TECHNICAL FIELD OF THE INVENTION
This invention relates in general to microstrip antennas
and, more particularly, to a conformal channel monopole array antenna.
BACKGROUND OF THE INVENTION
Antennas with ultra-wide bandwidth have usually been too
large to consider for arrays. Examples are spirals and log-periodic slots. They
are also often inefficient because they are backed with absorber-filled cavities.
The absorber attenuates the received RF power by one-half. Still other ultra-wideband
antennas such as flared notches are very deep, resulting in unacceptable intrusion
into, or protrusion from the supporting structure. On the other hand, antennas that
are compact and amendable to conformal flush-mounting, are usually very narrowband.
Examples are cavity-backed slots and microstrip patches. Their bandwidths are typically
limited to less than 10%, or 1.1:1. Furthermore, their bandwidth decreases when
they are used in arrays.
SUMMARY OF THE INVENTION
According to an embodiment of the present invention, a
conformal channel monopole array antenna includes a base plate having a continuous
electrically conducting channel formed therein, and a substrate coupled to the base
plate. The substrate has a plurality of radiating elements formed on a first surface
thereof. Each radiating element includes a radiating portion, a feed line, and an
end load. The feed lines are configured to couple to a beamformer.
Embodiments of the invention provide a number of technical
advantages. Embodiments of the invention may include all, some, or none of these
advantages. For example, in one embodiment, a compact, low-profile antenna has moderate
bandwidth and is suitable for line-source arrays. Its gain vs. frequency performance
is comparable to spirals and log-periodic slots, but its compact size allows many
radiators to be packed together, so that they are less than one wavelength apart
at the highest frequency of operation.
Some applications may accept reduced efficiency at the
edges of the operating frequency band. For this extended-frequency coverage, it
may still be necessary that the antenna have low voltage standing wave ratio (VSWR),
even at the band edges, to prevent oscillations on the line connecting the antenna
to the electronic circuitry. For these situations, an antenna according to one embodiment
of the invention allows a convenient method for including a resistive end load for
The present invention achieves ultra-widebandwith (up to
10:1) with moderately high efficiency while remaining very shallow (approximately
.05 wavelengths at the lowest frequency).
Other technical advantages are readily apparent to one
skilled in the art from the following figures, descriptions, and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
DETAILED DESCRIPTION OF THE INVENTION
- FIGURE 1 is an exploded perspective view of stripline construction of a line-source
array including a radome according to one embodiment of the present invention;
- FIGURE 2 is an exploded perspective view of microstrip construction of a line
source array according to another embodiment of the present invention;
- FIGURE 3 is an exploded perspective view of microstrip construction of a line
source array conforming to a curved surface according to another embodiment of the
- FIGURE 4 is an exploded perspective view of microstrip construction of a line
source array using split feeds according to another embodiment of the present invention;
- FIGURE 5 is an exploded perspective view of microstrip construction of a ring
array according to another embodiment of the present invention.
Embodiments of the present invention and some of their
advantages are best understood by referring to FIGURES 1 through 5 of the drawings,
like numerals being used for like and corresponding parts of the various drawings.
FIGURE 1 is an exploded perspective view of a conformal
channel monopole array antenna 100 according to one embodiment of the present invention.
In the illustrated embodiment, antenna 100 includes a base plate 102 having a continuous
channel 104 formed therein, a dielectric material 106, a substrate 108 comprised
of a first layer 109 having a plurality of radiating elements 110 formed thereon
and a second layer 111 having a pair of ground planes 112 formed thereon, and a
radome 114. The present invention contemplates more, less, or different components
than those illustrated in FIGURE 1. In addition, other embodiments of antenna 100
are illustrated below in conjunction with FIGURES 2 through 5.
Base plate 102 may be any suitable size and shape and may
be formed from any suitable material. For example, the material for base plate 102
may be any suitable metal or any suitable metal coating 118 on a non-metallic material,
such as plastic. Continuous channel 104 is an electrically conducting channel formed
along the length of base plate 102. The continuous nature of channel 104 extends
the bandwidth of antenna 100 by increasing the electrical volume therein. Although
channel 104 is illustrated in FIGURE 1 as having generally parallel and upright
walls 116, walls 116 may be sloped or may have other suitable configurations. The
depth of channel 104 is determined approximately by the following formula: 0.2*&lgr;o/sqrt(&egr;r),
where &lgr;o equals the center frequency wavelength and &egr;r
equals the relative permittivity of the dielectric material 106.
Dielectric material 106, which is optional for antenna
100, is illustrated in FIGURE 1 as being disposed within channel 104 and substantially
conforming to the shape of channel 104; however, alternate shapes that only partially
fill the channel are also contemplated by the present invention. In one embodiment,
dielectric material 106 is a material with low loss at microwave frequencies.
Substrate 108 is formed from first layer 109 and second
layer 111, which both may have any suitable size and shape and may be formed from
any suitable material, for example circuit card material may be utilized.
As described above, first layer 109 includes a plurality
of radiating elements 110 formed therein. Radiating elements 110 may be formed within
first layer 109 using any suitable fabrication method, such as photolithography.
Any suitable number of radiating elements may be formed on first layer 109 and they
may be spaced apart any suitable distance 125, usually less than one wavelength
at the highest frequency of operation for antenna 100. Each radiating element 110
comprises a radiating portion 120, a feed line 122, and an optional resistive end
Radiating portion 120 may have any suitable shape; however,
in the illustrated embodiment, the shape of radiating portion 120 is rectangular.
Other suitable shapes, such as triangular and elliptical may be utilized for radiating
portion 120. The function of radiating portion 120 is to radiate signals received
through feed line 122.
Feed line 122 may have any suitable shape and may couple
to radiating portion 120 in any suitable manner. Feed line 122 may receive the incoming
signals from any suitable source. For example, feed line 122 may receive signals
perpendicular through base plate 102 or may receive signals from components that
are formed in first layer 109, such as amplifiers and phase shifters.
Resistive end load 124 may also be any suitable shape and
may be coupled to radiating portion 120 in any suitable manner. Resistive end loads
124 generally function to absorb the ringing caused by the residual energy of antenna
100. A suitable choice of resistor provides low voltage standing wave ratio (VSWR)
over the operating bandwidth for antenna 100. In one embodiment, resistivity of
resistive end load 124 is chosen to minimize VSWR while maximizing the radiating
efficiency. Typically, resistance should be larger than the characteristic impedance
of feed line 122. However, if VSWR and bandwidth requirements allow, it may have
As described above, second layer 111 includes ground planes
112, which may be formed from any suitable material and formed in second layer 111
using any suitable method. Ground planes 112 may include a plurality of plated vias
126 and 127. Plated vias 126 are also formed in first layer 109 in order to couple
radiating elements 110 to continuous channel 104.
Radome 114 may be any suitable size and shape and may be
formed from any suitable material that is transparent to radio frequencies.
FIGURE 2 is an exploded perspective view of an antenna
200 according to another embodiment of the present invention. Antenna 200 is similar
to antenna 100 in FIGURE 1, except that it uses a single substrate layer instead
of two. Antenna 200 includes a substrate 208 having a plurality of radiating elements
210 formed therein. Radiating elements 210 include a radiating portion 220, a feed
line 222, and a resistive end load 224.
Radiating portion 220 functions in a similar manner to
radiating portion 120 in FIGURE 1. In one embodiment, radiating portion 220 is triangular
in shape; however, other suitable shapes for radiating portion 220 are contemplated
by the present invention.
Radiating portion 220 couples to feed line 222, which may
have any suitable length and any suitable shape. Feed line 222 includes a contact
via 228 that couples to a respective coaxial cable 232 in order to receive signals.
Resistive end load 224 may also have any suitable size and shape and may couple
to radiating portion 220 in any suitable manner. Resistive end load 224 functions
in a similar manner to resistive end load 124 FIGURE 1; however, in the illustrated
embodiment, resistive end load 224 includes a grounding pin 230 that couples to
base plate 202.
In order to couple coaxial cables 232 to respective feed
lines 222, a plurality of apertures 234 may be formed in base plate 202. Similar
to base plate 102 of FIGURE 1, base plate 202 includes a continuous channel 204
that is electrically conducting. Antenna 200 may also have a dielectric material
206 within channel 204 that is similar to dielectric material 106 of FIGURE 1. A
radome (not illustrated) may also be associated with antenna 200.
FIGURE 3 is an exploded perspective view of an antenna
300 according to another embodiment of the present invention. Antenna 300 is similar
to antenna 200 illustrated in FIGURE 2; however, antenna 300 in the embodiment illustrated
in FIGURE 3 includes components that are curved in order to conform to a curved
shape, such as an aircraft fuselage. Antenna 300 may include stripline radiating
elements, such as those shown in FIGURE 1, in lieu of the microstrip radiating elements
FIGURE 4 is an exploded perspective view of an antenna
400 according to another embodiment of the present invention. Antenna 400 is similar
to antenna 200 illustrated in FIGURE 2, except that in the embodiment illustrated
in FIGURE 4, antenna 400 includes a plurality of power dividers 402 each coupled
to respective pairs of feed lines 404. Each feed line 404 is associated with a radiating
element 401 also having a radiating portion 406 and a resistive end load 408. Each
power divider 402 has a contact portion 403 that couples to a respective coaxial
cable 409 for receiving signals.
Power dividers 402 function to split the feed power in
half, which leads to two separate radiating elements 401. This pairing up of radiating
elements 401 may allow a closer spacing for radiating elements 401, which prevents
grating lobes at higher frequencies for antenna 400. Although triangularly shaped
radiating portions 406 are illustrated in FIGURE 4, radiating portions 406 may have
any suitable shape.
FIGURE 5 is an exploded perspective view of an antenna
500 according to another embodiment of the present invention. In one embodiment,
antenna 500 is particularly suitable for direction-finding applications and may
be used in place of spiral antennas. In the illustrated embodiment, antenna 500
includes an annular channel 502 formed in a base plate 501, which may be any suitable
size and shape. Channel 502 is a continuous electrically conducting channel that
is disposed beneath a plurality of radiating elements 504 each radially extending
from a center 505 of a substrate 506. Radiating elements 504 are similar to radiating
elements of FIGURE 2 and include a feed line 508, a radiating portion 510, and a
resistive end load 512. Feed lines 508 also include a contact via 509 that couples
to a respective coaxial cable 514 for receiving signals therefrom.
Thus, embodiments of the invention provide antennas that
are compact, wideband, arrayable, efficient, and broad-beam. Some embodiments of
the antennas described above in conjunction with FIGURES 1 through 5 are low profile
for ease of installation on aircraft and missiles, and have bandwidths that exceed
a 5:1 ratio.
Although embodiments of the invention and some of their
advantages are described in detail, a person skilled in the art could make various
alterations, additions, and omissions without departing from the spirit and scope
of the present invention as defined by the appended claims.