ANTENNA ALIGNMENT IN A NEAR-FIELD FACILITY

Transcription

1 ANTENNA ALIGNMENT IN A NEAR-FIELD FACILITY Mart Hagenbeek and Arnold R.Boomstra Naval Maintenance Establishment, Division SEWACO Royal Netherlands Navy, P.O.Box , 1780 CA Den Helder, The Netherlands. Tel (+31) g.hagenbeek@mindef.nl ar.boomstra@mindef.nl ABSTRACT This paper will discuss the application of alignment techniques and tools in a near-field testfacility. Standard alignment telescopes are not directly applicable in a general purpose near-field set-up because of limited dimensions of such a facility, where a direct target is not available and is often to close to the antenna to be in the focus region of the telescope itself. Self-made optical tools will be presented to overcome this problem, including some estimates about the required and obtained accuracies. Using these tools is demonstrated as a fast and accurate way to align an antenna to the measurement set-up. Keywords: Near-field measurements, facilities, alignment, optical tooling. 1.0 Introduction Next to gain and sidelobe-levels, alignment is one of the major properties of an antenna. Most naval shipboard antennas are used as part of a complex combat system in which reaction time is critical, that means that designation from one radar to another one needs to be as accurate as possible to overcome long acquisition times. For integration of these subsystems each antenna is fitted with an additional optical telescope for alignment purposes. Alignment is done to make the pointing of the electrical axis to the same direction as the mechanical axis, mostly by using the optical axis as the reference. Nowadays it is not always necessary to actually align the electrical axis; it is acceptable to know the misalignment between mechanical and electrical axis and to correct in software for the errors somewhere else in the system. There are several ways of aligning antennas, using telescopes, mirror-cubes, tooling-balls or simply by measuring the distance from a reference to different points on the antenna-surface. Track-antennas for gunfire support on naval ships require measurement-accuracy s of about 0.1 mrad and that forces the responsible technicians to a lot of care during the process of alignment Far-field alignment 2.0 Alignment On board of Dutch naval ships standard telescopes are used, mostly removable ones that are placed in special dovetail-fittings [fig 1]. Three kinds of references for the definition of an antennaaxis can be found: Mechanical axis - defined by surface, shape, and construction of the antenna and its frame. Electrical axis - defined by feed (position w.r.t. reflector) or pattern/steering. Optical axis - defined by offset (translation) and alignment (rotation). Figure 1 - Alignment telescope

2 Alignment-checks are performed using a special collimation tower, equipped with various RF- and optical targets [fig.2]. Solution to overcome parallax is an offset between RFand optical targets identical to the offset between the phase center of the antenna and the optical axis of the telescope. This requires individual targets for each type of antennas as can be seen in fig Near-field alignment If an antenna is measured in a near-field antenna facility high accuracy s can be obtained, but careful attention has to be paid to possible errors, of which alignment is a major one. Example To demonstrate the effects of errors on alignment the most important numbers are given from the validation report of the Royal Netherlands Navy near-field facility using the ESA VAST-12 antenna. The NIST-error terms are sorted in the order of magnitude: term Error source Error 7 AUT alignment error 0, Systematic phase error 0, Probe Z position error 0, Mutual coupling (Probe/AUT) 0, Room scattering 0, Random amplitude/phase errors 0,001 Figure 2 - Collimation tower Parallax-errors are introduced when RF-origin and the optical-origin do not coincide, especially when the distances within the antenna-system (including optics) are not very close and the distance between antenna and target is not infinity. Figure 3 - Optical and RF-targets in collimation-rack The total rss alignment error is as large as 0,018 (0.3 mrad) which was far beyond the design goal. It can simply be seen that error-term nr 7, AUT alignment error, is by far dominant. It is the sum of the transfermirror (used because the optical cube was not visible from the theodolite in the test setup) and all instrumentation errors in the translation from scannersystem to the transfer-mirror. By careful design it was possible to align within 0.003, which brings all the errorterms within the same order of magnitude. The total rss error is then about 0,1 mrad as required. The conclusion was that it is very well possible to measure beam-pointing with high accuracy in a planar near-field facility. Some aspects are different from far-field measurements: 1. There is no well-formed beam available inside the test-setup. 2. The direction of main beam is normal to the planar scan-plane. 3. Reflecting objects in the vicinity of the EM-field can disturb the beam. 4. Antennas are frequently located at positions that are hard to reach. These points will be discussed in more detail.

3 Ad 1. Because there is no well-defined narrow beam at distances that occur inside a near-field facility, the antenna can not easily be pointed to an electrical target. The pointing direction of the beam is derived from the transform process, and is only available after a fully completed scan. Easily rotating the antenna to the maximum of the pattern is not possible. Ad 2. The pointing direction of the EM-beam is defined w.r.t. the scanplane of the planar near-field scanner, that is a virtual plane through the probe positions during a scan. Most of times this is normal to the antenna-pointing. That means that with additional tooling a translation of directions has to be made. Normally this is done with two theodolites, one of them is aligned with the scanplane, and the other one is aligned with the antenna. A solution to overcome these problems and have an accurate azimuth-alignment for different elevations was found in a procedure using a porroprism, which is described in the next chapter. 3. Elevation independent alignment Instead of the removable telescope a special porroprism mounted to a rectangular dovetail block is used [fig.5]. Ad 3. Normally no reflecting objects are allowed in the vicinity of the EM-field during pattern measurements. That means that no mirrors can be mounted on fixed positions on or behind the scanner. Another weak point of permanently mounting of mirrors is that there is often no absolute control over the stability of the surface where the mirror is mounted on. Ad 4. In a near-field site the antenna is preferably located in the center of the scan-plane. That means that it is frequently at an elevated position [fig.4]. Figure 5 - Porroprism on dovetail-fixture On this block electronic levels are used to align the antenna in two directions, elevation and roll, [fig.6]. Figure 4 - Elevated AUT-position That makes it very difficult to use the autocollimation technique for alignment of one theodolite to the antennatelescope, because there is no stable position possible standing on a scaffold! Figure 6 - Elevation- and roll-alignment using an electronic level

4 Finally, azimut alignment has to be done. For that the porroprism, which is mounted to the dovetail, is used. It has the property that all light-rays in a plane are reflected in the same plane, under the same angle for one direction (elevation), while it acts as a mirror for the other direction (azimuth). [fig. 7 and 8]. Unfortunately no supplier could be found to deliver a porroprism as a standard product, however, within our company there is a good optical department where expertss can make special lenses, prisms and other instruments, mainly used in periscopes. A prototype was designed and built by the optcal department and it was used to align various X-band track-antennas on different elevations. This demonstrated that the solution is very well usable in a near-field test setup. During this evaluation some critical points in the design were found with considerable effects on the accuracy. 4.1 Example Figure 7 - Operating principle of a porroprism el Figure 8 - Sideview porroprism The method is useful when the special tool is perfectly machined and aligned and when the alignment of the scannersystem to gravity is perfect or at least perfectly known. 4. Evaluation of prototype On a regular basis the complete scannersystem is aligned, including all axes of the planar scanner and the alignment of the z-stage positioner and the azimuth positioner for the antenna under test. So here will be focussed on the accuracies of the porroprism itself. To check the potential for alignment, a prototype was tested on the NF-facility. Error in azimuth (º) Elevation of antenna (º) As an example, the influence of the roll of the prism on the azimuth error was investigated in more detail. For this, an experiment was done to measure the error in azimuth-alignment for different elevations of the antenna for a given roll of the prism [fig 9]. Fig 9, Azimuth error for a given roll of the prism. This figure shows that for a given (small) roll of the prism, the error in azimuth is proportional with the elevation. This result shows that the tool (prism) has to be manufactured accurately to obtain neglectable errors in azimuth due to roll As an alternative, the results can be used to calibrate the prism and correct the azimuth alignment for incident angles that are known, which is the case in the NFmeasurement set-up.

5 4.2 Errors Errors and the sources of errors are analyzed. Errors are the result of internal deviations in the production, mounting and alignment of the different parts of the tool. The first group of errors is the specification of the prism itself like: top of the prism parallel to the front, measurements close to the Brewster angle, other effects in optics. Use of proper raw material (glass), proper manufacturing and proper use can minimize these errors. The second group of errors is related to mounting of the prism onto the dovetail-fixture. Misalignments will result in inaccuracies. For instance the deviation in azimuth as a function of misalignment in roll. In general, misalignment can be measured in separate experiments and measurement results can used for correction, see example in fig 9. The last group of errors is the mounting of the fixture in the slide, an integral part of the antenna. By giving attention to the roughness of the contact surface and proper tension after mounting, these effects can be minimized. measurement Facility fot the Royal Netherlands Navy 1998 AMTA Proceedings pp Conclusions For many antenna-applications alignment is important. Alignment in near-field facilities differs from farfield facilities, and needs special attention. Common alignment tools are not useful for alignment of antennas on different elevations. Special tooling needs to be carefully aligned and calibrated. Measurements and calibration can correct for some residual errors. 5. References [1] IEEE Standard Test Procedures for Antennas. ANSI/IEEE Std , Dec 19, [2] Newell, A.C., Error Analysis Techniques for Planar Near-field Measurements IEEE Transactions on Antennas and Propagation, Vol.AP-36, pp , June [3] Slater, D., Near-field antenna measurements, Artech House, 1991 [4] Moore, V. and Schluper, B., Precision Boresight Measurement for Doppler Radar Systems Measured on a Near-field Range 1998 AMTA Proceedings pp [5] Janse van Rensburg, D. X-band VAST-12 Antenna Validation Test Results on RNLN 9mx6m Planar Nearfield Test Range, March 2, 1998 [6] Hagenbeek, M. and Janse van Rensburg, D. Design and Validation of a General Purpose Near-field Antenna