Non-Dispersive Acoustic Transport Time Delay Beam Steering Antenna

This invention relates in general to RF beam forming apparatus and more particularly to non-dispersive beam steering apparatus for a phased array antenna operable at UHF frequencies.

Electronically controlled phased array antennas are generally known and are comprised of three main parts, namely the radiating elements, the phase shifters, and feed network coupled to a source of RF energy in the case of a transmitter. For reception, the feed network is replaced by a receiver. In order to electronically control the antenna elements so that the beam can be steered electronically in space, it is normally necessary to use many closely spaced individual radiating elements with individual phase shifters controlling the elements in a piecewise fashion. The phase shifters themselves heretofore have taken many forms and designs. The subject of phased array antennas, moreover, are broadly covered in many publications. A typical example of such teachings is provided in Chapter 21, "Phased Array Antennas--An Overview/Knittel", pp. 290-301 of a text entitled Radar Technology by Eli Brookner, published by Artech House, Inc., October, 1986.

It is an object of the subject invention, therefore, to provide an improvement in RF antenna apparatus.

It is another object of the invention to provide an improvement in the control of phased array antennas.

And it is yet another object of the invention to provide an improvement in the beam steering control of phased array antennas.

Briefly, the foregoing and other objects are achieved by a plurality of acoustic charge transport (ACT) tapped delay lines coupled to respective antenna elements of a phased array antenna assembly to control the beam steering of either a transmitted or received electromagnetic wave in the megahertz (MHz) range. Each delay line, moreover, is comprised of an ACT channel region which operates as a delay line and further including multiple signal output taps which can be selectively addressed for providing a predetermined delay. With each delay line being individually controlled, an improved technique for beam steering is provided.

These and other objects, features and details of the invention will become apparent in light of the ensuing detailed disclosure, and particularly in light of the drawings wherein:

FIG. 1 is an electrical block diagram illustrative of an acoustic charge transport delay line;

FIG. 2 is an electrical block diagram illustrative of an acoustic charge transport tapped delay line; and

FIG. 3 is an electrical block diagram illustrative of the preferred embodiment of the invention.

Referring now to the drawings and more particularly to FIG. 1, shown thereat is an acoustic charge transport (ACT) device configured as a delay line. An ACT device combines many of the performance and implementation features of charge couple devices (CCDs) and surface acoustic wave (SAW) devices fabricated in gallium arsenide (GaAs).

An ACT delay line comprises a high speed monolithic GaAs charge transfer device that is capable of providing RF signal delay. This function is achieved through the conversion of an analog input signal voltage to discrete charge packets that are translated through a semiconductor channel at a fixed velocity in accordance with a SAW launched on the channel and which are subsequently sensed at an output detection point. Charge packet transport is accomplished in a buried channel utilizing a piezoelectrically induced traveling wave electric field of a single frequency UHF surface acoustic wave that is generated directly in the GaAs. By analogy with the conventional charge coupled device, the propagating SAW function is a built-in clock signal that results in continuous charge transfer precisely at the characteristic SAW velocity, which is approximately 2864 m/sec. in a GaAs medium.

Shown in FIG. 1 is a typical proton isolated ACT delay line 10 formed on a GaAs substrate 11. An elongated ACT channel 12 is furthermore located on the substrate 10 and is bounded on either end by input and output regions 14 and 16 and including an input contact 13, a gate contact 15, and an output contact 17 coupled to the channel region 12. Adjacent the input region 14 there is formed an isolation region 18 fabricated by a proton isolation implant that renders the epitaxial layer outside the channel region 12 semi-insulating. Additionally, an SAW transducer element 20 and a SAW reflector 22 are positioned adjacent the input region 14.

In operation, the transducer 20 generates a relatively large, typically one volt, amplitude surface acoustic wave at an effective clock frequency determined by the characteristic periodicity cf the transducer. The delay line then consists of an input section including elements 13, 15 and 20 to the left of the isolation region 18, ACT delay element 12 to the right of the isolation section 18, and an output section including element 17 at the other end of the channel 12, all of which are illuminated by the SAW from the transducer 22.

Referring now to FIG. 2, an ACT delay line as shown in FIG. 1 can be formed into a programmable delay line 24 which includes an ACT delay channel region 12. The output section 16, however, is now comprised of a plurality of non-dispersive signal taps 26.sub.1 -26.sub.n to provide signals at the output of a serial chain of output gates 28.sub.1 -28.sub.n which are digitally controlled by an address buss 30. Non-dispersive absolute delays of 40 nsec. to 2.6 .mu.sec. or relative delays of 0 to 2.5 .mu.sec can be provided when operated at 360 MHz, for example. Typically, an array of taps 26.sub.1 -26.sub.n can include as many as 1024 taps spaced by 2.8 nsec. in time.

A plurality of ACT tapped delay lines 24.sub.1 -24.sub.n are utilized as shown in FIG. 3 to implement beam steering in a phased array antenna 32 comprised of a plurality of mutually spaced elements 34.sub.1, 34.sub.2 . . . 34.sub.n by coupling each of the tapped delay lines 24.sub.1 -24.sub.n to a signal splitter 36 in the case of a transmitter or a signal combiner, not shown, in the case of a receiver. As illustrated, an RF input signal Ein is split and fed to each of the antenna elements 34.sub.1 -34.sub.n. The signal fed to each antenna element is delayed by a predetermined time .DELTA.t.sub.o by a digital controller 38 coupled to a respective address bus 39 (FIG. 2), whereupon a resultant wave E(.THETA.) is generated and radiated at an angle .THETA. from the center line 40 of the array.

An array 1024 elements 34 operating at a frequency of 360 MHz, for example, would yield a beamwidth .THETA.B of:

where .lambda. is the wavelength and A is the aperture. Such an array would also yield a gain G of:

Where scanning is provided to an off broadside target, the instantaneous bandwidth is not limited by the array since beam scanning is not a function of frequency change. The following equation is descriptive of time delay beam steering achieved by such an array:

where E(.THETA.) is the antenna field pattern, E.sub.e is the RF input, .THETA. is the direction angle off of the array center line, and n is the number of antenna elements.

Thus what has been shown and described is a non-dispersive time delay beam former implemented by way of digitally controlled acoustic charge transport delay lines which can be digitally controlled to effect beam steering.

Having thus shown and described what is at present considered to be the preferred embodiment of the invention, it should be noted that the same has been made by way of illustration and not limitation. Accordingly, all alterations, changes and modifications coming within the spirit and scope of the invention are herein meant to be included.