Dual polarization electromagnetic power reception and conversion system

An antenna array for receiving dual polarized electromagnetic waves, comprised of a first thin-film printed circuit rectenna having a plurality of linear half-wavelength dipole antennae oriented in a first direction for receiving a first component of the dual polarized waves, and a second thin-film printed circuit rectenna parallel to the first rectenna, having a plurality of linear half-wavelength dipole antennae oriented in a second direction for receiving the second orthogonal component of the electromagnetic waves. A reflector screen is disposed parallel and behind the second rectenna, for reflecting incident electromagnetic waves transmitted through the first and second rectennae back to the first and second rectennae for reception thereby. The dipole antenna of the first rectenna are disposed in a predetermined pattern in relation to the second rectenna dipole antennae, the first and second rectennae are separated by one of either substantially zero distance or by a multiple half-wavelength distance, and the second rectenna and reflector screen are separated by a predetermined distance to effect substantial cancellation of transmission line shielding effects and mutual coupling, resulting in high efficiency signal reception.

BACKGROUND OF THE INVENTION 
This invention relates in general to the transfer of electrical power 
between two separated locations by means of transmitting and receiving 
electromagnetic waves, and more particularly to an antenna array for 
receiving dual polarized electromagnetic waves with high efficiency over a 
wide range of angles of incidence. 
Research in the area of remotely powered mobile systems has centered around 
the requirement for cost effective means to receive and convert 
transmitted electromagnetic power into direct current power when the 
transmitter and receiver are moving relative to one another. 
For example, it has been proposed that an electromagnetic power reception 
and conversion system could be implemented for transmitting propulsive and 
communications payload power in the 2.4-2.5 GHz microwave ISM band to a 
lightweight electrically-powered aircraft circling over a fixed ground 
antenna system for continuous periods of weeks or even months at a time. 
One prior art electromagnetic power reception and conversion system is 
described in U.S. Pat. No. 3,434,678, and is referred to as a linear 
rectenna. This device consists of an array of linearly polarized 
half-wavelength dipole antennae, each followed by a conversion system 
consisting of wave filters and rectifier circuits. 
In order to achieve high efficiency power collection with the linearly 
polarized rectenna described in the prior art patent, the transmitted 
electromagnetic field must itself be linearly polarized. In addition, the 
polarization orientation of this field must be maintained parallel to that 
of the rectenna dipoles, or vice versa, in response to changes in the 
orientation of the receiving rectenna relative to the power transmission 
antenna, or due to Faraday rotation of a polarized beam transmitted 
through the ionosphere, etc. In other words, expensive and complex 
polarization tracking equipment must be provided at either the 
transmitting antenna or the receiving rectenna in order for the system to 
operate properly with high efficiency power collection. 
An improvement in linearly polarized rectennae is described in an article 
entitled "Design Definition of a Microwave Power Reception and Conversion 
System for Use on a High Altitude Powered Platform" NASA/CR/156866, by 
W.C. Brown, published in 1981. According to the Brown article, a linearly 
polarized rectenna is disclosed in the form of a thin-film printed 
circuit. This type of linear rectenna has many desirable characteristics 
which were not possessed by earlier prior art rectennae constructed of 
discrete components; such as those described in a further article of W.C. 
Brown entitled "The History Of The Development Of The Rectenna", 
publication of the S.P.S. Microwave Systems Workshop, Rectenna Session, 
Lyndon B. Johnson Space Center, Houston, Texas, Jan. 15-18, 1980. 
With the exception of the inclusion of rectifier diodes, all components of 
the improved rectenna were etched on a single thin-film dielectric sheet. 
Therefore, when compared with earlier discrete component rectennae, the 
potential fabrication costs for volume production are very low. 
Furthermore, the structural weight of the thin-film rectenna is very low 
(eg. typically less than 100 grams-per-square-meter), and the thin-film 
fabrication is very flexible and can be conformed to curved surfaces such 
as aircraft wings, etc. 
However, the improved rectenna disclosed by W.C. Brown also suffers from 
the principle disadvantage of well known prior art discrete component 
rectennae, which is that for high efficiency power reception the 
polarization orientation of the field should be parallel to that of the 
rectenna dipoles, resulting in the necessity of expensive and complex 
polarization tracking equipment. 
A further prior art system is described in U.S. Pat. No. 3,681,769 which 
teaches the use of multiple phased arrays of orthogonal dipoles disposed 
on separate planes and interconnected via transmission lines for 
transmission of radio signals in a dual polarized beam. 
However, this prior art approach suffers from poor efficiency performance 
due to shielding effects caused by the transmission lines. As an example, 
when two thin-film rectennae of the type and dimensions described in the 
latter mentioned article by Brown are laid out in two orthogonal 
foreplanes as taught by U.S. Pat. No. 3,681,769, it can be readily shown 
that approximately 30-40% of the power in one polarization is prevented 
from being received by the transmission lines of the other foreplane. 
Furthermore, such phased arrays of orthogonally disposed dipoles are 
subject to extremely poor directivity when applied to systems in which the 
angle of beam incidence varies (e.g. in systems characterized by relative 
movement between the transmitting and receiving stations, such as in an 
electrically propelled airborne transportation system). This is because 
the directivity of such arrays is proportional to the ratio of the 
wavelength to the dimensions of the array. 
In addition, as the separation between the planes is reduced to 
electrically small values, as would often be necessary for conformal 
applications, mutual coupling between the dipoles and transmission lines 
is known to occur, thereby reducing the reception and conversion 
efficiencies even further. 
One approach to eliminating the prior art requirement for polarization 
tracking equipment has been to replace the linearly polarized dipole array 
rectenna with a circularly polarized microstrip antenna array, as 
described in U.S. Pat. No. 4,079,268. However, it is believed that such a 
proposed microstrip antenna array would be incapable of achieving the 85% 
or better reception efficiencies which are characteristic of linearly 
polarized thin-film dipole rectennae. 
SUMMARY OF THE INVENTION 
According to the present invention, a dual polarization system is provided 
comprised of one or more pairs of orthogonally disposed rectennae and a 
backplane, wherein each pair of rectennae is aligned according to a 
specific and predetermined pattern and separated by a predetermined 
distance, and the respective pairs of rectennae are separated from the 
backplane by predetermined amounts in order to compensate for the 
shielding effect of the transmission lines. 
More particularly, the transmission lines and dipole antennae of each pair 
of rectennae are oriented such that lines parallel to and midway between 
the transmission lines on a first one of the rectenna foreplanes are 
aligned with lines parallel to and midway between the dipoles on the other 
rectenna foreplane. Conversely, lines perpendicular to and midway between 
the dipoles of the other rectenna foreplane are aligned with lines 
parallel to and overlying the dipoles of the first rectenna. 
The system according to the present invention is capable of receiving and 
converting a high fraction of the total power in an incident dual 
polarization electromagnetic field irrespective of polarization 
orientation movement between the transmitter and rectenna receiving 
system. The system also exhibits reduced signal reception efficiency 
losses as compared to prior art multiple foreplane systems over a wide 
range of beam incidence angles. Moreover, the system has twice the power 
handling capability per unit area of the prior art single linear rectenna 
array.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning briefly to FIG. 1A, a microwave-powered aircraft V is shown in 
flight over a microwave transmitter T.sub.x. The aircraft has a microwave 
power receiver and converter A mounted on the aircraft V for receiving 
microwave energy from the transmitter T.sub.x and converting the received 
energy to useful power for driving a propeller P (or other propulsion 
means) as well as supplying payload power for operating on-board 
equipment. Although FIG. 1A illustrates wing mounted receiver/converters 
A, additional receivers can be mounted elsewhere on the aircraft for 
effecting a larger power reception surface. 
With reference to FIG. 1B, the dual polarization electromagnetic power 
receiver and converter of FIG. 1A is shown in accordance with the 
principles of the present invention in its most general form. X and Y 
oriented rectenna foreplanes 1 and 2 respectively, are disposed in 
parallel with one another for intercepting a portion of an electromagnetic 
beam 3 transmitted perpendicular to the foreplanes 1 and 2. 
According to the environment in which the present invention operates, a 
transmitter antenna (T.sub.x in FIG. 1A) emits dual polarized waves, (i.e. 
waves of two orthogonal polarizations) which can be unequal in either or 
both of amplitude and phase. This class of transmitted waves includes the 
well known cases of linearly and circularly polarized waves. Accordingly, 
the two orthogonal field components of the incident beam 3 can be resolved 
into components aligned into each of the two directions, X and Y. 
As shown in FIG. 1B, the rectenna foreplane 1 is comprised of an array of 
linearly-polarized half-wave dipole antenna elements 5A oriented parallel 
to the X direction. Therefore, the dipole antenna elements 5A are capable 
of selectively receiving the transmitted wavefield component which is 
oriented in the X-direction. 
The other orthogonal component of the transmitted wavefield, which cannot 
be received on the dipole elements 5A of foreplane 1, continues to 
propagate through the foreplane 1 and is incident on foreplane 2. The 
foreplane 2 is comprised of a second thin-film printed circuit rectenna 
comprised of a further array of linearly-polarized half-wave dipole 
elements 5B, oriented parallel to the Y-direction. Therefore, the 
additional dipole elements 5B can selectively receive the orthogonal 
component of the transmitted wavefield oriented in the Y-direction. 
The dipole elements 5A and 5B are connected via transmission line busses 8A 
and 8B respectively, as discussed in greater detail below with reference 
to FIGS. 2A, 2B and 2C. Also, the foreplane 1 is separated from foreplane 
2 by a distance of m.lambda./2, where m is an integer value (including 0) 
and .lambda. is the wavelength of the received microwave beam 3. 
Furthermore, foreplane 2 is separated from reflector plane 4 by a 
predetermined distance "p", as discussed in greater detail below. 
Turning to FIGS. 2A and 2B, the rectenna foreplanes 1 and 2 are shown 
positioned relative to one another according to a predetermined pattern, 
resulting in an increase in the overall dual polarization power reception 
efficiency over prior art multiple foreplane approaches. 
In particular, FIG. 2A illustrates the format and dimensions of the 
rectenna foreplane 1. Half-wave dipole antenna elements 5A are oriented in 
the X-direction and are configured in a repetitive spaced array of spacing 
l, each dipole element 5A being connected to wave filters 6A and rectifier 
circuits 7A, and to adjacent dipole elements 5A. The transmission lines or 
busses 8A are disposed orthogonal to the dipole elements, for collecting 
the converted power from each element of the array. 
FIG. 2B illustrates the identical components on the second rectenna 
foreplane 2. The half-wave dipole antenna elements 5B are oriented in the 
Y-direction. Thus, the arrangement on rectenna foreplane 2 is the same as 
that on foreplane 1 except that it is rotated 90.degree. relative to 
foreplane 1. Furthermore, the lines parallel to and midway between the 
transmission lines 8B on foreplane 2, (denoted by lines of symmetry B'B' 
in FIG. 2B), are aligned with the lines parallel to and midway between the 
dipole elements 5A in foreplane 1 (denoted by lines of symmetry B--B in 
FIG. 2A). Conversely, the lines perpendicular and midway between 
respective rows of the dipole elements 5A in foreplane 1 (denoted by lines 
of symmetry A--A in FIG. 2A) are aligned with the lines parallel to and 
along the dipole elements 5B of foreplane 2, (denoted by lines of symmetry 
A'--A' in FIG. 2B). 
In order to facilitate a better understanding of the novel characteristics 
of the present invention, the prior art concept of the "independent 
transmission line cell" will be explained. For a dual polarized beam, 
normally incident on the plane of a rectenna array (see FIG. 1), it is 
well known that the electromagnetic boundary conditions for each component 
of the two orthogonally polarized waves are not affected by the existence 
of idealized magnetic and electric walls erected along predetermined 
planes of symmetry perpendicular to the rectenna foreplanes. 
For example, the x-polarized plane wave is characterized by electric walls 
constructed on planes located at one half the distance between adjacent 
pairs of transmission lines, (eg. passing through lines A--A and A'A' in 
FIGS. 2A and 2B, respectively), and magnetic walls constructed on planes 
located at one half the distance between adjacent parallel dipole elements 
(e.g. through lines B--B and B'B' in FIGS. 2A and 2B, respectively). 
These imaginary electric and magnetic walls extending in front and behind 
the foreplanes 1 and 2 define identical transmission line cells enclosing 
each dipole element of the arrays. It has been determined mathematically 
that when considering the electromagnetic boundary conditions for 
orthogonally polarized waves, the field outside of the cell may be 
completely ignored and the array behavior determined from the behavior of 
a single transmission line cell, such as that represented by the hatched 
areas of FIGS. 2A and 2B, for the x-polarized component of the received 
wave. In other words, all mutual coupling due to neighbouring elements is 
automatically taken into account by the specific configuration of the 
foreplanes 1 and 2. 
From FIG. 2B it is seen that, according to the specific configuration of 
the present invention, the dipole elements 5B lie along the aforementioned 
electric walls and therefore do not affect the transmission line 
characteristics. The rectenna transmission lines 8B also appear as purely 
inductive strips across the electric walls of the cell. 
Similar cells can be constructed for analyzing the characteristics of the 
Y-polarized wave. 
By considering the configuration of foreplanes 1 and 2 according to the 
above-described concept of the independent cell, a series of foreplanes 
and reflectors can be equated for analytical purposes with a series of 
electric network elements connected by free space transmission lines as 
shown in FIG. 2C, whereby all of the electromagnetic field considerations 
of the rectenna structure can be translated and reduced to a simple 
electric network problem. 
Specifically, with reference to FIG. 2C, for the X-polarization, a 
transmission line cell becomes a transmission line 10a carrying power from 
a distant X-polarization microwave transmitter 12a. This transmission line 
10a is shunted at foreplane 1 by rectenna dipole elements 5A (terminated 
with a linear load), shunted at foreplane 2 by the transmission lines 8B 
which lie across the electric walls, and terminated by reflector plane 4 
at a distance "p" from the foreplane 2 (see also FIG. 1B). The 
characteristic impedance Z.sub.o represents the impedance of the 
transmission line 10a in free space. 
Similarly, for the Y-polarization, the transmission line cell becomes a 
transmission line 10b carrying power from a distant Y-polarization 
microwave transmitter 12b and is shunted at foreplane 2 by dipole elements 
5B and at foreplane 1 by inductive transmission lines 8A, and terminated 
by the reflector plane 4 forming a short circuit connection. 
It is then a standard network problem to show that when the two foreplanes 
1 and 2 are separated by a distance of m .lambda./2, where m may take any 
integer value, the effect of the foreplane transmission lines 8A and 8B 
may effectively be compensated for. This is accomplished by adjusting the 
reflector spacing "p" (FIG. 1) to capacitively balance the effect of the 
inductive strips at the rectenna foreplanes (i.e. the capacitive reactance 
of the short circuited transmission lines 10a and 10b at reflector plane 4 
is made equal and opposite to the inductive reactance caused by the 
transmission lines 8A and 8B such that all of the power in the 
transmission lines 8A and 8B is absorbed by the matched antenna load). It 
should be noted that "m" may take the value zero (i.e. for conformal 
applications), provided electrical isolation between the foreplanes 1 and 
2 is maintained. 
To confirm that the above-described objects of the invention have been met, 
tests were carried out using circularly-polarized transmitted waves. It 
was found out that a successful prototype of the embodiment of the present 
invention resulted in a reception efficiency degraded by less than 5% 
below that which could be obtained with a single linearly-polarized 
thin-film rectenna constructed according to prior art. However, according 
to the present invention, no expensive and complex polarization tracking 
equipment was required to maintain high efficiency reception in the event 
of rotational movement between the transmitter and receiver. 
Moreover, a successful prototype of the present invention has been 
incorporated into the world's first microwave powered aircraft which has 
now completed many test flights under rigorous conditions. All test 
flights have established the utility of the invention as well as the 
proven feasibility of remotely powered moving systems. 
A person understanding the present invention may conceive of other 
embodiments or variations therein. For example, whereas the disclosed 
embodiments relate to rectenna arrays having a square layout, existing 
prior art rectangular or triangular dipole element layouts may be 
reconfigured in a square layout embodying the principles of the present 
invention. Also, the restriction on foreplane separation may be eliminated 
if separate reflector grids are used for each polarization. 
The theory of operation of the present invention described above with 
reference to FIGS. 1B, 2A and 2B considered only the case of a beam 
normally incident on an array. However, in accordance with an important 
feature of the present invention, the method of compensation described 
above is applicable to any specified angle of incidence, suitable 
modifications being made to the transmission line cell characteristic 
impedance and lengths in FIG. 2C. 
The specified angle is usually chosen to be that which is most desirable 
for matching the antenna to its power conversion circuit over the 
operational range of beam incidence, and it (though not the polarization 
orientation) can often be strictly controlled, in order to maintain the 
impedance stability necessary for total energy absorption. Due to the 
analogy between an off-broadside angle of incidence and an inclined 
transmission line cell, the effect of the inductive strips may still be 
compensated for and the transmitted power received by the matched antenna 
load. 
In cases where the range of beam incidence cannot be carefully limited 
(e.g. banking of the aircraft V relative to the microwave beam in FIG. 1A, 
or movement of the reception system over long distances,) the variation in 
rectenna reception efficiency due to varying angles of beam incidence is 
reduced according to the transmission line compensation scheme of the 
present invention, with suitable selection of foreplane separation and 
reflector spacing. 
For example, for a dual polarization rectenna of foreplane separation 0.08 
.lambda. and a reflector plane 4 located 0.23 .lambda. behind foreplane 2, 
the efficiency of power reception has been computed to vary from 96% to 
80% as the angle of beam incidence varies from 0.degree. to .+-.50.degree. 
from broadside. This may be compared to a change in efficiency of from 
100% to 67% for prior art rectennae, over the same variation in angle of 
beam incidence. 
Hence, power transmission wavefields can be received according to the 
present invention over a wide range of incidence angles. 
Furthermore, once the dual polarization system is formulated in network 
terms according to the configuration of the present invention, the effect 
of changes or modifications to the system may be quantified and 
compensated for according to the aforementioned network model. For 
example, dielectric material may be inserted above or between the 
respective foreplanes for mechanical considerations, resulting in changes 
in the characteristic impedance Z.sub.o above and between the foreplanes. 
Also in certain applications the required DC power from a rectenna system 
may be more than can be handled by two foreplanes. Therefore, as shown in 
FIG. 3, multiple foreplanes (1, 1A...1N, 2, 2A...2N) for each polarization 
(separated by a multiple of half wavelengths), can be arranged to share 
the power absorbed in each polarization direction. However, the parallel 
conversion circuit impedances must be chosen to match the transmission 
line cell impedance as discussed above. 
Moreover, although the successful prototype of the present invention 
utilized a microwave power transmission, reception and conversion system, 
it is contemplated that systems could be developed using the principles of 
the present invention applied to power conversion of electromagnetic 
energy in other frequency bands (e.g. radio, laser, etc.). 
Also, whereas the successful prototype of the invention was implemented on 
a microwave powered aircraft, it is contemplated that the principles of 
the present invention may be applied to developing other land, air, sea or 
space-based transportation systems, or providing payload power to remote 
equipment, (e.g. high-powered radar, microwave repeater platforms, 
on-board sensors, etc.). 
These and other modifications or variations are believed to be within the 
sphere and scope of the present invention as defined in the claims 
appended hereto.