|Publication number||USH173 H|
|Application number||US 06/859,283|
|Publication date||Dec 2, 1986|
|Filing date||Apr 30, 1986|
|Priority date||Apr 30, 1986|
|Publication number||06859283, 859283, US H173 H, US H173H, US-H-H173, USH173 H, USH173H|
|Inventors||Kenneth D. Claborn, William C. Bailey|
|Original Assignee||The United States Of America As Represented By The Secretary Of The Army|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (27), Classifications (5), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The Government has rights in this invention pursuant to Contract No. DAAK80-80-C-0035 ordered by the Department of the Army.
This invention relates generally to phased array antennas and more particularly to the compensation of beam pointing errors resulting from frequency and temperature changes.
In airborne and ground based phased array antenna systems driven by a series-fed power divider and operating over a selected range of frequencies, beam pointing errors and undesired antenna sidelobes are known to be generated as a function of the expansion and contraction of the aperture as a function of temperature and more particularly over the range from -50° C. to +70° C. when operating at a predetermined frequency. Phased errors resulting from expansion and contraction of the manifold of the power divider are also known to exist as a result of temperature changes. Heretofore, compensation for these errors was accomplished by constructing the antenna components from relatively expensive material having a low temperature coefficient of expansion, a typical example being invar, an alloy of iron and nickel. This mechanical approach has been found to be relatively costly and, at most, an approximation.
Accordingly, it is an object of the present invention to provide an improvement in phased array antenna systems.
It is a further object of the invention to provide an improvement in the compensation for pointing angle errors introduced by dimensional changes of the antenna beam forming structure.
It is yet another object of the invention to provide electrical compensation for pointing errors and sidelobes due to temperature in a phased array antenna.
And it is still a further object of the invention to electronically shift the antenna beam forming angle to compensate for pointing angle errors and sidelobes introduced by dimensional changes of the phase antenna beam forming structure due to temperature changes when operated over a relatively large frequency band.
Briefly, the foregoing and other objects of the invention are accomplished by a method and apparatus implemented in an array beam steering unit whereby beam pointing errors and sidelobes resulting from temperature variations on the aperture and power divider of a series-fed phased array antenna operating over a large range of frequencies are compensated for electronically. A temperature sensor is placed on both the aperture and power divider of the array which includes uniformly spaced elements. Temperature signals corresponding to the temperature of the aperture and power divider are converted to digital signals which are applied along with a digital signal corresponding to the selected operating frequency to digital circuit means which generate address pointers for respective non-volatile programmed memories which read out digitized beam steering phase gradients and feed phase corrections, respectively, which are summed and applied at regular intervals to symmetrically located phase shifters coupled to respective elements of the array in accordance with a predefined sequence.
While the present invention is defined in the claims annexed to and forming a part of the specification, a better understanding can be had by reference to the following description when taken in conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram generally illustrative of the subject invention;
FIG. 2 is a diagram further illustrative of the phased array antenna shown in FIG. 1;
FIG. 3 is a block diagram further illustrative of the beam steering unit shown in FIG. 1; and
FIGS. 4A and 4B are diagrammatic illustrations of the programmable read only memories utilized in the beam steering unit shown in FIG. 3 for providing respective outputs for determining coarse phase gradients and feed phase corrections.
Referring now to the drawings, reference numeral 10 of FIG. 1 denotes a phased array antenna aperture including a plurality of uniformly spaced radiating elements 12 which generate a composite beam 14. The beam 14 defines a phase gradient φ as shown in FIG. 2 which is perpendicular to the direction of radiation. A beam pointing angle (θ) offset from antenna boresight 15 is defined by the equation: ##EQU1## where φ is the incremental phase between elements, λ is the wavelength in air and d is the physical spacing between elements. This is furthermore shown diagrammatically in FIG. 2. This figure additionally discloses a plurality of phase shifters 16 which couple RF energy to respective radiating elements 12 as shown in FIG. 1. The phase shifters are further divided into a left and right side set 18 and 20. The two sets of phase shifters 18 and 20 are coupled to and receive RF energy from a series center fed power divider 22. The power divider 22 comprises a dielectrically loaded stripline power divider and is fed RF energy from a power amplifier 24 coupled to the output of an RF signal source 26 which is operable at a selected one of a plurality of operating frequencies f consisting of, for example, 200 discrete channel frequencies within a portion of the C band of the electromagnetic spectrum.
The steering angle θ of composite beam is controlled by a beam steering unit 28 which generates left and right fine phase command signals, as will be subsequently explained, which are coupled to the left and right side phase shifters 18 and 20 at regular intervals in a predefined sequence of individual phase shifters.
The phase gradient φ for any steering angle θ can be defined by solving equation (1) for φ, or ##EQU2##
If it is assumed that φ and λ are invariant versus temperature, then θ is dependent on variations of element spacing d (FIG. 2) with temperature and d can be expressed as:
where d is the nominal spacing at the standard temperature (typically 20° C.), e is the coefficient of expansion in inch/inch/°C., and T is the temperature expressed in degrees centigrade. It is known, however, that phase gradient φ does change as a function of temperature. For a series center-fed power divider, a V-shaped gradient 14' results across the array as shown in FIG. 1. By substituting equation (3) into equation (1) and (2), and referencing the result to φ as computed at 20° C., an expression for the error Δθ as a function of temperature results which can be expressed as: ##EQU3## where θ20 is the beam pointing angle at 20° C.
This now leads to a consideration of the inventive concept of the subject invention wherein it is desired to compensate for any change in phase gradient with respect to temperature caused by both the antenna aperture 10 and the power divider 22 for a particular operating frequency. This involves adding a separate temperature sensor 20 and 32, respectively, to the physical structures of the antenna aperture 10 and the power divider 22. Both temperature sensors are operable to generate an analog electrical output signal which is fed to respective analog to digital (A/D) converters 34 and 36 which generate prescaled and quantized digital words which are used as inputs to the beam steering unit (BSU) 28. As shown in FIG. 3, the BSU 28 includes digital means for generating phase shifter drive signals which are applied one at a time to each of the phase shifters 16 in a predetermined sequence.
The compensation scheme involves the following consideration. If one examines equation (2) and rewrites it with d as a function of temperature normalized to 20° C. and replaces λ by c/f, where c is the velocity of propagation and f is the operating frequency, the expression for the phase gradient φ can be expressed as: ##EQU4## Expressed in this form, all terms are constant except f and T. Compensation accordingly comprises the selection of a proper set of beam steering gradients for a particular operating frequency or channel and sensed temperature and involves generating an address or pointer to a programmed digital memory which will now be explained.
Beam steering is accomplished by a method which is known as the COARSE/FINE scanning technique. With this technique, the steering phase for each antenna element 12 is calculated at discrete steps known as the coarse scan step. These phases are then applied at regular intervals to symmetrically located phase shifter pairs 16 as shown in FIGS. 1 and 2 according to a predefined sequence. Each pair of phase shifters in turn causes the beam 14 to move a fraction of the coarse scan step, and is known as the fine scan step.
Referring now to FIG. 3, disclosed there are the details of the beam steering unit 28. It comprises a digital compiler wherein analog voltages representative of each temperature i.e. of the antenna aperture 10 and power divider are preconditioned, and digitized through analog to digital converters 34 and 36 which generate 6-bit digital outputs. These digital temperature values are fed via data buses 38 and 40 to digital data latches 42 and 44 where the binary values are temporarily stored and updated periodically under the control of the scan gate control signal applied from a source, not shown. The channel RF frequency selected by the operator is also fed as an 8-bit binary word to a data latch 46. Both the quantized temperature and channel frequency data are thus both latched at the start of an angular guidance scan interval.
Disregarding feed phase correction for the present, the steering phase gradient φ for a beam pointing angle θ degrees from boresight 15, as shown in FIG. 2, can be expressed as equation (5) above. By lumping constants, equation (5) can be rewritten as:
φ=K sin θf[1-e(T-20° C.)] (6)
where θ is steering angle, f is operating frequency and T is temperature in degrees centigrade. Therefore,
φ=F(f, T) (7)
The determination of the coarse phase gradient can thus be calculated as a function of both frequency and temperature. This can be realized in view of the foregoing considerations. An uncompensated increase in frequency over some nominal values produces a pointing error in the same direction as that caused by an increase in the antenna aperture temperature over a given ambient temperature value. By inverting the analog temperature and prescaling the amplitude prior to digitizing, the quantized temperature may be directly added to the binary channel address forming an effective address or pointer that locates the coarse phase gradient value that satisfies the frequency and temperature parameters by effecting a shift in frequency or λ. Accordingly, the beam steering unit 28 includes a first PROM 48 which includes stored values of the term 2πd/λ which are read out in response to an 8-bit address pointer generated by an address computational logic block 50 and applied thereto via the digital data bus 52. The logic block 50 basically comprises a binary adder which sums the binary values of the temperature and frequency temporarily stored in the latches 42 and 46. The following Table A is illustrative of three resulting addresses for three different channel frequencies and three different temperatures, although it is possible for different frequencies and temperatures to provide the same coarse phase gradient address as shown in Table B.
TABLE A______________________________________FREQ. + TEMP. = COARSE PHASEChannel # Dig. Addr. °C. Dig. Value Gradient Addr.______________________________________0 00000000 0 011100 000111004 00000100 20 010100 0011000020 00010100 40 001100 00100000______________________________________
TABLE B______________________________________FREQ. + TEMP. = COARSE PHASEChannel # Dig. Addr. °C. Dig. Value Gradient Addr.______________________________________0 00000000 +20 010100 000101004 00000100 +30 010000 0001010020 00010100 +70 000000 00010100______________________________________
The PROM 48 is programmed to satisfy the function φ=F(f,T) for all of the required combinations of the frequency and temperature variables. Accordingly, a digital word corresponding to the value 2πd/λ is outputted for a particular memory address which is a function of temperature (-50° C. to +70° C.) and frequency (200 channels). Furthermore, as shown in FIG. 4A, the address pointer has a lower address number for increasing temperature, and decreasing frequency. A 16-bit binary output from the PROM 48 is fed via a 16-bit data bus 54 to a digital multiplier 56 which receives a 16-bit word from data bus 58 from a second PROM 60 which has a set of sin θ values stored therein for a plurality of beam steering command angles θ and which is fed to an address counter 62 under the control of a coarse clock input from a source, not shown. The multiplier 56 provides a 16-bit (2πd/λ) sin θ output signal in accordance with equation (2) on data bus 64 which comprises a compensated coarse phase gradient control signal for the phase shifters 16 shown in FIG. 2. In absence of any feed phase correction, the coarse phase gradient signal on data bus 64 is applied to a digital multiplier 66 where an element position number is multiplied therewith from an input from an antenna element position counter 68. The output of the multiplier is rounded to a 4-bit digital word (MSB=180°) corresponding to the fine phase drive signals applied to the appropriate phase shifter 16 in accordance with the sequence established. Thus each phase shifter pair receives its appropriate fine phase drive signal in turn with the fine phase received by a particular phase shifter being the 2's complement of the fine phase received by its symmetrically located mate.
In order to also provide for feed phase correction, the digital 6-bit temperature value of the power divider 22 temporarily stored in the latch 44 is fed to an address computational logic block 70 along with the 8-bit binary address of the operator selected channel frequency which is temporarily stored in the latch 46. The two binary values stored in the latches 44 and 46 are summed together in logic block 70 in the same fashion as shown in Table A to generate a feed phase correction address which appears as an 8-bit signal on data bus 72 for addressing a third PROM 74 which has a set of stored values of the term 2πd/λg where λg is the wavelength in the series-fed power divider and which can be expressed by the equation: ##EQU5## where λ0 is free space wavelength and E is the dielectric constant of the stripline dielectric.
The PROM 74 is similar to that of PROM 48 with the exception that a lower address is generated for decreasing frequency. A 16-bit digital word is fed out on the digital bus 76 where both the coarse phase gradient and the feed phase correction values are summed together in a binary adder 78. The output of the adder 78 is fed to the element position multiplier 66 whereupon the combined value of the coarse phase gradient and the feed phase correction is multiplied by the element position number to provide the respective fine phase drive signal for the appropriate phase shifter 16.
Thus what has been shown is a beam steering unit 28 which fetches digitized steering phase gradients and feed phase correction data from a pair of non-volatile memory storage units which are used to generate fine phase drive signals for a phased array antenna that is now compensated for with respect to both temperature and frequency.
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 modifications, alterations and changes coming within the spirit and scope of the invention are herein meant to be included.
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|U.S. Classification||342/372, 342/173|
|Mar 17, 1989||AS||Assignment|
Effective date: 19860403
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST. SUBJECT TO BE LICENSE;ASSIGNOR:BAILEY, WILLIAM C.;REEL/FRAME:005110/0917
Owner name: UNITED STATES OF AMERICA, THE, AS REPRESENTED BY T