# Patent

## Title
Antenna calibration

## Abstract

The present invention relates to antenna calibration for active phased array antennas. Specifically, the present invention relates to a built in apparatus for autonomous antenna calibration
Accordingly, the present invention provides a method of self-calibration of a plurality of calibration antennas comprising the steps of: (i) selecting two calibration antennas to be calibrated that have a common area in range of both calibration antennas; (ii) selecting at least one radiating element within range of the two calibration antennas; (iii) transmitted a known test signal from the one or more selected radiating elements; (iv) measuring a received signal at each of the two calibration antennas; (v) comparing the received signals at each of the two calibration antennas; and (vi) determining a correction coefficient for each calibration antenna based on the received signals at the said calibration antennas.

## Background
The present invention relates to antenna calibration for active, phased array antennas. Specifically, the present invention relates to a built in apparatus for autonomous antenna calibration and real-time RF performance monitoring.
A known method of calibrating an array antenna is to use calibration coupler manifolds 150, as shown in FIG. 1, at each of the elements 140 in the array.
Referring to FIG. 1, there is shown a known antenna element comprising a receiver 110, array cabling 120 and various active components 130. A calibration signal from a central source is split many ways in the manifold and a nominally-equal proportion is coupled into each element channel at some point behind the radiating element. The signal level at the receiver(s) 110 can then be adjusted accordingly to produce the desired performance characteristics for the array antenna.
When using a calibration coupler, a portion of the element channel 140 is not included in the calibration process. One problem with calibration coupler manifolds 150 is that they are relatively large devices and so cause problems in the design of an array antenna which incorporates them. Another problem with calibration coupler manifolds 150 is that the coupling factors at each channel have individual variability which needs to be removed to achieve optimum performance, i.e. the accuracy of antenna calibration is limited to the extent that the individual manifold outputs are known.
Alternatively, another known method for calibrating an array antenna is to use an external scanner. This involves placing an external scanning apparatus in front of the array face and scanning the properties of each radiating element of the array in turn by moving the scanner over each radiating element and measuring the radiation it produces and/or receives. It has many moving parts which require maintenance, especially because the equipment usually operates in exposed environments as this is where equipment employing phased array antennas is usually operated. In addition, this is a slow process and requires normal use of the equipment to stop while calibration is performed.
Accordingly, the present invention provides a method of self-calibration of a plurality of calibration antennas comprising the steps of: (i) selecting two calibration antennas to be calibrated that have a common area in range of both calibration antennas; (ii) selecting at least one radiating element within range of the two calibration antennas; (iii) transmitted a known test signal from the one or more selected radiating elements; (iv) measuring a received signal at each of the two calibration antennas; (v) comparing the received signals at each of the two calibration antennas; and (vi) determining a correction coefficient for each calibration antenna based on the received signals at the said calibration antennas.
An advantage of the present invention is that the calibration apparatus of the antenna array can be self-calibrated in the field, rather than needing to take the entire apparatus offline to set it up. Additionally, the present invention does not introduce extra equipment to the array, e.g. calibration coupler manifolds, that itself requires further calibration to prevent accuracy limitations.



Specific embodiments of the invention will now be described, by way of example only and with reference to the accompanying drawings that have like reference numerals, wherein:—
FIG. 1 is a schematic diagram of a known calibration coupler manifold;
FIG. 2 is a diagram of an array face with four calibration antennas mounted around the edge of the array face according to a specific embodiment of the present invention;
FIG. 3 is a diagram of an array face with four calibration antennas mounted around the edge of the array face showing the overlapping coverage areas of each calibration antennas according to a specific embodiment of the present invention; and
FIG. 4 is a diagram of an array face with four calibration antennas mounted around the edge of the array face showing the overlapping coverage areas of two calibration antennas according to a specific embodiment of the present invention;



A first embodiment of the present invention will now be described with reference to FIGS. 2 to 4:
In FIG. 2, there is shown an array face 250 having four calibration antennas 210, 220, 230, 240 fixed at each corner of the array face 250. The calibration antennas 210, 220, 230, 240 are low directivity open wave guide antennas in fixed, known, locations around the array face 250. The calibration antennas 210, 220, 230, 240 are mounted to allow a degree of overlap in coverage area of the array face 250 such that all portions of the array face 250 are covered by at least one calibration antenna 210, 220, 230, 240.
In FIG. 3, an example of the overlap in coverage areas 215, 225, 235, 245 between all of the calibration antennas 210, 220, 230, 240 is shown—the entire array face 250 is covered by at least one calibration antenna 210, 220, 230, 240. In FIG. 4, the respective coverage areas 215, 225 of just two of the calibration antennas 210, 220 is shown.
Initially, the calibration antennas 210, 220, 230, 240 need to self-calibrate: this is performed in pairs, using the overlapping coverage areas between each pair, in turn, to check each calibration antenna 210, 220, 230, 240 against a common antenna element in the array face 250. The self-calibration method is as follows:
Three antenna elements 410, 420, 430 in the region of the array face 250 that is within range of the two calibration antennas 210, 220 to be calibrated are arbitrarily selected. For illustration, the following procedure is described with the elements in transmit mode; the same procedure is carried out in receive mode, with the transmit and receive roles of the elements and the calibration antennas reversed. Each antenna element 410, 420, 430 radiates a known signal in sequence. The radiated signals are detected by both calibration antennas 210, 220. The received signals at each calibration antenna 210, 220 are compared to that of the other respective calibration antenna 220, 210 and the known radiated signal. The process then repeats with a different pair of calibration antennas 220, 230, selecting different antenna elements 430, 440, 450 to radiate the known signal. Once all neighbouring pairs of calibration antennas 210, 220, 230, 240 have been through this process, a calibration coefficient for each calibration antenna 210, 220, 230, 240 is determined to produce the same output at each calibration antenna 210, 220, 230, 240 for a given input. The calibration coefficient is the difference between the desired signal and the achieved detected signal and once applied will align the gains and phases of the array.
The calibration process that occurs during normal operation repeats the as follows, with reference to FIG. 3:
For illustration, the following procedure is described with the elements in transmit mode; the same procedure is carried out in receive mode, with the transmit and receive roles of the elements and the calibration antennas reversed. Each antenna element in the array 250 radiates a known signal in sequence. The radiated signals are detected by a designated calibration antenna 210, for example, in whose quadrant the particular element is situated. The received signal at the calibration antenna 210 is compared to desired response to the known radiated signal. The process then repeats with all remaining elements in the array, selecting different calibration antennas 210, 220, 230, 240 to radiate the known signal. Once all elements have been through this process, a calibration coefficient for each element is determined to produce the desired output at each calibration antenna 210, 220, 230, 240 for a given input.
Each array has a first pass scan performed when it is first assembled at, for example, the factory that has assembled the array. This first pass scan creates one or more first pass coefficients for either portion of the array and/or the entire array. Using the calibration antennas mounted around the array, once these have been self-calibrated, the values for these coefficients can be computed.
In a second embodiment, by incorporating the fixed auxiliary radiators of the above embodiment at intervals around the periphery of the array, a means of coupling RF energy into the antenna elements from the array is introduced. Test signals may then be routed to each of these radiators in turn, which illuminate the array elements at high angles of incidence. The elements' responses to these test signals may then by used as a guide to their operational condition. The test signals may be interspersed during normal operational transmissions and hence offer a continuous on-line monitoring process.
In the systems of the first and second embodiments of the present invention, the full RF chain is tested, comprising active antenna element (including attenuator and phase shifter functions), beamformer, transmit output power, receive gain, and attenuator and phase shifter accuracy on every element can be monitored.
It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

## Claims

1. A method of self-calibration of a plurality of calibration antennas comprising the steps of:
(i) mounting the plurality of calibration antennas around an antenna array;
(ii) selecting two calibration antennas to be calibrated that have a common area of the array in range of both calibration antennas, said selected two calibration antennas forming a pair of calibration antennas;
(iii) selecting at least one radiating element from the array within range of the pair of calibration antennas;
(iv) transmitting a known test signal from the one or more selected radiating elements;
(v) measuring a received signal at each of the pair of calibration antennas;
(vi) comparing the received signals at each of the pair of calibration antennas; and
(vii) determining a correction coefficient for each calibration antenna of said pair of calibration antennas based on the received signals at the said pair of calibration antennas.

2. A method of self-calibration according to claim 1, wherein steps (i) to (vi) are repeated for each pair of the calibration antennas having a common areas of the array in range.

3. A method of self-calibration according to claim 2, wherein a different radiating element is selected from the array for each pair of the calibration antennas having a common area of the array in range.

4. A method of self-calibration according to claim 3, wherein step (vii) is carried out after steps (i) to (vi) have been repeated, the correction coefficient for each calibration antenna being based on the received signals at more than one pair of calibration antennas for which it is one of the pair.

5. A method of self-calibration according to claim 2, wherein step (vii) is carried out after steps (i) to (vi) have been repeated, the correction coefficient for each calibration antenna being based on the received signals at more than one pair of calibration antennas for which it is one of the pair.