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(11) | EP 0 415 574 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Antenna array |
| (57) A log periodic antenna array, each consisting of e.g. five dipoles D1 - D5 are each
driven by a transmitter Tx by an amplifier A and phase shifter 5. Each antenna radiates
generally from one dipole only for example dipole D4. The Applicants have discovered
that the antenna may behave as if the radiation appears to emanate from the position
marked with a cross. Probe antennas P1 - P3 are used to derive error signals representing
the difference between the expected relative phase between a dipole and a probe antenna
and the measured relative phase between the dipole and the probe antenna, which error
signals are then used to apply corrections to the phase shifters PS. |
[0002] The invention relates particularly to phased antenna arrays in which the transmitter connected to each antenna of the array includes phase shifters, to enable the individual antennas to be fed at different phases relative to each other in order to steer the transmitted beam.
[0003] The applicants were concerned with antenna arrays for high frequency (HF) radar and in particular log periodic dipole arrays an example of which (with four elements and five dipoles to an element) is shown in figure 1. The series of dipoles D forming each element 1, 2, 3, 4 of the array were fed at the front end, parallel wires W conducting the energy to the dipoles behind, and the radiating signal appeared along the boresight B of the element. The elements could be energised at different frequencies, and only certain dipoles within each element were energised for any given frequency. While investigating such antennas, the applicants made the discovery that, for any given element of the array, the centre from which radiation appeared to be propagated, the so-called active region or phase centre did not coincide exactly with the particular dipole that was being energised, but could instead be displaced from the centre of the dipole either along the direction of the element or transverse to that direction, or both.
[0004] The invention provides apparatus for calibrating a transmit antenna array, which comprises two or more probe antennas spaced apart from each other and in the near or intermediate field region of the antenna array, and means for determining the location of a phase centre of one of the antennas of the array from the phase at the probe antennas of a signal transmitted by that antenna of the array.
[0005] The phase measurements at the probe antennas enables the phase centre of the antenna of the array that is excited to be determined. This knowledge can be used to apply phase shifts to the respective transmitter at the respective frequency to compensate for the deviation of the phase centre from its expected position.
[0006] Apparatus for and method of calibrating a transmit antenna array, constructed in accordance with the invention on, will now be described by way of example with reference to figure 2 of the accompanying drawings, which is a partly schematic, partly plan view of the array and apparatus.
[0007] A phased array of seven log periodic dipole antennas is shown. Each antenna consists of five vertically arranged dipoles D1 - D5. Only the dipoles for the antenna on the extreme left are numbered. The dipoles are fed in the conventional manner from a co-axial cable C which is connected via a transformer forming a balun (not shown) to the centre of the first dipole D1 of the array. Parallel wires W (shown in figure 1) running the length of the antenna are connected to the centre of the other dipoles.
[0008] The shortest element is D1 and the longest is D5. The direction in which the radiation is propagated is shown as the direction B in figure 1.
[0009] Power is fed to each antenna from its own transmitter Tx, the signal from which is amplified in a power amplifier A and fed to the co-axial cable C via a phase shifter PS. These are all arranged at a control location 5 remote from the antennas. It is a property of log periodic antennas that they radiate or, for that matter receive, over different frequencies, and for any particular frequency only one or two dipoles of the array are energised. The highest frequencies are radiated at the shortest dipole D1 and the lowest frequencies at the longest dipole D5. The antenna could typically operate in the HF band of from 3 to 30 MHz.
[0010] If all the antennas are driven with the same amplitude and phase, as might be expected the beam is directed forwardly. If the antennas are driven with linear phase slope e.g. the antenna on the left at 0° phase, the next driven at 10° phase, the next at 20° phase and so on, the beam is steered and is directed at an angle to the straight ahead position. It can be shown that to direct the beam at an angle of 0° relative to a straight ahead position at the centre of the array, the phase shift ∅ for each antenna is given by
where d is the separation of the antennas and λ is the wave length of the radiation being propagated.
[0011] In an HF array, which could be quite large, the lengths of the co-axial cables joining the transmitters Tx from the central location 5 to each antenna will in general be different, and hence the same signal generated by each transmitter Tx will in general have different amplitude and phase when it reaches the respective antenna. While it might in principle be possible to measure this and correct for it, the position is more complicated than this because when an antenna radiates, adjacent antennas pick up and re-radiate the signal
[0012] The Applicants have discovered that the effect of this is that the actual phase centre from which a dipole appears to radiate may be displaced from the centre of the dipole, not only along the length of the antenna, but also in a traverse direction relative to the dipole. For example, when it radiates, dipole D4 of the left hand antenna may appear to radiate from the adjacent position marked with "X".
[0013] In accordance to the invention, the Applicants provide three probe antennas P1, P2, P3 (field detecting probes) in the near or intermediate field region of the antenna. The boundary between the intermediate and far field region of the antenna is approximately a distance in front of the array given by
where D is the antenna aperture which is the separation between the antenna on the left and that on the right, and λ is the wave length of the radiation beam propagated.
[0014] These probe antennas are used in the following way. The antenna at the left of the array is energised at a particular frequency with the transmitter Tx operated at a known amplitude and phase, and the amplitude and phase of signals received by the probe antennas P1, P3 is measured. Assuming that the frequency is such that the dipole D4 radiates, the phase of the signal at P3 is compared with that of the transmitter. The processor will contain data indicating the expected phase shift between the dipole D4 and the probe antenna P₃ were the dipole D4 to radiate from its actual centre. The consequent error signal is calculated, and this enables an imaginary circle 6 to be drawn centered on P3 indicating the locus of points on which the actual phase centre must lie. The phase of the signal received at P1 is then compared with that transmitted by transmitter Tx, and this relative phase is again compared with the data in the processor which indicates from geometrical considerations the expected relative phase between the dipole D4 and the antenna P1 were dipole D4 radiating at its centre. A second error phase is derived and enables a second circular locus, this time centered on P1, to be calculated.
[0015] The intersection of circles 6 and 7 pinpoints the actual phase centre for that frequency for that antenna. Error signals can now be stored in the processor to be fed to the phase shifters PS whenever that frequency is radiated from that antenna to correct for the error between the actual phase centre and the expected phase centre. For example, the correction could be such that the phase of the transmitter is advanced so that the circle centered on P3 now passes through dipole D4.
[0016] The calibration is repeated for frequencies relevant to each dipole for the antenna at the left, and then the procedure is repeated for each other antenna.
[0017] The inclusion of P2 as well increases the accuracy of the detection, since a majority decision can be taken from the results obtained from the antennas. However, two probe antennas only could be used if desired, and equally more than three could also be used if desired.
[0018] The probe antennas are loops which are large enough to provide an output at the processor, but no larger than that in order not to affect the performance of the antenna array. Other probe antennas such as electrically short dipoles or monopoles may be used instead of loops.
1. Apparatus for calibrating a transmit antenna array, comprising two or more probe
antennas spaced apart from each other and in the near or intermediate field region
of the antenna array, and means for determining the location of a phase centre of
one of the antennas of the array from the phase at the probe antennas of a signal
transmitted by that antenna of the array.
2. Calibration apparatus as claimed in claim 1, in which the determining means is
arranged to compare the phase of a signal received at a probe antenna relative to
that of the transmitter with the phase at the probe antenna relative to that of the
respective dipole which would be expected from geometrical considerations.
3. Calibration apparatus as claim 2, in which the determining means is arranged to
derive an error signal from the actual and expected relative phases and adjust a phase
shifter connected to that antenna in dependence on the error signal.
4. Calibration apparatus as claimed in any one of claims 1 to 3, in which each antenna
of the array is a log periodic antenna.
5. Calibration apparatus as claimed in any one of claims 1 to 4, in which three probe
antennas are provided.
6. Calibration apparatus as claimed in claim 5, in which the probe antennas are arranged
in line broadside to the boresight of the antennas of the array.
7. A method of calibrating a transmit antenna array, comprising measuring the phase
of a signal transmitted by one of the antennas of the array at two or more positions
spaced apart in the near or the intermediate field region of the antenna array, and
determining the location of a phase centre of that antenna from those measurements.
8. A method as claimed in claim 7, in which an off-set is applied to the phase of
the signal transmitted by that respective antenna dependent on the location determined
for that phase centre.