Antenna with ground plane having cutouts

An antenna structure has a radiating element and a ground plane. The ground plane has a central region relatively closely spaced apart from the radiating element and a peripheral region extending away from the central region. The peripheral region comprises at least the conductive layer that extends radially beyond the radiating element and provides a sheet resistivity higher than that of the radiating element. Though physically small, the ground plane simulates an infinite ground plane, and the antenna structure reduces multipath signals caused by reflection from the earth.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to antenna structures and more particularly to a 
novel and highly effective antenna structure comprising a radiating 
element such as a patch antenna in combination with a ground plane 
constructed to enhance antenna performance. 
2. Description of the Prior Art 
There is a need for an improved antenna structure for use with a GPS 
receiver that receives and processes signals from navigation satellites. 
Antenna structures known heretofore that are capable of optimum 
performance are too bulky and unwieldy for use in small GPS receivers, 
especially hand-held receivers. Compact antenna structures that are 
conventionally employed with GPS receivers do not provide optimum 
performance. One problem is that they receive signals directly from 
satellites and, because of ground reflections, also indirectly. This 
so-called multipath reception causes time measurement errors that can lead 
to a geographical fix that is erroneous or at least suspect. 
A British patent publication No. 2,057,773 of Marconi discloses a large 
radio transmitting antenna including aerial wires supported in spaced, 
parallel relation by posts. The ground around the antenna is saturated to 
a depth of two or three meters with an aqueous solution of calcium sulfate 
to increase the conductivity of the ground and thereby improve its 
reflectivity. The ground is permeated to a distance two to three times as 
far from the antenna as the antenna is tall. In a typical case this can be 
from 50 to 100 meters from the boundaries of the antenna array. 
A European patent publication No. 394,960 of Kokusai Denshin Denwa 
discloses a microstrip antenna having a radiation conductor and a ground 
conductor on opposite sides of a dielectric substrate. The spacing between 
the radiation conductor and the ground conductor, or the thickness of the 
dielectric substrate, is larger at the peripheral portion of those 
conductors than at the central portion. Because of the large spacing at 
the peripheral portion, the impedance at the peripheral portion where 
electromagnetic waves are radiated is said to be close to the free-space 
impedance. 
A German patent publication No. DE 37 38 513 and its U.S. counterpart U.S. 
Pat. No. 5,061,938 to Zahn et al. disclose a microstrip antenna including 
an electrically conductive base plate carrying an electrically insulating 
substrate on top of which are a plurality of radiating patches. A 
relatively large spacing is established between the electrically 
insulating substrate and the base plate at lateral dimensions somewhat 
larger than lateral dimensions of the patches and also in the vicinity of 
the patches. The patches and spacings are vertically aligned through 
either local elevations of the insulating substrate or local indentations 
in the base plate. The feeder line is thus relatively close to the 
conductive base plate, and the radiating patch is farther away from the 
conductive base plate. This is said to improve the radiating 
characteristics of the patch. 
A German patent publication No. DE 43 26 117 of Fischer discloses a 
cordless telephone with an improved antenna. 
A European patent publication No. 318,873 of Toppan Printing Co., Ltd., and 
Seiko Instruments Inc. discloses an electromagnetic-wave-absorbing element 
comprising an elongate rectangular body of dielectric material having a 
bottom portion attachable to an inner wall of an electromagnetically dark 
room, and peripheral elongate faces extending vertically from the bottom 
portion. A set of the absorbing elements can be arranged in rows and 
columns on the wall. An electroconductive ink film is formed on the 
peripheral faces of the body and has a gradually changing surface 
resistivity decreasing exponentially lengthwise of the peripheral face 
toward the bottom portion. The incident electromagnetic wave normal to the 
wall provided with the rows and columns of absorbing elements is absorbed 
by a lattice of the electroconductive film during the travel along the 
electroconductive film. In order to avoid reflection of an incident 
electromagnetic wave at the boundary between the surrounding air and the 
absorbing element, the characteristic impedance at the top of the element 
through which the incident wave enters is close to the impedance of air. 
In order to avoid reflection at the boundary between the bottom of the 
element and the wall to which it is attached, the characteristic impedance 
at the bottom is close to that of the wall. The absorbing element is made 
of a plastic body with an electroconductive covering and having a variable 
resistivity or conductivity. 
The following prior art is also of interest: U.S. patents to Nelson U.S. 
Pat. No. 5,592,174 for GPS Multi-Path Signal Reception, Raguenet U.S. Pat. 
No. 5,248,980 for Spacecraft Payload Architecture, Franchi et al. U.S. 
Pat. No. 5,204,685 for ARC Range Test Facility, Kobus et al. U.S. Pat. No. 
5,170,175 for Thin Film Resistive Loading for Antennas, De et al. U.S. 
Pat. No. 5,132,623 for Method and Apparatus for Broadband Measurement of 
Dielectric Properties, Hong et al. U.S. Pat. No. 4,965,603 for Optical 
Beamforming Network for Controlling an RF Phased Array, Schoen U.S. Pat. 
No. 4,927,251 for Single Pass Phase Conjugate Aberration Correcting 
Imaging Telescope, and Bhartia et al. U.S. Pat. No. 4,529,987 for 
Broadband Micropstrip Antennas with Varactor Diodes. 
The prior art as exemplified by the patents discussed above does not 
disclose or suggest an ideal antenna structure for use in a GPS receiver 
that receives and processes signals from navigation satellites. What is 
needed in such an environment is an antenna structure that is very light 
and portable and adapted to hand-held units of the type used, for example, 
by surveyors. 
OBJECTS AND SUMMARY OF THE INVENTION 
An object of the invention is to overcome the problems of the prior art 
noted above and in particular to provide an antenna structure that reduces 
multipath signals caused by reflection from the earth, that is physically 
small yet simulates an infinite ground plane, and that is particularly 
adapted for use in a GPS receiver that receives and processes signals from 
navigation satellites. Another object of the invention is to provide an 
antenna structure that is suitable for hand-held units of the type used by 
surveyors. 
In accordance with one aspect of the invention, an antenna structure is 
provided comprising a radiating element and a ground plane for the 
radiating element having a central region closely spaced apart from the 
radiating element and a peripheral region extending away from the central 
region. The peripheral region is formed with at least one cutout having an 
area that increases as radial distance from the central region increases 
to provide an equivalent sheet resistivity that increases as radial 
distance from the central region increases. 
In accordance with an independent aspect invention, a method is provided 
comprising the steps of forming an antenna structure comprising a 
radiating element for receiving broadcast signals directly and, because of 
reflection of the signals, also indirectly with a time delay, and a ground 
plane. The ground plane has a central region closely spaced apart from the 
radiating element and a peripheral region extending away from the central 
region. The peripheral region is formed with at least one cutout having an 
area that increases as radial distance from the central region increases 
to provide a sheet resistivity that increases as radial distance from the 
central region increases. The antenna structure is employed to receive the 
broadcast signals. The signals received indirectly because of reflection 
are attenuated. 
Preferably, an antenna structure in accordance with the invention is 
characterized by a number of additional features: the radiating element is 
a patch antenna, the radiating element and the ground plane have the same 
shape (both square, both circular, both octagonal, etc.), and the 
radiating element is centered over the ground plane (it is also within the 
scope of the invention, however, for the radiating element and the ground 
plane to have dissimilar shapes). 
The ground plane has minimum linear resistivity adjacent the central region 
and maximum linear resistivity at the outer edge of the peripheral region. 
The ground plane can be planar, frustoconical and concave up or down, or 
frustopyramidal and concave up or down. The ground plane in some 
embodiments comprises a conductive portion in the central region, for 
example a disk made of or coated with aluminum. 
The ground plane ideally has a sheet resistivity substantially in the range 
of 0 to 3 ohms per square measured from dead center to a position adjacent 
the periphery of the radiating element and a sheet resistivity of 
substantially 500-800 ohms per square measured at the periphery of the 
ground plane. The sheet resistivity of the peripheral region thus exceeds 
that in the central region by several orders of magnitude, whereby the 
ground plane, though physically small, simulates an infinite ground plane. 
In the preferred method of practicing the invention, the received 
electromagnetic signals are GPS signals broadcast by navigation 
satellites.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1-3 are top schematic views of antenna structures 10-12 constructed 
in accordance with the invention; FIGS. 4, 4A, 5, 6 and 6A respectively 
show other features that can be incorporated in antenna structures in 
accordance with the invention. 
In FIG. 1, the antenna structure 10 comprises a ground plane 16 and a 
radiating element 22. Both the ground plane 16 and the radiating element 
22 are circular. In FIG. 2 both (17, 23) are square; and in FIG. 3 both 
(18, 24) are octagonal. In each of FIGS. 1-3 the ground planes 16, 17, 18 
are illustrated as planar, but, as FIGS. 4, 4A, 6 and 6A illustrate, they 
need not be. In FIG. 4, the ground plane 19 is frustoconical and concave 
up, and in FIG. 6 the ground plane 21 is frustoconical and concave down. 
In FIGS. 4A and 6A the ground planes are frustopyramidal and concave 
respectively up and down. In FIG. 5 the ground plane 20 is planar. The 
ground plane can have any of the shapes illustrated in FIGS. 
1-3--circular, square or octagonal--combined with any of the shapes 
illustrated in FIGS. 4, 4A, 5, 6 and 6A. Other shapes both in plan view 
and in side section are also within the scope of the invention, as those 
skilled in the art will readily understand. 
FIGS. 7-10 show embodiments of the invention wherein the radiating element 
and the ground plane have dissimilar shapes: respectively round/square in 
FIG. 7, square/round in FIG. 8, round/octagonal in FIG. 9, and 
square/octagonal in FIG. 10. Other combinations of dissimilar shapes will 
readily occur to those skilled in the art in light of this disclosure. 
While the radiating element used in many applications is preferably a 
patch, other radiating elements including a quadri filar helix or 
four-armed spiral on a cylindrical or conical (or frustoconical) support 
base are well known in the art and can be used in appropriate cases. In a 
quadri filar helix, typically each spiral arm is fed by a power divider 
with an integral phase shifter to give each arm a successive 90-degree 
shift (to 0.degree., 90.degree., 180.degree., and 270.degree.). 
At the center of the ground plane there is a conductive portion, which can 
be formed of a metal such as aluminum or of a nonconductive material such 
as a woven cloth or a plastic disk impregnated with, or having a coating 
of, aluminum, another metal, or another conductive material. Aluminum 
plates 28-30 are illustrated in FIGS. 4, 4A, 5, 6 and 6A (an aluminum 
plate is of course highly conductive). The aluminum plate has an outer 
diameter of, say, 5 inches (about 13 cm). 
In accordance with the invention, the ground plane has an outer diameter 
of, say, 13 inches (about 33 cm). 
Sheet resistivity is measured in ohms per square. Consider a sheet of 
homogeneous material of uniform thickness in the shape of a square having 
a potential applied across it from one edge to the opposite edge. The 
current that flows is independent of the size of the square. For example, 
if the size of the square is doubled, the current must flow through double 
the length of the material, thereby doubling the resistance offered by 
each longitudinal segment of the square (i.e., each segment extending from 
the high-potential side of the square to the low-potential side). On the 
other hand, doubling the size of the square in effect adds a second 
resistor in parallel to the first and identical to it, thereby reducing 
the resistance by half. The change in resistance caused by doubling the 
size of the square is therefore 2.times.0.5=1. In other words, changing 
the size of the square does not affect the resistance offered by the 
square. 
In contrast, the effective sheet resistivity varies in accordance with the 
present invention. The ground plane in the preferred embodiment of the 
invention has a sheet resistivity substantially in the range of 0 to 3 
ohms per square measured from dead center to a position adjacent the 
periphery of the radiating element and a resistivity of substantially 
500-800 ohms per square measured at the periphery of the ground plane. The 
resistivity of the peripheral region thus exceeds that in the central 
region by several orders of magnitude, whereby the ground plane, through 
physically small, simulates an infinite ground plane. 
The sheet resistivity of free space is 377 ohms per square. The sheet 
resistivity of the ground plane at the outer periphery is thus much higher 
than that of free space. 
FIG. 11 shows an antenna structure 40 in accordance with the invention. A 
radiating element as illustrated in any of the preceding figures is 
employed. FIG. 11 shows a ground plane 42 for the radiating element. The 
ground plane has a central region 44 closely spaced apart from the 
radiating element and a peripheral region 46 extending away from the 
central region 44. The peripheral 46 is formed with at least one cutout 48 
having an area that increases as radial distance from the central region 
44 increases. 
As FIG. 11 shows, the peripheral region 46 can be formed with a plurality 
of cutouts. Each cutout can be, for example, U-shaped, as shown in FIG. 
12A and FIG. 12A1, or V-shaped, as shown in FIG. 12B and FIG. 12B1. Each U 
or V has a narrow end 50 or 52 near the central region 44 and a wide end 
54 or 56 remote from the central region. As FIG. 12C and FIG. 12C1 shows, 
the cutouts can have edges 58 that form an exponential curve. As FIG. 12D 
and FIG. 12D4 shows, the cutouts can also be spiral-shaped. For example, 
the cutout may describe as spiral about a central point, the spiral 
subtending an arc of at least 360.degree. as measured from the central 
point or an arc of a multiple of 360.degree. (FIG. 12D-1). The spiral in 
that case can form loops that are closer together as radial distance from 
the central region increases (FIG. 12D-2) or that become wider (FIG. 
12D-3). 
As FIG. 12E and FIG. 12E1 shows, the peripheral region can formed with a 
plurality of cutouts, each cutout being a closed figure and the cutouts 
collectively having an area that increases as radial distance from the 
central region increases. In this case, the cutouts can be elliptical 
(FIG. 13A and FIG. 13A1), circular (FIG. 13B and FIG. 13B1), polygonal 
(FIG. 13C and FIG. 13C1), rectangular (FIG. 13D and FIG. 13D1), square 
(FIG. 13E and FIG. 13E1), or have any other closed shape. 
It is also possible for the peripheral region to have a first plurality of 
cutouts each extending from the central region to the periphery and a 
second plurality of cutouts interspersed with the first plurality of 
cutouts and each extending from a position spaced apart from the central 
region to the periphery (FIG. 13F). 
Ideally, resistivity measured from the inner edge to the outer edge has a 
resistive profile varying in accordance with the following formula: 
EQU R=3+4.9881((exp 1.258.times.)-1) (1) 
where R is resistivity in ohms per square and x is distance in inches 
measured form the inner to the outer edge of the ground plane. The graph 
is plotted in FIG. 14. 
The conductive center of the ground plane is 4.97 inches square (about 12.6 
cm square) and approximately covers the "hole" in the ground plane. From 
another standpoint, the ground plane extends radially out approximately 
from the edges of the conductive center of the ground plane. 
If a patch is employed as the radiating element, its dimensions will depend 
on the dielectric. If air is the dielectric, the patch can be, say, 2 
inches (about 5 cm) on a side. If a material of higher dielectric constant 
is employed, the size of the patch can be reduced to, say, 1.5 inches 
(about 3.8 cm) on a side. 
FIG. 14 shows the resistivity profile of the ground plane for the preferred 
embodiment of the invention. In equation (1) above, consider for example a 
position 2.4 inches measured radially outward from the inner edge of the 
ground plane. The resistivity is calculated from equation (1) as follows: 
1.258.times.=3.0192. 
exp 3.0192=20.475 (approximately) 
20.475-1=19.475 
4.9881.times.(19.475)=97.143 (approximately). 
Finally, 3+97.143=100 (approximately), yielding the point (2.4, 100) as 
illustrated in FIG. 14. A similar calculation produces the other points on 
the graph. 
FIGS. 15 and 16 show the antenna pattern without a ground plane (at the two 
GPS frequencies). FIGS. 17 and 18 show the antenna pattern in accordance 
with the invention at the two GPS frequencies. The important thing to 
notice is that the back lobes (the area under the curves on the bottom 
half of the plots) are reduced in FIGS. 17 and 18. The two lines on each 
plot represent the received signal strength of a right hand circular 
polarized (RHCP) signal and a left hand (LHCP) signal, corresponding to a 
GPS signal and a reflected signal. 
The antenna structure described above reduces multipath signals caused by 
reflection from the earth. The ground plane, though physically small, 
simulates an infinite ground plane because of its varying sheet 
resistivity. Signals reflected from the ground and impinging on the 
underside of the antenna structure are absorbed by the ground plane and 
dissipated as heat; they do not interact substantially with the antenna 
proper. The antenna is particularly adapted for use in a GPS receiver that 
receives and processes signals from navigation satellites. Because of its 
light weight, it is suitable for hand-held units of the type used by 
surveyors. 
While the preferred embodiments of the invention have been described above, 
many modifications thereof will readily occur to those skilled in the art 
upon consideration of this disclosure. The invention includes all subject 
matter that falls within the scope of the appended claims.