Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-03-02
2001-03-20
Negash, Kinfe-Michael (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C250S227120, C342S375000
Reexamination Certificate
active
06204947
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to array antennas and is particularly concerned with providing time delay steering to array antenna elements. This application is a co-pending U.S. patent application Ser. No. 09/017,099, filed Feb. 2, 1998.
2. Description of the Prior Art
An array antenna consists of a group of antenna elements uniformly spaced apart to form an array. The array can be used for transmitting a beam of microwave energy in a chosen direction or receiving a microwave signal from a particular direction. This beam steering is achieved by controlling the relative timing or phasing of the individual elements.
The most common means of steering a beam in an antenna array is to control the relative phase of the signal of the elements. For the case of a flat antenna array, if all the elements are operated in unison, the beam will be pointed in the boresight direction, which is the direction perpendicular to the plane of the array. If a linearly increasing phase shift is introduced across the face of the array, the beam will be deflected at some angle from the boresight direction. Such antenna systems, referred to as phased arrays, are employed in applications where it is required to steer the beam rapidly in space and where the use of parabolic dish antennas is not practical.
Controlling the relative phase of each of the antenna elements requires that each element contains a phase shifting device and that an electronic control system be used to control the phase of each of the elements. However, the wide scale use of phased arrays has been limited by the high cost of their complex circuitry. Furthermore, if the phase shifting circuit is adjusted to steer in a particular direction, this setting will only be valid for a particular frequency. Adjacent frequencies will be transmitted or received with directional errors, a phenomena known as “squint”. Therefore, known phase shifting techniques impose a limit on the frequency range of operation.
Another technique that is used to steer the beam in an array antenna is to control the relative timing of the transmitted or received signal at the array element. In the transmission mode, if the signal at each of the elements is emitted in unison, a wavefront is formed that is parallel to the plane of the array. The signal beam is directed perpendicular to the wavefront, therefore, when the signal is emitted from the antenna elements in unison, the beam is directed perpendicular to the plane of the array (the boresight direction). When the emission from the antenna elements is not in unison, but is varied in time along the array, the angle of the wavefront relative to the plane of the array changes and the beam is steered away from boresight. If, for example, the signal emission from any element relative to its nearest adjacent element is delayed a time t and each element is spaced a distance d apart, the steered angle &phgr; between the boresight direction and the beam direction is given by the formula sin &phgr;=tc/d, where c represents the velocity of electromagnetic propagation in space. True-time delay techniques allow antenna arrays to operate over extremely wide frequency ranges as the delay techniques are frequency independent.
The use of fiberoptic communication systems is known. A commercially available laser unit is used to convert a microwave signal to an amplitude modulated optical signal. The optical signal travels through the optical fiber to where it is converted back to a microwave signal by an optical detector and a microwave amplifier, which are commercially available.
Optical techniques have been suggested to control array elements. Schemes have been proposed to use a selection of optical fibers with lengths arranged in a binary or quadratic sequence and to switch in a series string combination to achieve a desired timing. This would result in a very complex control scheme employing thousands of optical fibers and optical switches for even the simplest array.
An optical commutator scheme using two sets of fiber optics, each having a parabolic distribution of lengths has been described in U.S. Pat. No. 5,347,288. By aligning these two sets of fibers and moving one set relative to the other, a linear and variable set of delay paths can be generated which can be incorporated into an antenna array to provide the timing needed to form and steer the beam. The optical commutator uses far fewer optical delay lines than any other scheme known. However, the optical commutator must employ a large number of fibers if it is required to steer the antenna array with great precision and to be able to adjust the pointing direction in small fractions of a degree.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a device that performs the steering and timing function for an antenna array. Another object of the present invention is to provide an array antenna which can be steered with great precision. Still another object of this invention is to provide coarse and fine steering adjustment.
In this invention, the timing scheme for a particular array antenna design is ‘hard wired’ by having two sets of optical fiber delay lines each built into a separate movable element. The optical fiber delay lines of the first movable element are of selected lengths that vary over a wide range and have first ends which are alignable to a set of input optical fibers. The input fibers are fed from one or more amplitude modulated light sources representing the signals of the antenna elements. The second ends of the optical fibers are alignable to a set of output fibers of selected lengths. These output fibers are connected to a second set of input fibers that in turn are alignable to the firs tends of a second set of fibers mounted on a second movable element. These movable fibers are of selected lengths that vary over a fine range and have second ends that are alignable to a second set of output fibers having selected lengths.
The second output optical delay lines are connected to one or more optical detectors that are capable of converting the optical modulated signal into an RF signal. The two movable sets of optical fibers and the three sets of stationary optical fibers together with the associated mechanism and components are referred here as a “multi-stage optical commutator”.
In the case of a transmitter, the parallel signals are fed to the respective radiating elements in an array. In the case of a receiver, the signal is combined with other parallel signals from other receiving elements to form the composite received signal.
As the first and second movable elements are moved, each of the parallel signals is transmitted through a selected optical delay path to the radiating element in the case of a transmitter. In the case of a receiver, signals from the antenna elements are passed through various parallel delay paths and then combined. By controlling the amount of time delay for each antenna element, a beam may be formed and steered.
Moving the first movable element causes the beam to steer in large, coarse increments in space. Moving the second movable element causes the beam to steer in fine increments in space.
Other objects and advantages of the invention will become apparent from the description of certain present preferred embodiments thereof shown in the drawings.
REFERENCES:
patent: 4028702 (1977-06-01), Levine
patent: 5325102 (1994-06-01), Page
patent: 5347288 (1994-09-01), Page
patent: 5856805 (1999-01-01), Page
patent: 5923291 (1999-07-01), Page
patent: 6002365 (1999-12-01), Page
H.C. Lin Patent Agent
Negash Kinfe-Michael
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