AN OVERVIEW OF TEST TECHNIQUES FOR CHARACTERIZING ACTIVE PHASED ARRAY ANTENNAS W.P.M.N. KEIZER TNO-FEL P.O. Box 96864 2509 JG The Hague The Netherlands E-mail:
[email protected] In this paper a review will be given of the microwave testing of active phased array antennas. It will be shown that due to the application of Transmit/Receive (T/R) modules in such antennas considerable more tests have to be performed to characterise completely their microwave performance than for conventional passive antennas. The major characteristics of three types of antenna test ranges, far-field, compact and near-field, will be discussed including their suitability for testing active phased array antennas.
1 Introduction The testing and evaluation of the far-field behaviour of antennas in terms of the antenna pattern, directivity, gain, boresight direction, beamwidth and so forth are performed in antenna ranges. The ideal condition for measuring far-field radiation characteristics of an antenna is the illumination of the Antenna Under Test (AUT) by a plane wave front with uniform amplitude and phase. Although this ideal condition is not achievable, it can be approximated by separating the AUT from the illuminating source at a large distance. Under this condition the curvature of the spherical phasefront produced by the source antenna is small over the test antenna aperture. If the separation distance R is equal to R = 2D2/λ , (D = diameter AUT and λ = wavelength) then the maximum phase error of the incident field from an ideal plane wave is about 22.5 deg. This separation is adequate for measurement of the pattern, directivity, gain, boresight direction and halfpower beamwidth, but is far too small for accurate low sidelobe testing. Low sidelobe testing requires a phase error much smaller than 22.5 deg for the plane illuminating wave, which can only be achieved when the separation R is much greater than 2D2/λ. The outdoor far-field range is best suited for achieving large separations between the AUT and the illuminating source. However, during recent years two other types of antenna test ranges, the compact range and the near-field range, have been developed which create over a very short distance a planar wave for testing antennas. The main features of each of these three types of test ranges will be covered by this paper. Besides, it will be shown that the experimental characterisation of active phased array antennas is much more elaborated than that of conventional passive antennas. A number of additional tests are needed as a result of the application of RF solidstate amplifiers in the T/R modules of active phased array antennas. This paper will identify these additional tests and will discuss the suitability of the three test ranges for characterising active phased array antennas. Included is also a description of the main features of a large compact range and a large near-field range, both installed in the Netherlands.
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Conventional antenna testing
Experimental far-field qualification of conventional antennas comprises following measurements: • • •
Far-field (FF) pattern, usually limited to E- and H-plane two-dimensional pattern cuts (co- and cross polar) Gain or directivity Halfpower beamwidth, E- and H-planes
A.B. Smolders and M.P. van Haarlem (eds.) Perspectives on Radio Astronomy – Technologies for Large Antenna Arrays Netherlands Foundation for Research in Astronomy - 1999
• •
Boresight direction Monopulse slope (only for monopulse tracking antennas)
to be made at a number of different frequencies constituting the frequency range of the antenna. Since most conventional antennas are passive and behave reciprocal, usually no separate testing is needed for transmit and receive operation.
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Active phased array antenna testing
The experimental qualification of the radiating properties of phased array antennas involves much more testing especially for the active ones equipped with T/R modules. This is related to the power amplifiers in the T/R modules generating the transmit signal during transmit operation, and the low noise amplifiers switched on during reception to amplify the received signal. Also scanning of the main beam which changes the antenna pattern, necessitates the testing of the far-field pattern at different scan angles for a number of different frequencies. An overview of the tests to qualify an active phased array antenna is given below. The non-reciprocal behaviour of active phased array antennas requires that a series of tests have to be performed for both receive and transmit operation of the AUT. These tests include: • • • • • • • •
Three-dimensional far-field patterns (co- and cross polar) Directivity Halfpower beamwidth Boresight direction Peak sidelobe level distribution and rms sidelobe level Spurious signal generation Switching time scanned beam Monopulse slope (not on transmit and only for monopulse tracking antennas)
High performance phased array antennas require the measurement of three-dimensional patterns and detailed knowledge about the peak sidelobe distribution and the rms sidelobe level. The last two parameters are not actual measurements, but can be derived through processing of the three dimensional FF results. The same applies for the directivity and the halfpower beamwidth. Verification of the proper operation of the power amplifiers in the T/R modules during transmit operation requires testing of the following parameters: • • •
Effective Radiated Power, (ERP) Pulse shape (only for pulsed transmitted signals) Three-dimensional far-field patterns for different pulse repetition rates and duty cycles (only necessary in case of pulsed transmit signals)
Also on receive some specific tests are needed which apply only for active phased arrays and are the consequence of the use of solid-state components in the receiving part of the T/R modules. These tests deal with the characterisation of the antenna on receive in terms of noise figure, dynamic range and linearity, and involve: • • • •
Antenna noise figure Input saturation level 1dB compression level at the antenna output Third order intermodulation distortion
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All above listed tests have to be performed at different frequencies, at various scan angles constituting the beam scan range and under operational conditions. The latter means that active phase array antennas have to be tested at ambient temperature ranges and at transmit power levels, average and peak, all typical for operational use.
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Antenna test ranges
Antenna test facilities are categorised as outdoor and indoor ranges, and limitations are associated with both of them. Outdoor ranges are not protected from environmental conditions whereas indoor facilities are limited by space restrictions. Due to the large separation between the AUT and the illuminating source, this condition limits the value of indoor facilities to testing large size antennas. 4.1 Outdoor far-field range Outdoor far-field ranges are usually designed to operate mostly over smooth terrains with the antennas mounted on towers or on roofs of adjacent buildings. The contributions from the surrounding environment in terms of unwanted reflections are usually reduced or eliminated. Outdoor ranges are used to test physically large antennas and therefore they accommodate a large separation between the AUT and the source antenna. In general, the pattern of an antenna is threedimensional. Because it is impractical to measure a three-dimensional pattern, a number of twodimensional patterns are measured and then used to construct the three dimensional graph. A two-dimensional plane cut of the three-dimensional far-field pattern of the AUT is measured by rotating the AUT in the azimuth plane. Other two-dimensional pattern cuts can be measured in an identical way, after the orientation of the AUT and that of the source antenna in the vertical plane has been changed. The minimum number of two-dimensional patterns is two, and they are usually chosen to represent the orthogonal principal E- and H-plane patterns. Since three dimensional pattern measurement is very time consuming for a far-field range, pattern testing is usually limited to the measurement of Eand H-plane cuts. 4.2 Compact range The requirement of an ideal plane wave illumination can be nearly achieved by utilising a compact range. A Compact Antenna Test Range (CTAR) is a collimating device which generates a nearly planar wavefront in a very short distance (10 to 20 meters) compared to the 2D2/λ distance required for a far-field test range. Compact antenna test ranges are essentially very large reflector antennas designed to optimise the planar characteristics in the near-field of the aperture. Compact range configurations are often designated according to their analogous antenna configurations: parabolic, Cassegrain, Gregorian and so forth. The major drawbacks of compact ranges are aperture blockage, direct radiation from the source to the test antenna, diffractions from the edges of the reflector and feed support, depolarization coupling between the AUT and the feed antenna of the CTAR, and wall reflections. The use of an offset feed eliminates aperture blocking and reduces diffractions. Du to the finite size of the CTAR reflector and the imperfections of their surfaces, the test zone fields they produce can only approximate plane waves. The quiet zone, the usable portion of the test zone, consists of nearly planar wavefronts. The size of the quiet zone is typically about 50% - 60% of the dimensions of the main reflector. The frequency of operation of a CATR is determined by the size of the main reflector and its surface accuracy. The low-frequency limit is usually encountered when the reflector is about 25 to 30 wavelengths in diameter resulting in large quiet zone ripples at the low frequency limit. At high frequencies, reflector surface imperfections contribute to the quiet zone ripple. Many CATR systems
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operate typically from 1 to 100 GHz. The way the antenna testing is made in a CATR, is almost identical to that of a far-field range and requires the rotating of the AUT in the azimuth plane. The three dimensional far-field testing in a compact range is therefore a very time consuming process, since it is performed in the same way is as for the far-field range. An example of a large CATR is the ESA/ESTEC Compact Payload Test Range (CPTR) located in Noordwijk, the Netherlands, [1]. It is a RF facility designed for both antenna, RCS and payload end-to-end testing on satellites in a climate controlled clean environment. The CPTR is a dual reflectors type range with an optimised geometry for cross polarisation reduction. The feed of this range is placed on a positioner to allow the change of the angle of incidence of the CPTR plane wave on the AUT. In this way plane wave scanning measurements of -3 to +3 deg can be performed. Table 1 summarises the main characteristics of this CATR. Reflector system Size aperture reflectors Dimensions quiet zone Scan range CPTR main beam Frequency range Gain accuracy Amplitude ripple Phase ripple Surface accuracy reflectors
Compensated Cassegrain System 9m x 8m 7m x 5m x 5m (W x H x D) -3 to 3 deg 1 - 35 GHz 0.16 dB ± 0.4 dB ± 4 deg 0.05 mm rms
Table 1 Characteristics ESA/ESTEC Compact Payload Test Range
An artist impression of the ESA Compact Payload Test Range is given in Fig. 1.
Fig. 1
Artist's impression of the ESA/ESTEC Compact Payload Test Range
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4.3 Near-field range The dimensions of a conventional FF test range can be reduced by making measurements in the nearfield, and then using analytical methods to transform the measured near-field data to compute the farfield radiation characteristics. These are referred to as near-field to far-field (NF/FF) methods. Such techniques are usually used to measure patterns, and they are nearly always performed indoors. Therefore, they provided a controlled environment and an all weather capability and the computed patterns are as accurate or even better as those measured in a far-field range. Furthermore the computed patterns are three-dimensional. The near-field test data (usually amplitude and phase distributions) are measured by a scanning a RF field probe over a pre-selected surface which may be a plane, a cylinder, or a sphere. The measured data are then transformed to the far-field using Fourier transform methods. The complexity of the Fourier transformation increases from the planar to the cylindrical, and from the cylindrical to the spherical surfaces. In general the planar system is better suited for high gain antennas, especially planar phased arrays, and it requires the least amount of computations and no movement of the antenna. Acquisition of planar near-field data is usually conducted over a rectangular x-y grid with a maximum near-field sample spacing of ∆x = ∆y = λ/2. The test antenna is stationary while the probe is moved to each grid location on the plane. The planar transformation to get the far-field from the near-field can be done with the computationally efficient Fast Fourier Transform (FFT) algorithm. Planar Near Field/Far-Field (NF/FF) techniques are well suited for measuring antennas which have low backlobes. The primary disadvantage of probing the near-field on a planar surface to calculate the far-field, is that the resulting far-field pattern is over a limited angular span. If the planar scanning surface is of infinite extent, one complete hemisphere of the far-field can be computed. Planar near-field testing is therefore usually only applied to directional antennas. The near-field technique provide the antenna designer information not previously available to him. For example, if a given far-field pattern does not meet the required specifications, it is possible to use the near-field data to pinpoint the cause. It is also possible to diagnose the antenna illumination by back-transform the computed far-field to the aperture of the AUT using an inverse FFT. This back transformation method reveals accurate information on the aperture field distribution of the AUT and can be used to determine the element excitations of phased array antennas. Applications of this technique include element failure diagnosis, phased array calibration and T/R module alignment. An example of a large near-field test facility installed in the Netherlands is the planar near-field range operated by SEWACO in Den Helder, [2]. It is a multi-purpose planar & cylindrical near-field test facility in use with the Royal Netherlands Navy. The planar scanner depicted in Fig. 2, has a total probe travel of 9m (X-axis) x 6.4m (Y-axis) and has a planarity of better than 25µ rms. This facility is designed for dealing with high radiated power levels up to several kilowatts from active phased array radar antennas. The near-field facility is housed in shielded room of which the inside is covered with RF absorbers to prevent reflections from the walls. Special attention was given to new designed high power absorbers in an area on the back wall. These absorbers are air-cooled and capable to withstand RF power levels more than 1.5 W/cm2. The RF instrumentation is based on a dual source HP8530 antenna receiver with remote mixers and allows operation up to 40 GHz. It has a pulse capability to allow pulsed RF measurements and is reconfigurable for AUT receive or AUT transmit. The main characteristics of the SEWACO near-field facility are given in Table 2.
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Fig. 2
Drawing of the SEWACO 9m x 6.4 m planar scanner.
Near-field scan type Frequency range Antenna type tested XY scanner size Scan speed for X, Y axes Scanner planarity (rms) Scanner planarity corrected (rms) AUT Z travel stage Maximum radiated RF power Gain uncertainty Side lobes uncertainty Boresight accuracy Cross polarization uncertainty
Planar and cylindrical 1 - 40 GHz Shipborne radar antennas 9m x 6.7 m 0.25 m/s 0.1 mm 0.025 mm 4.5 m Several kilowatts 0.25 dB 3 dB at -45 dB level 0.02 degrees 3 dB at -30 dB
Table 2 Main characteristics of the SEWACO near-field range
Fig. 3 shows the SEWACO planar near-field scanner testing a linear array.
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Fig. 3
Linear test array in front of the SEWACO near-field scanner
4.4 Main features far-field, compact and near-field test ranges The main features of the three types of antenna ranges discussed before, including their advantages and disadvantages are summarised below. Far-field Range • • • • • •
Outdoor, range must be fee of obstacles Large real estate, length range >2D2/ λ Instantaneous results for two-dimensional pattern cuts Rotation of AUT necessary Three-dimensional pattern testing very time consuming Subjected to adverse weather conditions
Compact Range • • • • •
Indoors, therefore all weather capability and shielded room assures secrecy Short range, plane wave obtained through illumination of large parabolic reflector Operation and results comparable to far-field range Very suited for RCS testing Due to ripples in plane wave not suited for accurate low sidelobe testing
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Near-field Range • • • • • •
Indoors, therefore all weather capability and shielded room assures secrecy Plane wave sequentially synthesised by moving the scanner probe Three-dimensional FF data after computer processing NF probe data No rotation of AUT (only planar NF) Time consuming data collection by NF probe High accuracy FF characteristics, pattern, directivity, peak direction, beamwidth, sidelobes
Since the far-field range requires a large real estate, nowadays the compact range as well as the nearfield range provide the most attractive solution in terms of investment costs. In Section 3 an overview was given of the tests which has to be performed for the qualification of active phased array antennas. Table 3 summarises the suitability of the three ranges described in Section 4 for testing active phased array antennas. According to the results of this table it seems that the near-field range is most attractive solution for testing phased array antennas. The test providing the most difficulties is the ERP measurement when a high transmit power will be generated by the phased array. In that case the far-field range is the only solution. The compact range is then hardly a valuable option, since during scanning the array the transmit beam can point to any direction in the anechoic room which means that the room must be completely covered by costly high power absorbers.
Tests for both Transmit and Receive Three-dimensional patterns Beamwidth Directivity Boresight direction Peak sidelobe distribution and rms sidelobe level Spurious signal generation Switching time scanned beam Transmit only Tests ERP Pulsed shape Three-dimensional patterns at different pulse repetition frequencies and duty cycles Receive only Tests Noise figure Input saturation level Third order intermodulation distortion Monopulse slope (only for monopulse tracking antennas)
Far-field Range -+ + + + +
Compact range -+ + + + +
Near-field Range ++ ++ ++ ++ ++ + (*) + (*)
++ ++ -
++ -
+ ++ (*) ++
Far-field range + + + +
Compact range + + + +
Near-field Range + (*) + (*) + (*) +
Legend: ++ best -worst (*) not an actual NF measurement Table 3
Comparison of FF, CTAR and NF test ranges for testing active phased array antennas
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The results in the column "Near-Field Range" and labelled with (*) are in fact not measurements made with the NF scanner. For these test the main beam is focused to a point in the anechoic room where the test source or the pick-up antenna is located, depending on the type of test to be performed.
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Conclusion
This paper discussed the possibilities of three types of test ranges for experimental qualification of phased array antennas. It is demonstrated that the qualification of active phased array antennas involves more tests than for conventional passive antennas. It seems that the planar near-field facility is most suited for testing high performance active phased array antennas.
References [1] J. Lemanczyk, Private communication, (1999). [2] M. Hagenbeek, D. Janse van Rensburg, “Design and Validation of a General Purpose Near-Field Antenna Measurement System for the Royal Netherlands Navy”, 1998 AMTA Symposium Proceedings, pp. 203 -208 (1998).
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