Microstrip antenna and antenna array

A microstrip antenna comprises a first dielectric layer having a permittivity of 2.5-12.5 carrying the ground plane on one face and a feeder-resonator on the opposite face; a second dielectric layer thereover having a permittivity of 2.2-2.5 and carrying on its outer face a radiator electromagnetically coupled to the feeder-resonator; and spacing means spacing the second dielectric layer from the first a distance of up to seven times the thickness of the first, and providing a permittivity between the two dielectric layers which is approximately that of air. Matching of the antenna is obtained by varying the spacing between the two dielectric layers. The gain of the radiating element is better than 7.5 dbi for bandwidth of 15%. Sidelobe level is less than 15 dB in azimuth and elevation plan. The radiation pattern of the antenna is symmetric in both azimuth and elevation plan.

BACKGROUND OF THE INVENTION 
The present invention relates to microstrip antennas, and also to 
microstrip antenna arrays. 
Microstrip antennas have been enjoying a growing popularity lately. They 
possess attractive features such as low profile, light weight and small 
volume, combined with capability of conforming to complex bodies and low 
production cost. In addition, the benefit of a compact and low cost feed 
network may be attained by integrating the microstrip feed structure with 
the antenna on the same substrate. This is especially useful in arrays. 
These antennas, however, have a narrow bandwidth, of the order of 2-4 
percent. 
SUMMARY OF THE PRESENT INVENTION 
An object of the present invention is to provide a microstrip antenna 
having an increased bandwidth. 
According to a broad aspect of the present invention, there is provided a 
microstrip antenna comprising: (a) a first dielectric layer carrying on 
one face an electrically-conductive element serving as the ground plane, 
and on its opposite face an electrically-conductive element serving as a 
feeder-resonator; (b) a second dielectric layer over said first dielectric 
layer and of a lower permittivity than said first dielectric layer, said 
second dielectric layer carrying an electrically-conductive element 
serving as a radiator electromagnetically coupled to said feeder resonator 
of the first dielectric layer; and (c) spacing means spacing said second 
dielectric layer from said first dielectric layer and providing a 
permittivity therebetween which is lower than that of said second 
dielectric layer. The radiator is parallel to, concentric with, and of 
larger dimensions than, the feeder-resonator so as to completely overlie 
it. The spacing between the two dielectric layers is equal to at least 
twice the thickness of the first dielectric layer, and the permitivity of 
the space is between 1 and 2.2. 
In the preferred embodiments of the invention described below, the first 
dielectric layer has a permittivity of 2.5-12.5, the second dielectric 
layer has a permittivity of 2.2-2.5, and the permittivity of the space 
between the two dielectric layers is 1-2.2, preferably as close to 1 (that 
of air) as possible. 
In one described preferred embodiment, the spacing means comprises spacer 
elements at discrete points between the two dielectric layers to provide 
mostly an air spacing therebetween; and in a second described embodiment, 
it comprises a layer of foamed plastic material. Impedence matching is 
effected by varying the dimensions of this spacing means, and it is 
possible to thus space the two dielectric layers up to seven times the 
thickness of the first dielectric layer carrying the ground plane and 
feeder-resonator. 
Electromagnetically-coupled microstrip radiators have been previously 
described in the literature, e.g., H. G. Oltman and D. A. Huebner, 
"Electromagnetically Coupled Microstrip Dipoles." IEEE Trans. Antennas 
Propagat, Vol. AP-29, No. 1, pp. 151-157, January 1981. Described in this 
publication are constructions wherein dipole radiating elements are 
closely stacked to microstrip feed lines. Bandwidths obtained in this 
manner were between 2.5.degree. and 5.5.degree. for a VSWR of 1.92 or 
better. Other "piggyback" antennas described in this literature, for 
example, R. J. Mailloux, J. F. McIlvenna, and N. P. Kernweis, "Microstrip 
Array Technology," IEEE Trans. Antennas Propagat, Vol. AP-29, No. 1, pp. 
25-37, January 1981; S. A. Long and M. D. Walton, "A Dual-Frequency 
Stacked Circular Disc Antenna," IEEE Trans. Antennas Propagat, Vol. AP-27, 
No. 2, pp. 270-273, March 1979, have operated as dual-frequency radiators. 
As distinguished from these constructions, however, the antennas of the 
present invention, as described below, appear to merge the two different 
frequency ranges into a single wider range. 
At this point, reference may be made to the publication, A. Sabban, "A 
Wideband Two Layer Microstrip Disc Antenna Array." (RAFAEL, Haifa, 
Israel). Electrotechnology for Development. Proceedings of MELECON 81, the 
First Mediterranean Electrotechnical Conference. Tel-Aviv, Israel. May 
24-28, 1981, describing earlier work leading to the present invention. 
This publication does not describe, among other things, providing the 
spacing "air" layer between the two dielectric layers, but rather effects 
impedence matching by sliding one dielectric layer with respect to the 
other. Whereas the experimental constructions described in that 
publication increased the bandwidth for VSWR 2:1 by 10%, the present 
invention, including the "air-gap" spacing layer has been found to 
increase this bandwidth up to about 15%. 
Further features and advantages of the invention will be apparent from the 
description below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The microstrip antenna illustrated in FIG. 1 comprises a first dielectric 
layer 2 and a second dielectric 3 overlying dielectric layer 2 and spaced 
therefrom by a substantially air-gap spacing 4. 
More particularly, dielectric layer 2 carries on its underface an 
electrically-conductive layer 21 serving as the ground plane, and further 
carries on its opposite face an electrically-conductive element 22 serving 
as a feeder-resonator. The permittivity of dielectric layer 2 is 
preferably from 2.5 to 12.5. 
Dielectric layer 3 is formed on its outer face, namely, that opposite to 
dielectric layer 2, with an electrically-conductive element 31 serving as 
a radiator which electromagnetically coupled to the feeder-resonator 22 of 
dielectric layer 2. The permittivity of dielectric layer 3 is lower than 
that of dielectric layer 2, being preferably within the range of 2 to 2.5. 
Radiator element 31 is concentric to but of a large dimension than the 
feeder-resonator 22. Both may be conveniently formed by printed-circuit 
techniques. 
In the construction illustrated in FIG. 1, the air-gap 4 between dielectric 
layers Z and 3 is formed by a plurality of spacer elements 41 at discrete 
points, e.g. at the corners, between dielectric layers 2 and 3, to provide 
mostly an air spacing between them. The spacer elements 41 should also be 
of low permittivity, since the permittivity of the complete space 4 
between the two dielectric layers 2 and 3 should be less than that of 
dielectric layer 3, and preferably as close to that of air (permittivity 
of 1), as possible. 
The thickness of the air gap 4 may be varied to match the impedence of the 
antenna with respect to the circuit to which it is connected. Preferably, 
this thickness should be at least four times that, and no greater than 
seven times that, of the thickness of dielectric layer 2. Particularly 
good results have been obtained when the thickness of this air gap layer 4 
is six times that of the dielectric layer 2. 
The feeder-resonator element 22 and radiator element 31 may take various 
configurations, such as discs of circular, square, or rectangular 
configuration. Best results have been obtained when these elements are 
symmetric, e.g. concentric discs. Particularly good results have been 
obtained when the diameter of the feeder-resonator element 22 is 
approximately 0.9 times the diameter of the radiator element 31. 
The spacing between the radiator element 31 and feeder-resonator 22 may be 
adjusted experimentally for the best impedence match over a wide frequency 
range. As a practical matter, the impedence value at the center frequency 
may be close to 50 ohms, thus making convenient integration with their 
feed network and the RF head possible. 
The feeder-resonator element 22 is designed as a resonator at the center 
frequency. Its substrate may be about 0.01, the wavelength thickness, and 
it may be fed by a single line for linear polarization, or by a 
combination of two lines in phase quadrature and with angular separation 
of 90.degree. between them for circular polarization. 
Table 1 below sets forth experimental results obtained with a microstrip 
antenna according to the above-described construction as illustrated in 
FIG. 1. 
For a VSWR (Voltage Standing Wave Ratio) of 2:1 or better, results listed 
in Table 1 show bandwidths of 9 to 15 percent, depending on the 
configuration. Also shown in Table 1 are beamwidths, gain values, and 
sidelobe levels for all versions. One may note that the aperture 
efficiency of these antennas is better than that of ordinary microstrip 
antennas whose beamwidth is of the order of 85.degree.-90.degree.. 
TABLE 1 
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Sidelobe 
Antenna 
Fre- Beamwidth levels Pola 
Geom- quency Band- H-Plane Gain H-Plane 
Polari- 
etry Band width (Degrees) 
(dbi) 
(dB) zation 
______________________________________ 
Circular 
S 15% 72 7.9 -22 Linear 
disc 
Circular 
S 11.5% 78 6.6 -14 Linear 
annular 
disc 
Rectan- 
S 9% 70 7.4 -25 Linear 
gular 
Square S 9% 72 7 -22 Linear 
Circular 
S 10% 72 7.5 -22 Cir- 
disc cular 
Circular 
X 15% 72 7.5 -25 Cir- 
disc cular 
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The antenna described above is most suitable for use as an array element. 
The feeding elements may be etched jointly with the power dividing network 
as an integrated structure, leading to a very compact, lightweight and low 
loss design. This is particularly important for mm wave applications. An 
array designed with this element achieves a lower sidelobe level as 
compared to conventional elements, owing to this element's higher 
directivity. 
FIG. 2 illustrates a variation which also includes two dielectric layers 
102 and 103 separated by an air-gap layer 104, layer 102 carrying a ground 
plane 121 on one face and a feeder resonator element 122 on the opposite 
face, and dielectric layer 103 carrying a radiator element 131 on its 
other face, all as described above with respect to FIG. 1. In FIG. 2, 
however, the air-gap 104 is produced by a layer of foamed plastic or 
rubber, such that the permittivity of this layer 104 is less than that of 
layer 103, preferably approach that of air, namely "1." 
For purposes of example, FIG. 3 illustrates an antenna array, including 16 
elements 200, in each of which the feeder-resonator and radiator are of 
the disc-configuration, as described above. In this example, the spacing 
between the elements in the array was chosen as 0.78.lambda., so as to 
attain the sidelobe level of -21 dB at the E-plane. In contrast, a 
cos.theta. element would bring about a sidelobe level of -16 dB with the 
same spacing. Results for several versions are shown in Table 2. 
TABLE 2 
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No. of Elements 
Frequency Band 
Bandwidth Gain (dbi) 
______________________________________ 
16 Ku 10% 18.5 
32 Ku 9.4% 20.5 
16 C 13.7% 17 
64 Ka 10% 23.5 
______________________________________ 
FIG. 4 illustrates a variation in the antenna of FIG. 1, in which variation 
the radiating element, therein designated 231, is formed on the inner face 
of the upper dielectric layer 203, rather than on its outer face as in 
FIG. 1. The remaining elements of the antenna, namely, the lower 
dielectric layer 202, the ground plane 221, the feeder resonator 222, and 
the spacer elements 241, are the same as the corresponding elements in the 
FIG. 1 antenna, the FIG. 4 antenna also defining an air gap 204 between 
the two dielectric layers 202 and 203. The advantage in the FIG. 4 
variation is that the radiating element 231, as well as the feeder 
resonator 222, is not exposed externally of the antenna and is protected 
by its dielectric layer 203. 
It will be appreciated that the variation illustrated in FIG. 4, namely, of 
providing the radiating element 231 on the inner face of the upper 
dielectric layer 203 rather than on its outer face, could also be 
incorporated in the FIG. 2 variation wherein the air space is formed by 
the foamed plastic layer 104. 
Following are examples of the materials that can be used for the various 
elements of the described antennas: 
For the lower dielectric layer 2 in FIG. 1 (and the corresponding layers 
102 and 202 in FIGS. 2 and 4, respectively), there may be used the 
ceramic-polytetrafluoroethylene-composite "RT-DUROID" (Reg. T.M.) 6010, 
supplied by Rogers Corporation, having a permittivity of 10.5; or "Epsilam 
10" (Reg. T.M.) supplied by 3 M Company, having a permittivity of 10.2. 
For the upper dielectric layer 3 in FIG. 1 (and the corresponding layers 
103 and 203 in FIGS. 2 and 4, respectively), there may be used the 
glass-microfiber-reinforced-polytetrafluoroethylene "RT-DUROID" (Reg. 
T.M.) 5880, supplied by Rogers Corporation, having a permittivity of 2.2, 
or "3 M Brand A-6098 Teflon" (Reg. T.M.) glass-cloth-laminate-type GT, 
supplied by 3M Company, having a permittivity of 2.5. For the spacer 
elements 41 in FIG. 1 (and 241 in FIG. 4), there may be used 
polytetrafluoroethylene "CuFlon" (T.M.) supplied by DuPont, having a 
permittivity of 2.1, or glass-reinforced-polypropylene, having a 
permittivity of 2.36. For the foamed plastic layer 104 in FIG. 2, there 
may be used foamed-polyurethane having a dielectric constant of 1.04-2.4, 
or foamed-polypropylene having a dielectric constant of 2.2-2.4. 
It will thus be seen that the above-described antennas effect matching by 
varying the spacing between the two dielectric layers and provide a gain 
of the radiating elements better than 7.5 dbi for bandwidth of 15%, the 
sidelobe level being less than 15 dB in azimuth and elevation and the 
radiation pattern being symmetrical in both azimuth and elevation planes. 
Many other variations and applications of the invention will be apparent.