The antenna of an active phased array system must be able to steer its beam so that the system can obtain information about the surroundings in different directions. It is also desirable that the antenna suppress signals from other directions than the direction in which the system is currently transmitting and receiving. A phased array antenna comprises a number of transmitting/receiving elements, usually arranged in a planar configuration. Each element, or group of elements, is driven by a transmit/receive (T/R) module which controls the phase and the amplitude of the corresponding antenna element.
On transmission of a signal from a phased array antenna, the signal is divided into a number of sub-signals, and each sub-signal is fed to one of the modules. The modules comprise signal channels guiding the sub-signals to the antenna elements. Each signal channel comprises controllable attenuators or amplifiers and controllable phase-shifting devices for controlling the amplification and the phase shift of the modules. The signals transmitted through the antenna elements interfere with each other. By selecting suitable values of the relative amplification and the relative phase-shifting between the modules and by utilizing the interference of the transmitted signals, the directional sensitivity of the antenna can be controlled.
During reception in a phased array antenna, the opposite procedure takes place compared to transmission. Each antenna element receives a sub-signal. The modules comprise signal channels for reception and through these signal channels the sub-signals are collected in a single point in which all sub-signals are added to form a single composite signal. The signal channels for reception also comprise amplifiers and phase shifters, and the directional sensitivity of the antenna for reception can be controlled in a corresponding way as for transmission, by varying the amplification and phase-shifting of the modules.
In order to obtain the desired directional properties of the antenna, it is desired to minimize the side lobe levels of the antenna. To enable low side lobe levels with an electrically controlled phased array antenna, high accuracy of the amplification and the phase shift in the modules is required. In practice, this is achieved by introducing a calibration function in the antenna system. Central to the calibration concept is the compensation of the various contributions of cables, attenuators, phase shifters, regulators and other parts in the transmit/receive channels which respond differently at different temperatures, for each antenna element and at each radio frequency. The calibration procedure is required to determine what controls should be applied to the transmit/receive modules in order to obtain the desired current distribution on the antenna aperture.
Phased array antenna architectures typically include a calibration network, whose purpose is to provide injection of a predetermined calibration signal to each antenna element and to the T/R module connected to it. An example of this type of calibration network is described in U.S. Pat. No. 7,068,218 to Göttl et al. Göttl et al's calibration procedure utilizes, in addition to the operational transmit/receive channels, also an auxiliary injection network, whose contribution must be known in advance. This is determined using the concept of the calibration ratio, which measures the ratio between signals injected externally (in principle from infinity) to those injected internally.
Much prior art relates to phased antenna calibration and the determination of calibration ratio. Of the many different approaches that are known in the art, all presently fall into one of two categories. Some methods use an external calibration signal that is disposed at infinity so that the respective amplitudes and phases of the external calibration signals injected into each antenna element are the same. This, of course, greatly simplifies the determination of calibration ratio, but is not feasible when there is insufficient space between the external calibration source and the phased array antenna, such as when a phased array antenna is recalibrated in the field.
The other approach disposes the external calibration source proximate to each antenna element in turn, while ensuring that the distance from the external calibration source to each antenna element is the same and that the external calibration source is exactly aligned to the optical center of each antenna element. This also ensures that the respective amplitudes and phases of the external calibration signals injected into each antenna element are the same, but requires critical and consequently complex alignment and is both time-consuming and expensive.
When the calibration reference signal is derived from a distant source such as a satellite, the signal emanates from infinity so that its wavefront is effectively equidistant from all the antenna elements. It therefore arrives in the same phase at all the antenna elements. But it is not always practical to use a distant source for the calibration source, particularly when space is at a premium as is often the case in field calibration. In satellite applications, for example, the required band of frequencies is not guaranteed and even if the required band of frequencies is provided—the signal may not reach the desired intensity. Prior art approaches that employ so-called near zone calibration are known to feed a planar calibration signal successively to the antenna elements.
For example, U.S. Pat. No. 6,084,545 (Lier et al.) discloses a near-zone calibration arrangement for a phased-array antenna that determines the phase shifts or attenuation of the elemental control elements of the array. The calibration system includes a probe located in the near zone, and a calibration tone generator. According to the concept of reciprocity, the near zone calibration procedure can be applied to transmit or receive modes as well. In case of receive calibration mode, a probe sequentially moves from one antenna element to another, keeping the same electro-magnetic coupling conditions (distance from antenna plane, polarization, orientation etc.) and transmitting the same test signal. A receive antenna array has a switching arrangement, providing appropriate RF-module/antenna element connection to the measurement unit via controllable phase shifter/attenuator. The near-zone calibration goal achieves the same signal parameters (phase and amplitude) coming from each RF-module (and appropriate probe locations) by applying control signals to the appropriate phase shifters and attenuators.
Regardless of whether near zone or far field calibration is performed, when a calibration network is factory-calibrated, sets of calibration values must be pre-assigned to each antenna. These values cannot be determined in the field and are apt to be inapplicable to a replacement antenna element, so that if an antenna element is replaced in the field, such an approach is fraught with difficulty.
In summary, far zone calibration allows the calibration signal to be fed simultaneously to all the antenna elements from a common source and ensures that it will arrive at the same phase at all the antenna elements; but is not suitable for use in confined spaces, such as when re-calibrating antenna elements in the field. On the other hand, near zone calibration requires that in order for the external calibration signal to arrive at the same phase at all the antenna elements, it must be fed to each antenna element sequentially and this requires precise alignment which is time-consuming and expensive.
The methods for calibration ratio acquisition described above, are costly and inaccurate in those cases where the antenna cannot be assumed to be bare i.e. where the antenna is electromagnetically coupled to an interfering structure. The accurate calibration ratio should take into account electro-magnetic coupling to all near-by interfering structures. Even if an interfering structure is absent at some point in time, this may be no longer true when the antenna is deployed later in some other place where the assumption of a bare antenna no longer holds. One example is a phased array cellular antenna located in some urban environment where some physical obstacles interfere with the antenna. Even worse, these obstacles might not have been present at the time of antenna installation. Another example is an antenna mounted on aircraft, or on a tank or ship. In this case an aircraft wing may interfere with an antenna mounted on the fuselage of the aircraft. By the same token masts on a marine vessel may also interfere with antennas installed on it. There is a plethora of examples where such interference might be significant.
U.S. Pat. No. 7,119,739 to Struckman describes a method for near field to far field DF antenna array calibration.
The disclosures of all publications and patent documents mentioned in the specification, and of the publications and patent documents cited therein directly or indirectly, are hereby incorporated by reference.