Amplitude Phase Figure 1
Phasefront of a small X-band weather radar antenna shown as a grey scale amplitude (left) and phase (right) image.?title=Test title for frist image! A near-field measurement system can be alternately thought of as a compact range (or collimator) with a synthetic (phased array) aperture instead of a real (parabolic dish) aperture. The synthetic aperture is formed by moving the probe antenna to positions which correspond to the element positions in a phased array. Beam forming for the phased array requires a combination of phase shifters (for beam steering) and summers. For a planar set of sample points, this is accomplished by a computer using a 2D Fourier transform to perform simultaneous beam steering to many different far-field angles.
All nearfield measurement systems (planar, cylindrical or spherical) derive the equivalent farfield antenna performance through 2 basic steps:
A nearfield measurement system consists of 3 functional subsystems:
An RF system is required to measure the complex gain and phase through a path which includes the antenna under test and the probe antenna. The commercial version currently uses an HP-8510B, Wiltron 360 or other vector network analyzer to perform this measurement.
When the initial near-field measurement system was built, no network analyzers were available. As an alternate, a simple microwave interferometer was built from surplus components. A 12.725 GHz crystal phase locked source provided the test frequency. The receiver consisted of a line stretcher, mixer and several other components. The receiver could only measure the inphase or quadrature signal component at a given time so two measurement passes were required. The line stretcher was set for a 1/4 wavelength difference between the 2 passes. The interferometer is shown with the near-field scanner in figure 2 and a block diagram is shown in figure 3.
Figure 2 Near-Field Scanner with Microwave Interferometer
Figure 3 Microwave Interferometer Block Diagram
This design worked reasonably well for the intended application but suffered from several problems. The need for 2 measurement passes was not desirable and more importantly, this design was quite sensitive to mixer leakage and 1/f noise.
These problems were eliminated by adding a computer controlled QPSK modulator which phase modulated the transmitted signal. This effectively converted the receiver into a superheterodyne configuration which drastically reduced the 1/f noise and leakage. AM to PM and PM to AM conversion due to QPSK modulator amplitude/phase unbalance was eliminated by a combination of a software model and statistical calibration method. This system worked quite well.
The data acquisition and processing was implemented on a Compaq 386/20 (IBM PC compatible) computer. The scanner was controlled by outputing pulses from a parallel port directly to the step motor translator. The receiver used a single board ADC to provide a computer interface. The near to far-field and holographic transforms were accomplished by a factored DFT. This algorithm provided the advantage of being moderately fast, simple and outputing directly in angle space. The algorithm also supports both raster and plane polar scan patterns. The software for the nearfield measurement system can be broken down into 2 major groups:
Figure 4 Near-Field Measurement System
Representative robot performance:
Maximum scan .................. 24. 18. inches
Maximum velocity .............. 30. 30. ips
Static positioning accuracy ... 0.002 0.002 inch
Dynamic positioning accuracy .. 0.010 0.010 inch
XY orthogonality .............. 5.0 arcsec
Representative system performance :
(X-band phased array)
side lobe noise level .......... -50. dB
Gain measurement uncertainty .. 0.3 dB
Boresight accuracy ............ 0.05 deg
Measurement time .............. 2. minutes
Processing time ............... 2. minutes
A small and portable near-field measurement system can perform many useful operations in addition to basic nearfield antenna measurements. A list of applications include:
Phase center position
Autotrack bias, scale factor,lin.
Reflector surface distortion
Feed position errors
Microwave holographic measurements
Figure 5 Far-Field / Nearfield Comparison for X-band Weather Radar Antenna
Figure 6 Waveguide Array Farfield Pattern
2. Holographic methods can be used to provide focused microwave images of antenna and radome defects. Figure 7 is a holographic back-projection on a small X-band phased array antenna, showing the position of an intentionally blocked slot.
Amplitude Phase Figure 7 - Holographic Back Projection
3. Because it is portable, it can be easily setup and used to perform diagnostic tests of anechoic chambers. As such, it can be used to image and locate multipath interference within compact ranges and anechoic chambers.
Figure 8 Contour Plot of Scattering In Anechoic Chamber
4. The portable near-field measurement system can be used to image leakage emissions at specific frequencies from electronic systems. This concept treats the leakages from the electronic system as a series of antenna elements. Specific emission areas can be detailed by a holographic back projection to the equipment surface.
5. Material properties such as the leakage of mesh reflector materials are readily handled by near-field measurements. Holographic techniques can be used to image specific regions of degradation.
6. The system requires a capability to precisely determine the position of the microwave probe antenna. As such, the system can also determine the precise position of other payloads. For example a touch probe could be used to rapidly make dimensional measurements in the field.
New uses for near-field measurement systems occur when the system is made small, portable and low cost. These new uses include measuring antenna performance on the flight line, in an office environment or for educational environments. Because of the portability, imaging anechoic chamber multipath, leakage and other chamber error sources is also possible.
1. G. Hindman and D. Slater, Error Suppression Techniques for Near-Field Antenna Measurements, 1989 Antenna Measurement Techniques Association Symposium, Monterey, Ca. Oct 9-13, 1989.