Small antennas such as microstrip patch antennas

In an antenna having a conductor of a length L and a dielectric material with a dielectric constant .epsilon..sub.r1 contacting the conductor, a matching dielectric layer .epsilon..sub.r2 less than .epsilon..sub.r1 matches the dielectric constant to free space. Preferably .epsilon..sub.r2 =.sqroot..epsilon..sub.r1 , L=.lambda..sub.o /(2.sqroot..epsilon..sub.r1 ). The depth d of the second dielectric is a quarter wavelength in the matching layer. Multiple matching layers with successively decreasing dielectric constants forms embodiments. In one embodiment the resonant conductive arrangement is a microstrip patch antenna with the dielectric material supporting a patch and matching layer covering the dielectric material.

FIELD OF THE INVENTION 
This invention relates to micro-dimensioned electromagnetic radiators, and 
particularly to microstrip patch and other small antennas. 
BACKGROUND OF THE INVENTION 
A small antenna is defined as a conducting radiator with overall dimensions 
of less than .lambda..sub.o /2, where .lambda..sub.o is the wavelength of 
the propagating signal in free space. The properties of a class dipole 
antenna with a length of .lambda./2 are described in detail in the book by 
John D. Kraus, "Antennas", McGraw Hill 1988. 
Efforts to shrink the length of the resonating dipole antennas have 
resulted in small antennas known as microstrip antennas constructed of 
dipoles or patches deposited on dielectric substrates. Microstrip antennas 
are described in the Proceedings of the IEEE, Vol. 80, No. 1, January 1992 
in the article entitled "Microstrip Antennas" by David M. Pozar. 
An object of the invention is to improve small antennas. 
SUMMARY OF THE INVENTION 
According to an aspect of the invention, an antenna includes a resonating 
conductive arrangement having an overall dimension L, a first dielectric 
contacting the conductive arrangement along the dimension L and having a 
dielectric constant .epsilon..sub.r1, and a second dielectric covering the 
first dielectric and having a dielectric constant with a value 
.epsilon..sub.r2 between the value .epsilon..sub.r1 and an ambient 
dielectric constant. 
These and other aspects of the invention are pointed out in the claims. 
Other objects and advantages will become evident from the following 
detailed description when read in light of the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 illustrates an antenna AN1 embodying the invention and using the 
fundamental dipole antenna structure. The arrangement permits shrinking of 
the physical conductor dimensions of a classic dipole antenna with a 
length of .lambda./2 without substantially altering the antenna 
characteristics, and increasing its efficiency. 
In order to shrink the length of the resonating dipole by a factor S 
(shrinking factor), a dipole DI1 connected to lead wires W1 is embedded in 
a small sphere SP1 composed of core dielectric material. This spherical 
volume is termed the "the near field sphere". The relative dielectric 
constant of the material in the near filed sphere SP1 is .epsilon..sub.r1. 
The central sphere SP1 is surrounded by a spherical shell SP2 with a 
relative dielectric constant .epsilon..sub.r2 =.sqroot..epsilon..sub.r1 . 
The shell SP2 is embedded in free space with a relative dielectric 
constant .epsilon..sub.r3 =1. The shell SP2 with dielectric 
.epsilon..sub.r2 is termed the "matching shell" or "matching layer." The 
matching layer SP2 matches a low impedance to a high impedance load or 
vice versa. The lead wires WI1 serve for connection to a receiver or 
transmitter (not shown). The relative dielectric constant .epsilon..sub.r1 
of the core dielectric material of sphere SP1 results in a shrinking 
factor S=.sqroot..epsilon..sub.r1 . 
The length L of the resonating Half-wavelength dipole DI1 is 
##EQU1## 
with a corresponding shrinking factor S=.sqroot..epsilon..sub.r1 . The 
value .lambda..sub.o is the center wavelength of the resonating antenna in 
free space. 
The thickness d of the matching shell SP2 is a quarter-wavelength within 
the dielectric medium SP2 with the relative dielectric constant of 
.epsilon..sub.r2, namely .lambda./4 or .lambda..sub.0 /(4 
.sqroot..epsilon..sub.r2 ). This matching dielectric constant 
.epsilon..sub.r2 is the geometric mean between .epsilon..sub.r1 and 
.epsilon..sub.r3, and is given by .epsilon..sub.r2 
=.sqroot..epsilon..sub.r1 .epsilon..sub.r3 =.sqroot..epsilon..sub.r1 
where .epsilon..sub.r3 =1.0 in free space and close to 1.0 in ambient air 
with the result d=.lambda..sub.o /.sqroot..epsilon..sub.r2 =.lambda..sub.o 
/(4.sup.4 .sqroot..epsilon..sub.r1 ). 
Thus for example: If the frequency f.sub.o =1 GHz and .epsilon..sub.r1 =38, 
.lambda..sub.o =0.33, m=12", .epsilon..sub.r2 =.sqroot.38, and 
d=.lambda..sub.o /(4.sup..sqroot.6.2)=1.2". In this case 
L=12/(2.times.6.2)=0.97" 
The matching shell SP2 reduces the effects of substantial reflections and 
other disadvantages arising from the dielectric mismatch between the shell 
SP1 and free space. Preferably, the thickness d of the matching shell SP2 
is one quarter wavelength of .lambda. or .lambda..sub.o /(4.sup.4 
.sqroot..epsilon..sub.r1 ) so that incoming waves are 180.degree. out of 
phase with the reflections that occur at the boundary of the matching 
shell and free space, and therefore cancel reflections from that boundary. 
In effect the matching layer introduces a gradual change in dielectric 
constant from sphere SP1 to sphere SP3 and that limits reflections. This 
has the effect of broadening the bandwidth propagated. 
The dielectric constant .epsilon..sub.r2 of the matching layer SP2 is 
chosen as the geometric means between .epsilon..sub.r1 and 
.epsilon..sub.r3, namely .epsilon..sub.r2 =.sqroot..epsilon..sub.r1 
.epsilon..sub.r3 =.sqroot..epsilon..sub.r1 , because this spreads the 
change in dielectric constant uniformly among the boundaries SP1-SP2 and 
SP1-SP3. 
According to an embodiment of the invention, additional quarter wavelength 
dielectric spheres or layers cover the sphere SP2. 
The dielectric constants of these added layers decrease from the dielectric 
constant .epsilon..sub.r1 of the sphere SP1 to the dielectric constant of 
the sphere SP3, namely .epsilon..sub.r3 =1. This provides gradual changes 
in dielectric constants. Preferably, the dielectric constant of each of 
all n overlying matching layers, including the sphere SP2, is then the 
next lower (n+1)/p-th root of .epsilon..sub.r1 where .epsilon..sub.r3 =1. 
This spreads the change in dielectric constant uniformly among the 
boundaries between spheres SP1 and SP3. Increasing the number of matching 
layers improves the efficiency even further and broadens the bandwidth. 
The addition of the matching layer SP2 favorably affects the radiation 
resistance R.sub.r of the antenna AN1. As shown in the aforementioned book 
"Antennas" by John D. Kraus, the radiation resistance of a dipole antenna 
is 73 ohms. With a single matching layer SP2 as shown in FIG. 1, the 
radiation resistance R.sub.r of the antenna AN1 reduced by a factor 
.sqroot..epsilon..sub.r1 from the resistance of 73 Ohms. Hence, in 
addition, to shrinking the physical size of the radiation system, the 
invention achieves a reduction of the radiation resistance to R.sub.r 
=73/.sqroot..epsilon..sub.r1 . 
The radius of the near-field sphere SP1 satisfies the condition 
1/(2.pi.).sup.2 &lt;r/.lambda.&lt;(2.pi.). This will cover the volume where the 
stored electromagnetic reactive energy is dominant and exceeds the 
radiated energy per signal cycle. 
FIGS. 2 and 3 are cross-sectional and plan views of a microstrip patch 
antenna embodying the invention and applying the aforementioned 
matching of a radiating structure to free space. Here, a conductive ground 
plane GP1 supports a near field dielectric substrate layer DL1 which 
embeds a patch resonator PR1. A matching dielectric layer DL2 overlies the 
layer DL1. 
The conductive patch resonator PR1 is rectangular in shape with a length 
L=.lambda..sub.o /(2 .sqroot..epsilon..sub.r1 ) and a width w. A conductor 
CO1 connects the patch resonator PR1 to the edge of the antenna for 
connection, with a connection to the ground plane GP1, to a receiver or 
transmitter (not shown). The near field substrate layer DL1 serves the 
same purpose of the sphere SP1 and has a relative dielectric constant 
.epsilon..sub.r1. To embed the patch resonator PR1, the near field 
substrate layer DL1 is thicker than the spacing of the patch resonator PR1 
to the ground plane GP1. The distance d.sub.2 between the patch resonator 
PR1 and the matching dielectric layer DL2 is preferably L/2.pi.. This 
approximates the radius of the sphere SP1 if the dipole DI1 is nearly 
equal to the radius of the sphere SP1. 
The matching dielectric layer DL2, serves the same purpose as the matching 
layer SP2 of FIG. 1 and has a relative dielectric constant 
.epsilon..sub.r2 =.sqroot..epsilon..sub.r1 . 
The thickness of the quarter-wave matching layer is given by 
##EQU2## 
According to another embodiment of the invention, additional matching 
quarter wavelength (in thickness) layers are placed over the matching 
dielectric layer DL2. In such cases, as in the case of the sphere, n 
matching layers each have dielectric constants that decrease sequentially 
from .epsilon..sub.r1 to 1 in the layers starting with the layer DL2. 
Preferably the layers have dielectric constants of the next lower of the 
(n+1)/p-th root of .epsilon..sub.r1, where p=n, . . . 2, 1 for each layer 
further from the substrate. This spreads the change in dielectric constant 
uniformly among the boundaries between the layer DL1 and free space. It 
spreads the changes of dielectric constants at the boundaries, and causes 
cancellation of reflections within each quarter wavelength layer because 
of the 180.degree. phase displacement between wave and reflection. It 
increases efficiency and other characteristics such as bandwidth. 
Another embodiment of the invention appears in the cross-sectional view of 
an antenna in FIG. 4. In this embodiment the plan view (not shown) is 
the same as in FIG. 3. Here, the near-field substrate layer is designated 
DL4 instead of DL1 as in FIG. 3. The cross-sectional view of FIG. 4 
differs from FIG. 2 only in that in FIG. 4 the thickness of the near-field 
substrate layer DL4 is equal to the height of the patch resonator PR1 
above the ground plane GP1. The relative dielectric constants are the same 
as in FIGS. 2 and 3. The thickness of the quarter wave matching layer DL2 
is also the same as in FIG. 2. 
FIG. 5 is a cross-sectional view of an antenna using a patch generator as 
shown in FIGS. 2 and 3 but with a quarter wavelength matching layer DL12 
and additional quarter wavelength matching layers DL13 and DL14. The layer 
DL1 is split into two dielectric layers having the same dielectric 
constant and receive the patch resonator PR1 between them. The dielectric 
constants decrease .epsilon..sub.r1 at the layer DL1 toward 1. Here, the 
dielectric constants of the layers DL12, DL13, and DL14 are .sup.4 
.sqroot..epsilon..sup.3.sub.r1 , .sqroot..epsilon..sub.r1 , .sup.4 
.sqroot..epsilon..sub.r1 . 
FIG. 6 is a cross-sectional view of an antenna using a patch generator as 
shown in FIG. 4 but with a quarter wavelength matching layer DL22 and 
additional quarter wavelength matching layers DL23, DL24, and DL25. Here, 
the dielectric constants of the layers DL22, DL23, DL24, and DL25 are 
.epsilon..sub.r1.sup.4/5, .epsilon..sub.r1.sup.3/5, 
.epsilon..sub.r1.sup.2/5, and .epsilon..sub.r1.sup.1/5. 
In operation, the antenna AN1, , and connect via wire lines W1 and 
conductors CO1 to respective receivers or transmitters (not shown). In the 
receive mode, for the length L, they respond to frequency ranges centered 
on the frequency f.sub.o having a wavelength .lambda..sub.o =2L 
.sqroot..epsilon..sub.r1 , (f.sub.0 =C.sub.0 /(2L .sqroot..epsilon..sub.r1 
) where C.sub.0 =velocity of light in free space. 
In the transmit mode, they radiate over frequency rangers centered on the 
same frequency. The matching dielectric layers prevent the waves, as they 
propagate through one medium of one dielectric constant, from encountering 
a medium with a vastly different dielectric constant. Each such encounter 
results in reflections that limit the efficiency and other characteristics 
of the radiation, such as the bandwidth. The matching layers interpose one 
or more media of intermediate dielectric constant, with each dielectric 
constant being the geometric mean between the dielectric constant of 
adjacent layers, such as .sup.n+1 .sqroot..epsilon..sup.p.sub.r1 , where n 
is the number of matching layers, p is the sequential number of any 
matching layer ending with the layer next to the substrate, and 
.epsilon..sub.r1 is the dielectric constant of the substrate layer. 
Because the thickness of each matching layer is one quarter wavelength of 
the matching layer medium, or .lambda..sub.o /(4.epsilon..sub.r1) if the 
layers are equal, the waves entering the matching layer are 180.degree. 
out of phase with waves reflected in the medium and hence cancel the 
reflection. 
Because .lambda..sub.o =2L .sqroot..epsilon..sub.r1 , f.sub.0 =C.sub.0 /(2L 
.sqroot..epsilon..sub.r1 ), the thickness of the matching layers may be 
chosen by the preferred relationship d=L/(2 .sqroot..epsilon..sub.r1 ). 
According to an embodiment of the invention this relation may vary over a 
tolerance of .+-.30%. 
In making antennas, such as the patch antennas and , the length L 
and the dielectrics DL1 and DL2 are chosen depending on the desired center 
frequency preferably on the basis of (equation). According to an 
embodiment of the invention, the relationship may vary over a range of 
.+-.30% because of the bandwidth of the resonator. The dielectrics SP2, 
DL2, and DL4 and the distance d are chosen on the basis of the dielectrics 
SP1 and DL1 as well as the center frequency f.sub.o by way of a preferred 
relationship such as .lambda..sub.o /(4 .sqroot..epsilon..sub.r1 ). 
According to an embodiment of the invention this relationship may vary 
over a tolerance of 30%. 
Because .lambda..sub.o =2L .sqroot..epsilon..sub.r1 , f.sub.0 =C.sub.0 /(2L 
.sqroot..epsilon..sub.r1 ) the thickness of the matching layers may be 
chosen by a preferred relationship d=L/(2 .sqroot..epsilon..sub.r1 ). 
According to an embodiment of the invention this relationship may vary 
over a tolerance of 30%. 
The values of the dielectric constants and thicknesses need not be exact 
but may vary. Within the matching layers, any dielectric constant between 
the dielectric constant of the substrate and free space improves the 
operation as long as they approach the dielectric constant of free space 
the closer they are to the free space in the antenna. 
The invention results in a smaller antenna that retains the efficiency of a 
larger antennas, or put otherwise, produces antennas of greater efficiency 
other than antennas of equal size. 
The invention also prevents a collapse of the bandwidth observed for 
conventional antennas if their size is substantially reduced from 
.lambda..sub.o /2. 
An embodiment of the invention incorporates the disclosure of our 
aforementioned concurrently-filed copending application entitled "High 
Efficiency Microstrip Antennas" by making the thickness of the conductor 
sufficiently small to reduce shielding and losses caused by the skin 
effect and make currents at the upper and lower surfaces couple with each 
other and make the conductor partially transparent to radiation. In one 
embodiment the thickness is between 0.5.delta. and 4.delta.. Preferably 
the thickness is between 1.delta. and 2.delta. where .delta. is equal to 
the distance at which current is reduced by 1/e., for example 1.5 to 3 
micrometers at 2.5 gigahertz in copper. According to an embodiment, 
alternate layers of dielectrics and radiation transparent patches on a 
substrate enhance antenna operation. 
An embodiment of the invention incorporates the disclosure of our 
aforementioned concurrently-filed copending application entitled "Antennas 
With Means For Blocking Currents In Ground Planes" by making dielectric 
components extend between top and bottom surfaces of a ground plane in a 
resonant microstrip patch antenna over a distance of 
one-quarter-wavelength of a resonant frequency of the antenna. The 
components form quarter-wave chokes within which waves cancel with 
reflected waves and reduce currents in the bottom surfaces of the ground 
plane. This reduces back lobe responses. 
The content of our co-pending applications entitled "High Efficiency 
Antennas" and "Antennas with Means for Blocking Currents in Ground Planes" 
both filed concurrently herewith, and assigned to the same assignee as 
this application, are hereby made a part of this application as if fully 
recited herein. 
While embodiments of the invention have been described in detail, it will 
be evident to those skilled in the art that the invention may be embodied 
otherwise without departing from its spirit and scope.