GB2171849A - Improvements in or relating to the alignment of phased array antenna systems - Google Patents

Abstract

In order to align or calibrate all the antenna elements 22 of a phased array, that is, to provide for one particular direction equal amplitude and phase for all the individual output signals, two stages of alignment are involved. First, one antenna is established as a reference, and then, taking each of the others in turn and making a one-to-one comparison with the reference, gain command values at G2, G3, etc. are established which give equal outputs to the reference antenna. These values are recorded. Secondly, again considering one antenna as a standard and working to the same constant output signal, phase command values P2, P3, P4 etc. are established to give phases which are each exactly in antiphase to the phase for the reference value P1. The calibration results may be stored in a computer memory. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the alignment of phased array antenna systems This invention relates to a method of aligning phased array antenna systems eg for radar purposes. Such an antenna system comprises a number of antenna elements each fed through a signal channel. Each such channel includes a module comprising a variable gain control and a variable phase control. This enables the phase of radiation transmitted from each antenna element to be adjusted in relation to the rest of the elements so that the total beam of radiation transmitted from the antenna system has a given direction. Varying said gain and phase controls suitably, enables the beam direction to be varied without altering the position of the antenna system physically.Conversely the antenna system can be made to receive radiation from any of a number of given directions, again without physical movement of the system.

In order that beam direction may be selected readily, either for transmitting or receiving, an array must be caiibrated, or aligned.

That is to say, a way must be established for setting the gain and phase controls in each channel feeding the antenna system in order to obtain a sufficiently accurate beam direction and shape. The background to the requirements for accurate calibration arrangements are set out in a number of the following items of generally relevant art.

M Skolnik (Ed), Radar Handbook, p 11-36 ff, 1970.

J K Smith, N-Bit L-Band Phase Shifters for Phased Arrays, MTTS-83.

W W Schrader, RF Analysis of the Cobra Dane Antenna using Self Monitoring Techniques, IEEE Int Radar Conf, Radar 80.

D K Alexander, R P Gray, Computer-Aided Fault Determination for an Advanced Phased Array Antenna, Univ of Illinois Antenna Applications Symposium, 1979.

E Hung, N Fines, R Turner, The In-Situ Calibration of a Reciprocal Space Fed Phased Array, IEE Conf. Publ. 216, Proc. Radar 82.

W Sander, Experimental Phased Array Radar ELRA, IEE Proc Vol 127, Part F, August 1980.

H D Griffiths, J R Forrest, A D Williams and C Pell, Digital Beam-forming for Bistatic Radar Receiver, IEE Conf Publ 219, Proc ICAP 83.

P Barton, Digital Beamforming for Radar, IEE Proc Vol 127, Part F, August 1980.

The present invention provides a logical and relatively simple method of aligning a phased array antenna system, applicable to arrays of any number of elements, and whether in the transmitting or receiving mode.

According to the invention a method of aligning a phased array antenna system having a plurality of elements each fed by a channel including variable gain control and variable phase control comprises the steps of: energising the channel feeding one of the elements and recording the command to the gain control of that element to produce given signal levels at the channel output; recording similarly the gain command for each of the other chanels, when individually energised, to produce the given signal levels; selecting one said channel as a reference channel and energising it at a given gain and phase to provide a given amplitude at the channel output; energising a single other said channel to provide the same output amplitude and recording the command to the phase control of that channel to set the output thereof in antiphase with the reference channel; and similarly recording the required command to the phase control of each of the rest of the other said channels when individually energised and compared with said reference channel; the recorded commands being thereafter applicable to adjust gain and phase control of each said channel to provide a desired direction and shape of beam to be radiated from the antenna system.

In this specification the term "command" is to be understood to signify a stimulus applied to a gain or phase control for the adjustment thereof; which stimulus may be, for example, a mechanical movement, a change in fluid pressure, a change in voltage or current, depending on the individual requirement of the control mechanism employed; with the proviso that the command be recordable and unambiguous.

The invention will be further described with reference to the accompanying drawings in which Figure 1 shows schematically the channels through which the elements of an array antenna are fed, Figure 2 illustrates the procedure of phase alignment.

The alignment method is applied to the adjustable portion of a phased array antenna system, said portions being adjustable for the purpose of producing a beam of given direction from the antenna system, energy being supplied from a generator of suitable power and frequency for radar purposes. Said adjustable portions comprise (Fig. 1) in a channel 10, feeding an element of the antenna array, a module 12. Each module incorporates a gain control 14 and a phase control 16. The gain and phase controls need not be separate within the module, but can be arranged in a single item of apparatus, eg a modulator. In Fig. 1, to avoid complication, only three channels 10 and modules 12 have been shown.

The chain lines indicate other channels of a similar nature, which, in practice, may number up to a hundred or considerably more. Commands affecting the gain controls in the different channels are applied through gain control actuating means indicated schematically by G1,G2... GN. Commands to the phase controls are indicated analogously by P1, P2,... PN. The channels are supplied, in use, through an input from a conventional source of RF power, (not illustrated). The individual channels are fed through a splitter means 20, also of conventional kind. The ouotput ends of the channels are taken to the individual radiative elements, 22, of the antenna system.By use of the gain controls any given channel or channels can be energised when required, the rest of the channels being de-energised, and substantially completely isolated from the energised channels. For example, for an array having a hundred modules a degree of isolation of 50dB would be required between channels.

In carrying out the method of aligning an array antenna system, the gains in all the channels feeding the array are aligned, or calibrated first. The first channel, which may be arbitrarily chosen, is switched on through the splitter 20. The input to the channel being known, the output is measured and compared with the input, using a conventional instrument 23, in Fig. 1, to provide the gain of the intervening channel. The said instrument can be of a quite simple nature, since it has only to indicate power levels, and not phase differences. The gain may be calibrated against the control input to the appropriate gain control actuating means G1 for any required range of command values. The calibration results may conveniently be recorded in a computer memory.For the purpose of making the measurements the instrument 23 may be applied for example, through a probe arranged to sample the radiation output of an element 22 of the antenna system; or to a coupler of conventional kind associated with each channel 10 between the module 12 and the respective element 22.

If the instrument 23 used to measure the gain is not precise in its linearity of response, the command to Gi can be varied until a suitably chosen instrument reading is reached at the output from the channel, and the command to G1 can then be recorded as the result for that channel.

The same procedure is applied to the rest of the channels, up to and including channel N, in turn. The command, or range of commands, G1 to GN are recorded for the corresponding range of gains for all the channels.

The next procedure to be followed is for the phase alignment of the array system. One channel, say the first channel, is selected as a reference channel. This one channel is energised and the gain G1, and phase P1, control actuating means are appropriately commanded to produce a given gain and phase state in that channel. The vector representing the output voltage of that channel is indicated at 24 in Fig. 2, arbitrarily to represent a phase of 180 say. This reference channel is maintained energised at the chosen gain and phase throughout the phase alignment procedure. A second channel is then seiected and energised. The gain in the second channel is adjusted, using the data already acquired, to be equal to that in the reference channel.An instrument of conventional kind, indicated by 30, which may in practice be the same as instrument 23, and which must be capable of providing an accurate null reading, is connected between the outputs of the reference channel and the said second channel, to read the difference between the voltages appearing at those outputs. The command to phase control actuating means P2 of the second channel can be varied, hence varying the phase of the output voltage from the second channel. The vector for this output voltage is indicated at 26 in Fig. 2. Varying the phase of the voltage swings the vector 26 through an angle represented in Fig. 2 by f). The difference between the two vectors 24 and 26 is represented by a third vector 28.As the angle 6 approaches zero, so the magnitude of the vector 28 also approaches zero, and hence the reading of the instrument 30 connected between the two channel outputs also reaches zero, or a very low minimum. This indicates that the vectors 24 and 26 are in precise antiphase. The command actuating means P2 is then recorded.

The second channel is de-energised and isolated, and a third channel energised, when the procedure of phase alignment is repeated; and so on through all the channels to channel N.

The accuracy of the method of alignment is determined, in the main, by the degree of fineness with which the gain and phase controls can be adjusted. It is feasible to carry out the adjustment automatically, using an on-line computer, having in its memory the record of commands applied to gain and phase controls while the method of alignment was carried out. The most complex aspect of the method is adjustment of phase while seeking the zero, or minimum, indicating precise antiphase between reference channel and a channel under test. One search routine which may advantageously be applied is the binary chopping system, in which the number of iterations required to achieve a minimum is given by the number of bits (of information) to which the phase is defined. Typically about ten iterations would be required for both gain and phase adjustment in one channel. Each interation comprises a measurement and a decision.

Measurement time would be of the order of 1 to 2 microsecond; and the decision time, depending on the detailed construction of the computer, of the order of 0.5 to 5 microsecond. By way of example, the phase and gain response of a hundred-element antenna array could be aligned in not more than about 5 millisecond. This impiies alignment at one frequency only.

It is to be noted that because alignment of gain is carried out first in the method, the minimum sought in phase alignment will be sharply defined, and can be determined with good accuracy.

A general advantage of the method of the invention is that the antenna array system is itself used as the measurement tool for its own alignment; the channels being aligned with each other, rather than against some external reference. Only raltively simple and cheap external instrumentation is required.

This provides an important economy, in the form of greater simplicity, potentially increased reliability, and financial saving as compared with earlier methods of alignment which have been applied.

It is a further advantage of the invention that the method of alignment is equally applicable to an antenna array for receiving a beam of radiation as for transmitting a beam.

It may be noted that the technique has been demonstrated, and details appear in a paper presented at the IEEC Conference on Antennae and Propagation in April 1985.

Claims (7)

1. A method of aligning a phased array antenna system having a plurality of elements each fed by a channel including variable gain control and variable phase control which method comprises the steps of: energising the channel feeding one of the elements and recording the command to the gain control of that element to produce given signal levels at the channel output; recording similarly the gain command for each of the other channels, when individually energised, to produce the given signal levels; selecting one said channel as a reference channel and energising it at a given gain and phase to provide a given amplitude at the channel output; energising a single other said channel to provide the same output amplitude and recording the command to the phase control of that channel to set the output thereof in antiphase with the reference channel; and similarly recording the required command to the phase control of each of the rest of the other said channels when individually energised and compared with said reference channel; the recorded commands being thereby applicable to adjust gain and phase control of each said channel to provide a desired direction and shape of beam to be radiated from the antenna system.

2. A method according to claim 1 in which a condition of antiphase between the output of said single other channel and the output of said reference channel is determined by mea suring the vector difference between said outputs and varying the command to the phase control of said other channel until the said vector difference reaches at least a minimum value.

3. A method according to claim 1 in claim 2 in which measurements of gain and phase are made through a probe arranged to sample the radiation output of each element of the antenna system.

4. A method according to claim 1 or claim 2 in which measurements of gain and phase are made through a coupler associated with each channel and arranged between said element and the associated gain and phase controls.

5. A method according to any one of the preceding claims in which the commands to the gain control and phase control for each channel are recorded in a memory of a computer.

6. A method according to claim 5 in which the computer is used to adjust gain and phase controls of an antenna system.

7. A method of aligning a phased array antenna system substantially as hereinbefore described.