A coplanar stripline antenna including a layer of dielectric material supporting a lower ground plane of conductive material on one side of the layer of dielectric material, a patch of conductive material on the other side of the layer of dielectric material, and an upper ground plane of conductive material on the other side of the layer of dielectric material and with the upper ground plane substantially surrounding and spaced from the patch of conductive material. Electrical signals are fed to the antenna between the patch of conductive material and the upper ground plane. A number of patches of conductive material may each be surrounded by the upper ground plane to form an antenna array and with the patches interconnected by coplanar stripline fed at a point equidistant from each patch.

The present invention is directed to a coplanar stripline atenna that is 
formed from printed circuit board construction techniques. The structure 
of the antenna is very thin and because of this thinness the antenna may 
be conformal so as to follow the shape of the surface to which the antenna 
is mounted. For example, the antenna may be mounted on the outside surface 
of an airplane and may conform to the outside surface of the airplane. 
Because the antenna is very thin and conforms to the outside surface, the 
antenna does not present any significant resistance to air and does not 
significantly disturb the aerodynamics of the airplane. 
There are basically four different types of electrically thin microwave 
transmission lines that can be formed from printed circuit board 
construction. These are generally stripline, microstrip line, slot line 
and coplanar stripline. Stripline or triplate line is the earliest and 
probably most widely used configuration and includes an inner conducting 
strip between two outer ground planes. Microstrip line is a single 
conducting strip spaced from a ground plane. Slot line is formed by a slot 
in the first plane spaced from a second ground plane. Finally, coplanar 
stripline is a conducting strip spaced from a surrounding ground plane and 
with the strip and surrounding ground plane located in the same plane. 
Coplanar stripline may also use a second ground plane spaced from the 
first ground plane. 
In the prior art these different forms of microwave transmission lines have 
been modified in structure so as to produce conformal antennas of 
different configurations. For example, stripline has been used to produce 
a slot antenna which when properly designed and constructed has provided 
desirable electrical performance. Microstrip line has also been used to 
produce antennas but only at a reduction in electrical performance. Slot 
line antennas have not been extensively studied but this type of structure 
has considerable electrical problems. 
Coplanar stripline has not been used extensively and is not normally used 
to provide an antenna structure. There has been one proposal for a 
log-periodic coplanar stripline antenna, but this antenna structure did 
not include a lower ground plane as part of the antenna structure. 
The present invention is a coplanar stripline antenna which includes a 
lower ground plane closely spaced from the upper ground plane and which 
has several advantages over the conventional stripline and microstrip 
antennas of the prior art. Specifically, the coplanar stripline antenna of 
the present invention has low losses, low fringing, low mutual coupling, 
high gain for a given size, good variation in achievable impedance levels, 
and low likelihood of launching trapped waves in the dielectric slab. In 
addition to the above, the coplanar stripline antenna of the present 
invention is mechanically simpler than stripline antennas and is no more 
mechanically complicated than microstrip or slot line antennas. 
The coplanar stripline antenna of the present invention includes a 
conducting strip spaced from but in the same plane as an upper ground 
plane. Spaced from the conducting strip and the upper ground plane is a 
second lower ground plane which is in the fringing field between the 
conducting strip and the upper ground plane. The coplanar stripline 
antenna of the present invention may be excited with electrical signals 
between the conducting strip and the upper ground plane. This type of 
excitation results in better confinement of the E-field lines which in 
turn results in less fringing and also reduces the E-field intensity in 
the dielectric medium. 
The coplanar stripline antenna of the present invention may also be formed 
as an array of antenna elements and with the individual antenna elements 
fed with electrical signals by coplanar striplines. The feed point may be 
located at a position equidistant from each separate antenna element and 
with the signals coupled through a coaxial connector located on the bottom 
ground plane and with the signals fed to the conducting strip. The lower 
and upper ground planes are coupled to each other and to the coaxial 
connector.

FIGS. 1 and 1(a) illustrate stripline or triplate line which is the 
earliest and probably the most widely used configuration for printed 
circuit type transmission line. A very large number of microwave 
components are currently produced in the stripline form. The advantages of 
stripline include its excellent containment of fields, its wide range of 
impedance levels, and its predictable electrical characteristics. 
Stripline is normally energized between the center conducting strip and 
the outer two ground planes. Since both ground planes are equally 
important in defining the transmission path, the two ground planes must be 
kept at the same electrical potential for proper performance. Because of 
this, shorting pins or wires are normally installed between the ground 
planes but this use of shorting pins increases the cost and also reduces 
the reliability since the integrity of the shorting pins is important. 
The stripline transmission line of FIGS. 1 and 1(a) includes a center 
conducting strip 10 equidistant between two ground planes 12 and 14 and 
with the layers of dielectric material 16 and 18 insulating the central 
conductor 10 from the ground planes. Shorting pins 20 are shown extending 
between the ground planes so as to ensure that the ground planes are at 
the same electrical potential. 
FIG. 2 is a perspective view of a slot antenna formed from stripline. 
Specifically, the stripline includes a center conducting strip 30 and with 
ground planes 32 and 34 spaced from the center conducting strip by layers 
of dielectric material 36 and 38. A slot 40 is formed in one of the ground 
planes so that the structure of FIG. 2 forms a slot antenna. 
In order to ensure proper performance of the antenna of FIG. 2, a ring of 
shorting pins 42 must be used around the slot so as to define a cavity 
backing. The resulting cavity is a high Q structure and is quite sensitive 
to spacing between the ground planes and to the electrical integrity of 
the shorting pins 42. For example, over wide temperature ranges, stripline 
antennas are well known for erratic behavior unless careful mechanical 
design has gone into the structure. 
Even though stripline antennas have many desirable electrical properties, 
they tend to be more costly to manufacture than single board structures 
because stripline antennas require a greater number of fabrication 
operations. Stripline antennas are also thicker in cross-section than 
single board structures. The stripline antennas are difficult to build and 
since, in order to obtain proper results, the registration between the two 
boards must be very accurate. Also, as indicated above, it is necessary to 
use shorting pins and this is an additional procedure which adds to the 
cost of the antenna. It would be desirable to provide for an antenna 
structure which has or even exceeds the desirable electrical properties of 
stripline antennas but with the elimination of the mechanical problems of 
stripline antennas described above. 
FIGS. 3 and 3(a) are a topview and a cross-sectional view of a microstrip 
line which is second in use to stripline for thin transmission line 
structures. The main advantage of microstrip line is simplicity since it 
consists of a single conducting strip 50 spaced from a single ground plane 
52 by a layer of dielectric material 54. 
FIG. 4 is a perspective view of a microstrip line antenna structure which 
consists of a rectangular area or patch 60 which extends from a center 
conducting strip 62. The patch 60 is spaced from a ground plane 64 by a 
layer of dielectric 66. The problems associated with all components formed 
with microstrip structure are related to the fact that the microstrip line 
structure is a semi-open system. Consequently, feed line radiation and 
cross-coupling or mutual interaction occurs between nearby transmission 
lines and antenna patches. Even through microstrip antennas have 
considerable problems, these antennas have found applications in systems 
where moderate electrical performance can be tolerated. As indicted above, 
these problems which relate to cross-polarization and coupling between 
adjacent elements makes for a less efficient and less desirable antenna 
than stripline antennas. 
FIGS. 5 and 5(a ) are a top view and a cross-sectional view of a slot line 
transmission line which includes a pair of upper conducting planes 70 and 
72 spaced by a slot 74. The planes 70 and 72 are supported on a layer of 
dielectric material 76. The specific structure shown in FIGS. 5 and 5(a) 
includes a lower ground plane 78, but normally a lower ground plane is not 
present in a slot line transmisin line. In order to be consistent with the 
previous descriptions, such a lower ground plane is shown. 
The slot line of FIG. 5 is generally energized by connecting a coaxial line 
at right angles across the gap or slot 74 so as to produce balanced 
excitation. No current is fed to the lower ground plane 78. Although the 
slot line shown in FIGS. 5 and 5(a) and the microstrip line shown in FIGS. 
3 and 3(a) appear to be duals, microstrip and slot lines are not duals 
because they are not energized in a dual manner. 
Slot line stuctures have several problems when used in antenna systems. The 
slot line will radiate a substantial power of its length approaches 
one-half wavelength. In addition, slot lines do not propagate a TEM mode. 
Thus, the field in the slot is elliptically polarized and this complicates 
the design of power dividers and raises the level of cross-polarized 
energy producted by a slot line antenna. An additional problem with slot 
line is difficulty in providing an effective transition from slot line to 
a 50 ohm coaxial cable on the lower ground plane. The connecting of a 
coaxial line at right angles across the gap as indicated above would be 
very difficult ot realize without projecting above the surface of the 
upper ground plane. For these reasons, the slot line structure would not 
be recommended for conformal antenna applications. 
As shown in FIG. 6, a slot line antenna would include an upper ground plane 
80 supported on a layer of dielectric material 88 and with a rectangular 
antenna slot 82. The antenna slot is fed by a slot line 84. A lower ground 
plane 86 is included but normally, as indicated above, slot line 
transmission line does not include a lower ground plane. 
FIGS. 7 and 7(a) are a top view and a cross-sectional view of a coplanar 
strip line which includes a center conducting strip 100 and with upper 
ground planes 102 and 104 spaced from the center conducting strip 100. The 
upper ground planes and conducting strip are supported by a layer of 
dielectric material 106. The structure shown in FIGS. 7 and 7(a) includes 
a lower ground plane 108. Normally, in coplanar stripline no lower ground 
plane is used. If there is such a lower ground plane, it is spaced very 
far from the center conducting strip 100 and the upper ground plane 
members 102 and 104 so as not to form a substantial part of the 
transmission line electrical system. 
In the antenna of the present invention, the lower ground plane is spaced 
close to the center conducting strip 100 and the upper ground planes 102 
and 104 so as to be within the fringing field and form a part of the 
electrical system. Specifically, the use of the lower ground plane helps 
to create a unidirectional antenna. If the lower ground plane were not 
present, the antenna would be bidirectional and this is not desirable for 
a conformal antenna since such antennas should be unidirectional since 
they are often used on the outside surface of an airplane. 
In addition, it is desirable to include the lower ground plane as opposed 
to using the outside surface of the airplane itself as the ground plane, 
since the outside surface of the airplane would not be at the same 
controlled distance from the other elements in the antenna, and could lead 
to varying electrical characteristics. The use of the ground plane close 
to the other elements so as to form a unidirectional antenna also provides 
that the electrical characteristics of the antenna of the present 
invention are reproducible. 
FIG. 8 is a perspective view of a coplanar stripline antenna in accordance 
with the teachings of the present invention. In FIG. 8 an upper ground 
plane 110 substantially surrounds but is spaced by a slot 116 from a 
rectangular conducting patch 112 to form the antenna. A center conducting 
strip 114, in combination with the surrounding portions of the upper 
ground plane 110, form a coplanar stripline transmission line which is 
used to feed electrical signals to the antenna 112. The electrical path of 
the slot 116 extending around the patch 112 is approximately one 
wavelength of the resonant frequency radiated by the antenna. 
The various conducting elements including the upper ground plane 110, the 
antenna patch 112 and the center conductor strip 114 are all supported on 
a layer of dielectric material 118. In addition, a lower ground plane 120 
is also supported by the layer of dielectric material 118. As indicated 
above, the lower ground plane 120 is close to the upper ground plane so 
that it is substantially within the fringing field between the antenna 
patch 112 and the outer surrounding upper ground plane 110. In this way, 
the antenna produces a unidirectional radiation pattern which is desirable 
for the conformal antenna structure of the present invention. In 
additiion, the lower ground plane 120 by being close to the upper ground 
plane and forming part of the electrical system provides for a uniform 
structure which is reproducible. Also, the skin of the airplane, if the 
antenna is attached to an airplane, does not affect the characteristics of 
the antenna since the lower ground plane 120 forms the lower surface with 
which the remaining portions of the antenna structure coact. 
FIG. 8 also shows the use of a dielectric material such as a paint 122 
which may be used in the slot 116 so as to tune the resonant frequency of 
the antenna. For example, this dielectric paint may contain titanium 
dioxide. The dielectric paint will tune the resonant frequency of the 
antenna since the E-fields are concentrated in the gap 116 and any 
dielectric material will interact with the E-fields to affect the resonant 
frequency of the antenna. 
As indicated above, the log-periodic antenna structure had been previously 
realized in coplanar stripline. However, this antenna structure was 
considerably different and did not include the lower ground plane. The 
coplanar stripline antenna of the present invention has numerous 
advantages over the conventional stripline antenna shown in FIG. 3 and the 
microstrip and slot line antennas shown in FIGS. 4 and 6. The coplanar 
stripline antenna of the present invention has low losses, low fringing, 
low mutual coupling, high gain for a given size, good variation in 
achievable impedance levels, low likelihood of launching trapped waves in 
the dielectric member and is mechanically simpler than the stripline 
antenna. 
Coplanar stripline is normally excited between the narrow center conducting 
strip and the upper ground planes which upper ground planes are closely 
spaced to the center conducting strip. For example, the coplanar stripline 
antenna shown in FIG. 8 would be excited between the center conducting 
strip 114 and the surrounding portions of the upper ground plane 110. This 
results in a better concentration of the E-field lines with less fringing 
and also reduces the E-field intensity in the layer of dielectric material 
118. 
FIG. 9 illustrates a first embodiment of a coplanar stripline antenna in 
accordance with the present invention. In FIG. 9, an upper ground plane 
150, substantially surrounds and is spaced from an antenna element 152 by 
a gap 154. The gap 154 between the antenna element 152 and the upper 
ground plane 150 has a length of approximately one wavelength for the 
resonant frequency of the antenna. A center conducting strip 156 which is 
also spaced from the ground plane 150 is used to feed the antenna element 
152. The elements 150, 152, and 154 are all supported on a layer of 
dielectric material 158 and with a closely spaced ground plane 160 forming 
the lower ground plane. A coaxial cable 162 has its inner conductor 164 
connected to the center conducting strip 156. The outer portion of the 
coaxial cable is connected to the upper ground plane 150 at positions 166. 
The antenna of FIG. 9 was designed to radiate a single lobe slot type 
pattern and FIG. 10 illustrates the radiation pattern from the antenna of 
FIG. 9 at the radiation frequency. The pattern cut is through the feed 
line 156 and normal to the plane of the slot 165 which cut would be an 
E-plane cut and with the polarization of the transmitting source parallel 
to the feed line 156. As shown in FIG. 10, the E-plane pattern is shown by 
a solid line and the H-plane pattern is shown by a dotted line. The 
E-plane pattern is broader than the H-plane pattern which corresponds to 
the usual behavior of slot antennas. Cross-polarization is generally more 
than 20 dB down and the antenna is well matched to the impedance of the 
coaxial line 162 at the designed frequency. 
FIG. 11 illustrates a coplanar stripline antenna array of four antenna 
elements which provides excellent electrical performance characteristics. 
The antenna array of FIG. 11 includes an upper ground plane 200 
substantially surrounding four antenna elements 202, 204, 206, and 208. 
Each antenna element is spaced from the upper ground plane 200 by gaps 210 
through 216. A coplanar stripline conducting strip 218 is used to feed all 
of the antenna elements and the upper ground plane, the antenna elements, 
and the coplanar stripline feed member all supported on a layer of 
dielectric material 220. A lower ground plane 222 is also supported on the 
layer of dielectric material 220. The gaps 210 through 216 may also 
include additional dielectric material such as a paint including 
dielectric material as shown in FIG. 9 so as to tune the resonant 
frequency of the antenna. As indicated above with reference to FIG. 9, 
this dielectric material may be paint containing titanium dioxide. 
In order to properly feed the antenna array of FIG. 11, a coaxial connector 
may be supported on the lower ground plane. Specifically, as shown in FIG. 
12, a coaxial connector 224 includes an outer connecting shell portion 226 
which is positioned against the lower ground plane 222, and with screws 
228 extending from the upper ground plane 200 to lock the outer shell of 
the connector 224 in position. The screws 228 connect the outer shell 
portion 226 of the connector 224 to the upper ground plane 200. An inner 
conductor 230 of the connector 224 extends through the layer of dielectric 
material 220 and is coupled to the feed line 218. A dielectric member 232 
insulates the inner conductor 230 from the outer shell portion 226. 
As shown in FIG. 11, the feed point between the conductor 230 and the 
coplanar stripline 218 is at a point equidistant from all four antenna 
elements 202 through 208. This ensures an equal radiation from the various 
antenna elements in the array. 
FIG. 13 illustrates a radiation pattern which is measured in a similar 
manner to the radiation pattern of FIG. 10. The E-plane pattern is shown 
by the solid line and the H-plane pattern is shown by the dotted line and 
the pattern cut is similar to that described above with reference to FIG. 
10. As can be seen in FIG. 13, the E-plane pattern is broader than the 
H-plane pattern which again is the normal behavior for this type of 
antenna. The gain of the antenna of FIG. 11 is considerably greater than 
the gain of the antenna of FIG. 9, which is to be expected, since the 
antenna of FIG. 11 includes an array of four antenna elements as opposed 
to the single antenna element of the antenna of FIG. 9. 
The conformal coplanar antenna of the present invention provides a 
significant improvement over conventional microstrip and stripline 
antennas. It is of a single board construction and does not require the 
additional shorting pins of the stripline antenna structure. The antenna 
of the present invention has higher efficiency, higher gain and less 
fringing than the microstrip antennas. The antenna of the present 
invention thereby provides for a superior antenna for applications 
requriring very thin conformal antennas such as those used on airplanes. 
It is to be appreciated that although the invention has been described with 
reference to particular embodiments, other adaptations and modifications 
may be made and the invention is only to be limited by the appended claims 
.