Calibration system and method for phased array antenna using near-field probe and focused null

A system for calibrating at least one element of a receive array antenna,

• said array antenna forming a finite plane of said elements, the plane including a center point and extreme edges,
• said system comprising:
• a signal generating means for generating a coherent reference phase signal,
• said coherent reference phase signal coupled to a zeroth two-way splitter,
• said zeroth two-way splitter splitting said coherent reference signal into a null signal carried to a horn probe,
• said horn probe including a face, said horn probe rigidly positioned normal to the plane of said array antenna and at a distance in the near-field of said array antenna,
• said face of said horn probe rigidly positioned over the center point of the plane thereby forming an angle between the center point and a point on one of the extreme edges of the plane of said array antenna,
• said horn probe therein positioned over the center point of said array antenna corresponding to said null,
• said horn probe, the center point, and the point on the extreme edge of the plane of said array antenna thereby forming a plane normal to the finite plane of said elements;
• said horn probe transmitting at the center point of said array antenna the null beam received by said array antenna from said signal source to said at least one element, the null beam received by said array antenna coupled as a signal to a first attenuator,
• said first attenuator coupled to a first two-way splitter,
• said first two-way splitter receiving the signal from said array antenna through said first attenuator;
• said zeroth two-way splitter splitting said coherent reference phase signal into a reference signal, said reference signal carried as a signal to a second attenuator,
• said second attenuator coupled to a second two-way splitter,
• said second two-way splitter receiving the signal from said zeroth two-way splitter through said second attenuator;
• a first mixer, said first mixer including as input ports a radio-frequency port and a local oscillator port, said first mixer including as an output port an intermediate frequency port;
• a second mixer, said second mixer including as input ports a radio-frequency port and a local oscillator port, said second mixer including as an output port an intermediate frequency port;
• said first two-way splitter splitting the signal received from said null probe through said first attenuator into a first signal, substantially 0 degree phase-shifted, which is coupled to said radio-frequency port of said second mixer, and into a second signal, substantially 90 degree phase-shifted, which is coupled to said radio-frequency port of said first mixer;
• said second two-way splitter splitting the signal received from said reference probe through said second attenuator into a first signal, substantially 0 degree phase-shifted, which is coupled to said local oscillator port of said second mixer, and into a second signal, substantially 0 degree phase-shifted, which is coupled to said local oscillator port of said first mixer;
• said first mixer combining the substantially 90 degree phase-shifted signal of said first two-way splitter and the substantially 0 degree phase-shifted signal of said second two-way splitter to yield an output signal at said intermediate-frequency port of said first mixer, said output signal at said intermediate-frequency port of said first mixer coupled to a first low-pass filter from which emerges a quadrature base-band component signal at substantially 0 degree frequency of the signal received from said null probe;
• said second mixer combining the substantially 0 degree phase-shifted signal of said first two-way splitter and the substantially 0 degree phase-shifted signal of said second two-way splitter to yield an output signal at said intermediate-frequency port of said second mixer, said output signal at said intermediate-frequency port of said second mixer coupled to a second low-pass filter from which emerges an in-phase base-band component signal at substantially 0 degree frequency of the signal received from said null probe;
• a processor,
• said processor receiving as an input signal said quadrature base-band component signal at substantially 0 degree frequency of the signal received from said null probe and as an input signal said in-phase base-band component signal at substantially 0 degree frequency of the signal received from said null probe,
• said processor processing said quadrature base-band component signal and said in-phase base-band component signal into data representing calibration reference values of said at least one element of said array antenna;
• a recorder for recording said data representing calibration reference values of said at least one element of said array antenna, said recorder receiving said data from said processor;
• and
• a beam steering controller for selecting the phases on said at least one element of said array antenna to form the difference null, and to vary the phase of said at least one element of said array antenna.

A method of calibrating at least one element in one of a (a) transmit array antenna and (b) receive array antenna, said method comprising the steps of:

• forming a difference null with errors for said at least one element being calibrated in phase state n, thereby yielding the in-phase base-band component, In, and the quadrature base-band component, Qn of said difference null;
• estimating the difference null without the element being calibrated by averaging over N phase states, by solving for
• ε=1/NΣ(I+jQn);
• estimating the field of the element in phase state n by subtracting the null without the element from the null with the element;
• calculating field, En, of the element in phase state n by solving:
• En=(In+jQn)−ε=xn+jyn;
• computing the absolute value of the estimate of the field of the element in phase state n, |En|, and the phase of the estimate of the field of the element in phase state n, arctan(yn/xn);
• and
• processing results of the calibration of the element being calibrated upon determining that all desired array elements have been measured.