, an axial
ratio that substantially equals unity and have far field patterns with low
side lobe levels.
A modification of the composite antenna of FIG. 8 is shown in FIG. 9,
wherein a helical conductor 64 is wound around antennas 60 and 62.
Conductor 64 is supported by insulator rods 66 and 68 which are connected
to ground plate 36. Additionally, conductor 64 is connected to ground in
any suitable manner. Conductor 64 has approximately the same pitch and one
half of the length of antennas 60 and 62. It has been demonstrated
experimentally that conductor 64 may be positioned along the axis of
cylinder 18 to cause the composite antenna to have reduced side lobe
levels.
As shown in FIG. 10, in a third form of the embodiment, helical antennas,
analogous to antennas 60 and 62 (FIG. 8) described hereinbefore, are
comprised of a cylindrical insulator 70 that has a metal clad outer
surface 71 which is etched to provide helical gaps 72 and 74. Insulator 70
is an assembly that has an end which abuts ground plate 36 in a manner
similar to assembly 30 described hereinbefore. It should be appreciated
that clad surface 71 provides a path for current with low ohmic loss
because it covers most of the surface of insulator 70.
Preferably, gaps 72 and 74 have the tapered pitch referred to hereinbefore,
whereby clad surface 71 defines a pair of helical antennas with the
tapered pitch. The antennas defined by the clad surface may be connected
to feed lines (not shown) as described hereinbefore. Because of the
tapered pitch of gaps 72 and 74, the defined antennas match the impedance
of the feed lines to free space.
The tapered pitch of gaps 72 and 74 cause the defined antennas to be
relatively wide at the end thereof distal from the feed lines. Because of
the large relative width, a longitudinal component of current may flow
through the defined antennas in the direction of the axis thereof. The
longitudinal component is undesirable because it does not cause a
radiation of a circularly polarized electromagnetic wave. The longitudinal
component of current is substantially eliminated by an inclusion of gaps
76 and 78 in the defined antennas near the ends thereof distal from the
feed lines.
It should be understood that in an alternative embodiment, helical antennas
may be provided where a turn of the helical antenna is acircular;
elliptical, for example.
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