Communications: directive radio wave systems and devices (e.g. – Directive – Including a steerable array
Reexamination Certificate
2001-05-03
2003-01-14
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a steerable array
C342S174000, C342S360000
Reexamination Certificate
active
06507315
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to array antennas and more specifically to characterization of element patterns and amplifier characteristics in array antennas.
In an array antenna, an “active element” immersed in an array environment will behave differently from the case where the antenna element is removed from the array. This problem arises from mutual coupling between the antenna elements. Therefore if one is to have an accurate model for predicting its performance, the antenna element must be measured when the antenna element is placed in the array environment. In the prior art, the process is typically done by applying a source to the “active element,” terminating the rest of the array elements, and then measuring the given active antenna element pattern.
Using the method of the prior art, single element pattern characterization measurements are used to determine each of the antenna element patterns. For an array of N elements, this is accomplished by exciting one array element and terminating all other N−1 array elements, such that only the desired array element is radiating energy. Only one of N array elements is measured at a time. Therefore, this is called the single antenna element approach. Using the single antenna element approach, all N antenna elements are measured sequentially. This process can be used to measure any array element pattern with the array element immersed in the array environment, which is, in general, different from an isolated array element, thus accounting for the mutual coupling interactions among array elements.
One problem with the prior art approach is that it is very time consuming since antenna elements are measured sequentially and the positioner will be required to go through the desired movement cycle once for each active array element. This is extremely inefficient and impractical when the positioner movement and data acquisition cycle must be repeated N times. A second disadvantage is that, in some cases, it may be difficult, impractical, or impossible to shut off all but one array element in the array under test. Removing signals from all but one array element may become a time consuming and expensive process, involving removal of a cable and replacement with a termination. If one is to rely on turning antenna elements off using digitally controlled radio frequency (RF) on/off switches, RF isolation may not be sufficient to allow for measurements to be performed to a suitable level of accuracy.
In a similar problem, the characterization of the amplitude and phase of each antenna element against signal level, frequency, and ambient temperature is crucial to create “look-up” calibration tables. This is particularly important in multi-beam active array antennas to characterize the nonlinear behavior of the amplifiers, and to compare them with theoretical models such as the Shimbo model; see O. Shimbo, “Transmission Analysis in Communication Systems,” Computer Science Press (1988). The current technique is to characterize each antenna element one at a time by disconnecting, turning off, or attenuating the other elements in the array. This is again the single antenna element approach so the technique is very time consuming, and therefore results in high parts integration and test time, which in turn adversely impacts the total assembly costs.
To further illustrate the limitations of the prior art, active phased-array antennas typically have a requirement to determine array element patterns while the antenna element is in the array environment. These data are needed for scaling factor constants which take into account that the antenna elements are at different distances from the calibration probe. The scaling factor constants are used in the near-field calibration system described in U.S. Pat. No. 6,084,545, issued Jul. 4, 2000 in the name of Lier et al. to take control circuit encoding (CCE) measurements of each of the array elements in an array antenna; see U.S. Pat. No. 5,572,219, issued Nov. 5, 1996 in the name of Silverstein et al. In other applications, accurate element patterns are needed for in-orbit far-field calibration where measurements of the main beam and sidelobes are taken for remote sensing of aperture deformation. For an array of 1000 elements, to efficiently obtain array element patterns for all the array elements, while the array elements are immersed in the array environment, 1000 cables must be disconnected and reconnected, the antenna rotated on a point, either spherical or planar, and the probe moved over the desired positioning range 1000 different times. This is a very time consuming and expensive process.
It can be understood then that the processes for measuring array element patterns and amplifier characteristics must be repeated for each of the array elements in the array antenna. The methods using the prior art are costly and inefficient since they are limited to measurements of a single array element at a time. Therefore, there is a need for performing antenna element pattern and amplifier characteristic measurements in a factory or diagnostic setting that allows all antenna elements and amplifiers to be characterized in an accurate, efficient and cost-effective way.
SUMMARY OF THE INVENTION
The system and method of the present invention described herein discloses a positioning device which allows movement of the antenna with respect to a calibration probe or movement of the calibration probe with respect to the antenna. It is the intermittent movement of the antenna and probe with respect to each other between measurement cycles which significantly improves implementation of the calibration procedure by permitting multiple simultaneous control circuit encoding (CCE) measurements of each of the array elements in an array antenna. The method is demonstrated experimentally using a near-field probe positioner to rapidly measure all 16 element patterns in a 2×8 array of horns.
Similarly, the system and method of the present invention described herein discloses changes in the level of signals transmitted by the amplifiers in the elements of an array antenna system in conjunction with the use of orthogonal coding measurements. Changes in the level of signals transmitted significantly improves implementation of the process of determining amplifier characteristics by permitting simultaneous measurement of the amplifier characteristics of each of the array elements in an array antenna.
The present invention comprises a system for characterizing the patterns of a plurality of elements located in an array antenna, with each of the plurality of elements including at least one of (either or both) a phase shifter and an amplitude attenuator. The antenna includes a signal port for each individual beam which the array antenna generates, and a control signal input port to which control signals are applied for control of the phase shifters and amplitude attenuators. A plurality of antenna elements comprise a beamformer, a plurality of beamformers form the array antenna. The system for characterizing the patterns of a plurality of elements located in the array antenna system comprises: a probe positioned within the field of the array antenna, and positioning means for changing the relative position and orientation between the probe and the antenna. The system also includes a calibration radio-frequency source which is (a) coupled to at least one of the signal-ports of the array antenna when the array antenna is oriented as a transmit antenna, and (b) coupled to the probe when the array antenna is oriented as a receive antenna, with the calibration radio-frequency source generating a calibration signal. An orthogonal code generating means is applied to a plurality of antenna elements of at least one of the beamformers to sequentially set at least one of the phase shifters and amplitude attenuators (either one or both) with a plurality of sets of values. Each of the sets of values imposes a coding on the calibration signal to thereby sequentially generate calibration signals encoded with
Ashe Jeffrey M.
Lier Erik
Purdy Daniel S.
Lockheed Martin Corporation
Mull Fred H
Scully Scott Murphy & Presser
Tarcza Thomas H.
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