Radio frequency antenna and mixer array

A radio frequency millimeter-wave signal antenna and mixer system includes a dielectric substrate having a generally planar upper surface, and a ground-plane layer of conductive material disposed below the upper surface. This ground plane layer includes a plurality of spaced-apart low conductivity sections or windows to allow passage therethrough of radio frequency signals received at a surface of the substrate. A plurality of antennae are disposed on the upper surface of the substrate, with each antenna being positioned over a respective one of the windows in the ground-plane layer. A plurality of mixers are also disposed on the upper surface of the substrate, each adjacent a respective antenna for receiving radio frequency signals therefrom and for producing intermediate frequency signals. A plurality of output conductors are formed on the upper surface of the substrate, each being coupled to a respective mixer to extend therefrom alongside the antenna from which respective mixer receives radio frequency signals. The output conductors carry intermediate frequency signals from the respective mixers to the edge of the substrate.

BACKGROUND OF THE INVENTION 
This invention relates to a radio frequency antenna and mixer system 
composed of a plurality of antennae and associated mixers, especially 
suitable for fabrication on a substrate utilizing semiconductor 
fabrication techniques. 
Imaging systems are designed for detecting or "viewing" a scene and then 
reproducing it on some type of display device. For example, existing 
imaging systems include radar systems, infrared night vision scopes, and 
the like. 
Typically, such systems have operated at electromagnetic frequencies 
outside of the millimeter-wave portion of the spectrum, i.e., not using 
electromagnetic energy having a wavelength of between ten millimeters and 
one millimeter, for frequencies of between 30 GHz and 300 GHz. Problems 
with lower frequency imaging systems, such as microwave radar systems, 
include lack of resolution. Problems with higher frequency imaging 
systems, such as infrared night vision scopes, include the difficulty of 
signals in this frequency range penetrating most fog, smoke, haze, rain, 
etc., and therefore the inability to produce a clear image under such 
circumstances. 
It has been recognized that the use of millimeter-wave imaging systems, if 
they could be constructed in a compact, lightweight, inexpensive fashion, 
would provide a number of advantages including the ability of signals in 
this frequency range being able to penetrate poor atmospheric conditions, 
while also providing high image resolution. One problem in implementing 
compact, lightweight, inexpensive millimeter-wave imaging systems has been 
the lack of technologies and component parts which could operate 
effectively at millimeter-wave signal frequencies. 
An example of one such component part or subsystem is an antenna and mixer 
array for intercepting radio frequency signals and processing the same for 
ultimate reproduction of the image represented by the signals. In 
particular, reflected or naturally emitted radio frequency signals from a 
target object would be detected by an array of receivers, including an 
array of antennae, the frequency of the received signals would be 
down-converted to an intermediate frequency, and then such intermediate 
frequency signals would be processed, using digital signal processing 
techniques to form an image of the target. Little, if anything, has been 
done to provide an antenna and mixer array in a fully solid state 
configuration, for operating in the millimeter-wave range. 
It is an object of the invention to provide a radio frequency antenna and 
mixer array for receiving a plurality of radio frequency signals and for 
developing intermediate signals therefrom, using circuitry capable of 
operating in the millimeter-wave range of the electromagnetic signal 
spectrum. 
It is another object of the invention to provide such an array which may be 
readily fabricated on a semiconductor substrate using semiconductor 
fabrication techniques. 
It is also an object of the invention to provide such an array which is 
compact, lightweight and relatively inexpensive. 
It is a further object of the invention to provide such an array in which 
inter-component electrical signal interference is minimized, and which 
allows for separate outputs for each antenna/mixer combination. 
SUMMARY OF THE INVENTION 
The above and other objects of the invention are realized in a specific 
illustrative embodiment of a radio frequency antenna and mixer system for 
receiving and down-converting millimeter-wave r-f signals to i-f signals, 
where the system includes a base having a generally planar upper surface 
and a lower surface, and formed of a material having low conductivity, 
with a layer of conductive materials disposed below the upper surface to 
act as a ground plane (return for signals being carried by circuitry on 
upper surface of the base). The layer includes a plurality of spaced-apart 
low conductivity sections or windows to allow passage therethrough of r-f 
signals received at the surface of the base. A plurality of antennae are 
each disposed in a collinear arrangement on the upper surface of the base, 
over a respective one of the sections or windows in the layer, and a 
plurality of mixers are each disposed on the upper surface of the base, 
adjacent a respective antenna for receiving r-f signals therefrom and for 
producing i-f signals. A plurality of conductors are each disposed on the 
upper surface of the base and are coupled to a respective mixer to extend 
therefrom alongside the antenna from which said respective mixer receives 
r-f signals. The conductors are for carrying i-f signals from the 
respective mixer to another circuit for utilization. 
In accordance with another aspect of the invention, each of said mixers 
includes a mixer circuit for heterodyning a r-f signal received from a 
respective antenna, with a local oscillator (l-o) signal, to produce an 
i-f signal which is supplied to a respective conductor. Each mixer also 
includes another conductor disposed on the upper surface of the base to 
extend to the mixer circuit from a direction generally away from the 
direction in which the i-f signal carrying conductor extends from the 
mixer circuit. This other conductor provides l-o signals from a local 
oscillator source to the mixer circuit.

DETAILED DESCRIPTION 
Referring to FIG. 1 there is shown a combination radio frequency antenna 4 
and mixer 8 suitable for use in an array of antennae/mixers fabricated on 
a semiconductor substrate such as shown in FIGS. 2, 3 and 4. The antenna 4 
is a so-called half-wave 60 degree "bow-tie" dipole antenna, having two 
halves 4a and 4b. The antenna, of course, is for receiving and coupling 
incident radio frequency signals into the mixer 8. As the incident radio 
frequency energy impinges on the antenna 4, the two halves 4a and 4b are 
alternately oppositely polarized in a well-known manner. 
The antenna 4 is coupled by a two conductor balanced line 12, via 
capacitors 16a and 16b (each disposed in a different one of the conductors 
of the balanced line 12) to a pair of semiconductor diodes 20a and 20b. 
The diodes 20a and 20b serve as the non-linear elements of the mixer 8 and 
might illustratively be Schottky-barrier diodes. The cathode of diode 20a 
is coupled to the capacitor 16aand the anode thereof is coupled to the 
cathode of diode 20b to form a junction or node 24. The anode of diode 20b 
is coupled to capacitor 16b. The received radio frequency signals drive 
the conductors of the balanced line 20 alternately to opposite polarities 
to thereby alternately switch the diodes 20a and 20b. 
Coupled between the node 24 and a local oscillator (l-o) input source 28 is 
a high pass filter 32. The local oscillator input source 28 provides a 
local oscillator signal to the mixer 8 where it is heterodyned with the 
received radio frequency signal, at the node or junction 24. Such 
heterodyning is well-known for down-converting the frequency of a received 
radio frequency signal to a lower intermediate frequency. A receiver which 
performs this function is known as a super heterodyne receiver. The 
intermediate frequency signal thus developed is supplied via a low pass 
filter 36 to an intermediate frequency output line 40. 
The capacitors 16a and 16b act as high pass filters to prevent local 
oscillator and intermediate frequency signals from reaching the antenna 4. 
The high pass filter 32 allows local oscillator signals to pass 
therethrough to drive the diodes 20a and 20bbut prevents intermediate 
frequency signals from reaching the local oscillator input signal source 
28. Similarly, the low pass filter 36 filters out and prevents local 
oscillator signals and radio frequency signals from reaching the 
intermediate frequency output line 40, while, of course, allowing 
intermediate frequency signals to reach the output line. 
An array of antennae/mixers of the type shown in FIG. 1 are shown in FIGS. 
2-4, formed on a planar structure, in particular a semiconductor substrate 
50. Utilizing a planar structure facilitates fabrication using 
semiconductor fabrication techniques which, it is well-known, are 
relatively inexpensive and allow for constructing compact circuitry. To 
allow for processing signals in the millimeter-wave portion of the 
spectrum, the substrate 50 advantageously is made of gallium arsenide. 
The component parts of the antenna/mixer array are formed on an upper 
surface 50a of the substrate (FIG. 2), with a conductive ground plane 54 
formed just below the surface 50a, to provide ground return for signals 
being carried by the components on the surface. If the substrate 50 is 
made of gallium arsenide, which is a dielectric, the ground plane 54 could 
be formed of a layer of doped gallium arsenide. Formed in the ground plane 
54 is an array of non-conductive sections or "windows" 58, the function of 
which will be described momentarily. The windows 58 could simply be 
undoped areas in the ground plane 54. 
FIG. 3 shows a top plan view of a sixteen element antenna/mixer array 
formed with solid state components upon the substrate 50. Each of the 
antenna/mixer elements are constructed in accordance with the FIG. 1 
configuration. FIG. 4 is a magnified view of a portion of the 
antenna/mixer array of FIG. 3, and will be used to identify the component 
parts of an antenna/mixer element. Each such element includes a planar 
half-wave 60 degree "bow-tie" dipole antenna 4 disposed on the surface of 
the substrate 50, over a dielectric window 58 (shown as white in FIGS. 3 
and 4). The shaded area of the substrate 50 represents the ground plane 54 
located below the surface of the substrate. Each of the antennae is 
located over a corresponding dielectric window so that radio frequency 
signals received at the surface 50b (FIG. 2) may pass through the 
dielectric substrate 50 and through the windows 58 to impinge upon the 
antennae. Otherwise, conductive material such as the ground plane 54, if 
positioned between a radio frequency source and an antenna, would 
interfere with and intercept the radio frequency signals so that it would 
not reach the antenna or would be significantly attenuated upon reaching 
the antenna. 
The antennae are disposed on the substrate 50 in a collinear arrangement, 
i.e., the antennae are aligned along a linear locus as best seen in FIG. 
3. Such a configuration results in significantly less coupling between the 
antennae and thus less interference between the receipt and processing of 
radio frequency signals. Of course, radio frequency signals could be 
received at the upper surface of the substrate 50 by the antenna 4. 
The antenna half elements 4a and 4b (FIG. 4) are coupled by respective 
conductors of a balanced line 12 to capacitors 16a and 16b respectively. 
The capacitors 16a and 16bin turn, are coupled respectively to diodes 20a 
and 20b. The diodes 20a and 20b are coupled to a node or junction 24 which 
is coupled to a high pass filter 32 by an unbalanced line 34, and also 
coupled to a low pass filter 36. The high pass filter 32 is coupled by 
conductors 38 and Wilkinson dividers 42 ultimately to a local oscillator 
source 28 (FIG. 4). A Wilkinson divider is a well-known conductor 
branching component. The low pass filter 36 is coupled to an output line 
40 which extends between adjacent antennae to exit the substrate. It will 
be noted that the output line 40 extends from the mixer in a direction 
opposite that in which the local oscillator conductor 38 extends. This 
configuration minimizes interference between the local oscillator signals 
and the intermediate frequency signals, and obviates the need for 
extending conductors or transmission lines through the substrate. The 
configuration of the collinear arrangement of the antennae and extending 
the intermediate frequency output lines between the antennae serves to 
minimize coupling between the antennae and the intermediate frequency 
output line. 
The width of the conductors of the balanced line 12 and the spacing 
therebetween are selected to minimize the impedance discontinuity 
encountered by radio frequency signal energy passing from the antenna to 
the mixer. Typically, the width of these conductors is in the millimeter 
range, e.g., 5 to 20 mm, and the gap spacing therebetween is less than the 
width, all depending on the wave length. 
With the antenna/mixer array configuration described above, a compact, 
lightweight and cost effective antenna/mixer system is provided for 
receiving and processing millimeter-wave radio frequency signals. The 
array may be fabricated using semiconductor fabrication techniques, and 
intercomponent coupling and interference is minimized. The dielectric 
windows 58 serve to focus the radio frequency energy into the array and in 
particular onto the antennae which are positioned over the windows. 
It is to be understood that the above-described arrangements are only 
illustrative of the application of the principles of the present 
invention. Numerous modifications and alternative arrangements may be 
devised by those skilled in the art without departing from the spirit and 
scope of the present invention and the appended claims are intended to 
cover such modifications and arrangements.