The present invention relates to the field of antennas, and more particularly, to the field repair and replacement of phased array antennas.
For phased array antennas, such as electronically scanned array (ESA) antennas, there is an emerging requirement to utilize modular arrays, in which standardized units or portions of the antenna (e.g., sub-arrays or a radio frequency (RF) feed network) are replaceable in the field as part of mission support. Driving this requirement is the desire to simplify and reduce the cost of repair or replacement of part of the antenna, for example, by reducing the size and cost of spares. Further, after replacement, the phase and amplitude of the antenna elements of a newly replaced sub-array, or those corresponding to a newly replaced feed network, must be calibrated (a process typically called phase-up). Thus, there is a desire in the art to eliminate the need to remove the entire antenna from the platform and either utilize special test equipment (STE) in the field or return it to the factory for recalibration or phase-up.
One conventional approach utilizes near field techniques through the use of a portable RF absorber aperture cover with an embedded horn feeding a network analyzer. The cover is placed over the aperture and a coarse measurement of the phase and gain of the replaced elements is made and used to align the new elements to the rest of the array. Another similar technique has horn antennas mounted on the edges of the aperture and the signals are processed within the system.
Still another approach is taught in U.S. Pat. No. 5,657,023 issued to Lewis et al., the entire content of which is incorporated herein by reference. Lewis provides for phase-up of array antennas of a regularly spaced lattice orientation, without the use of a nearfield or farfield range. The technique uses mutual coupling and/or reflections to provide a signal from one element to its neighbors. This signal provides a reference to allow for each antenna element to be phased-up with respect to one another.
Referring to FIG. 1A, as taught in Lewis et al., a line array includes antenna elements 1-5. The sequence begins by transmitting from element 1 as shown in FIG. 1A as transmission T1, and simultaneously receiving a measurement signal R in element 2. A signal T2 is then transmitted from element 3, and a measurement signal is received in element 2. The phase and gain response from element 2 in this case (reception of the transmitted signal from element 3) is compared to that for the previous measurement (reception of the transmitted signal from element 1). This allows the transmit phase/gain differences between elements 1 and 3 to be computed. While still transmitting from element 3, a receive measurement is then made through element 4. The differences in receive phase/gain response for elements 2 and 4 can then be calculated.
To finish the example depicted in FIG. 1A, a signal T3 is transmitted from element 5 and a receive signal is measured in element 4. Data from this measurement allows element 5 transmit phase/gain coefficients to be calculated with respect to transmit excitations for elements 1 and 3.
The result of this series of measurements is computation of correction coefficients that when applied allow elements 2 and 4 to exhibit the same receive phase/gain response. Further, additional coefficients result that when applied, allow elements 1, 3 and 5 to exhibit the same transmit phase/gain response. Typically, the coefficients can be applied through appropriate adjustment of the array gain and phase shifter commands, setting attenuators and phase shifters.
In a line array of arbitrary extent, the measurement sequences of transmitting from every element and making receive measurements from adjacent elements continues to the end of the array. Thus the calibration technique can be applied to arbitrarily sized arrays. Receive measurements using elements other than those adjacent to the transmitting elements may also be used. These additional receive measurements can lead to reduced overall measurement time and increased measurement accuracy.
For an odd element receive phase-up the second series of measurements is aimed at phasing up the odd numbered elements in receive and even numbered elements in transmit. These measurement sequences are similar to those described above for the even element phase-up, and are illustrated in FIG. 1B.
First, a transmit signal from element 2 provides excitation for receive measurements from element 1 and then element 3. This allows the relative receive phase/gain responses of elements 1 and 3 to be calculated.
A transmit signal from element 4 is then used to make receive measurements from element 3 and then element 5. This allows the relative receive phase/gain response of elements 3 and 5 to be calculated. Also, the relative transmit response of element 4 with respect to element 2 can be calculated. All of the coefficients can then be used to provide a receive phase-up of the even elements and a transmit phase-up of the odd elements.
To complete the overall phase-up utilizing conventional practices, the interleaved phased-up odd-even elements need to be brought into overall phase/gain alignment. Coefficients are determined, which, when applied, achieve this alignment.
However, in accordance with the technique described in Lewis et al. each individual antenna element is measured and calibrated, which can be time consuming and energy wasting.