2012 Loughborough Antennas & Propagation Conference 12-13 November 2012, Loughborough, UK

978-1-4673-2220-1/12/$31.00 ©2012 IEEE

An Optimized Radar Reflector Antenna Pair for Field Testing

Rodolfo Guidi, Antonio Sarri, Luca Fiori IDS Ingegneria Dei Sistemi S.p.A.

Via Enrica Calabresi, 24 - Loc. Montacchiello 56121 Pisa (PI) - Italy

r.guidi@idscorporation.com

Andreina Armogida RTW Ride The Wave S.r.l.

Via Roma, 25 56126 Pisa, Italy

aarmogida@rtw.it

Abstract—This paper presents the process carried out to design, manufacture and optimize the performance of a radar reflector antenna pair; this antenna assembly is to be integrated into a measurement system, used to evaluate Radar Cross Section (RCS) in field testing.

Keywords- radar reflector; Broadband antenna; optimal design; full wave methods; RCS; field testing.

I. INTRODUCTION The assessment of Radar Cross Section (RCS) and relevant

diagnostic signatures (i.e. high resolution range profiles, radar images), for existing and forthcoming aerospace and defense applications, requires a continuous improvement of measurement systems in several performance areas, such as operational bandwidth, sensitivity etc. In the particular case of field testing, typically when the target under test is a full scale platform in its operative environment, some further requirements such as deployability and flexibility become of primary importance. In addition, the ability to manage clutter, becomes of outmost importance for designing and developing measurement systems. To push performances to the state of the art, great care is to be taken in the whole system design as well as in the specific design of each subsystem.

This paper presents the workflow carried out to design, manufacture and optimize the performance of a radar reflector antenna pair.

It is shown that, as expected, when the constraints of deployability ( reflector dimensions), measurement flexibility, (bandwidth and polarization), clutter rejection (sidelobes), sensitivity (two way antenna gain) must be met altogether, conventional approaches based on asymptotic methods, tend to fail and full wave methods must be used at least for final assessment. A lot of hard work was done to improve existing methods to make them able to analyze the finer detail using full wave approaches, in a reasonable time.

The design is the result of the collaboration between IDS and RTW. The antenna pair consists of two radar reflectors operating in the 4-18 GHz nominal band, providing H and V linear polarization channels (on different ports).

The general design guideline aimed to achieve an optimal overall antenna performance across the entire band, so as to obtain a reasonable trade-off in terms of:

• Lowest possible VSWR at feeding coaxial ports;

• Highest possible gain;

• Shaping of main lobe vs. frequency;

• Highest possible suppression of side lobes;

• Lowest possible isolation between the two antennas (when using one antenna to transmit and the other to receive, possibly in continuous wave operation).

Maximum allowed dimensions and weight were also specified for the antenna pair.

Finally, cost issues should be taken into account as well, leading to the choice of a combination of full custom and commercial off the shelf parts for the manufacturing.

The paper is organized as follows: Section II presents a summary of design requirements. Section III reports the results of the preliminary design carried out by RTW by means of commercial design tools. Section IV shows the results of the experimental characterization of the prototype. Section V reports the optimization steps along with the sensitivity analysis carried out by IDS using the full wave methods of the IDS ADF framework This additional analysis enabled more accurate modeling of the antenna assembly. Conclusions are drawn in Section VI.

II. SUMMARY OF DESIGN REQUIREMENTS As anticipated, the general criterion for the definition of the

antenna requirements is to achieve optimal overall antenna performances to be matched to the specific application.

In the case of measurement systems dedicated to the assessment of radar cross sections (RCS) and relevant diagnostic signatures, when the target under measurement is a full scale platform in its operative environment, the performances to be accurately defined and designed are mainly antenna gain, to guarantee a suitable measurement sensitivity in terms of SNR for long range measurement (e.g. up to 15km), and beamwidth to guarantee a tapering less than 3dB all over the target extension (e.g. up to 150 mt) [1].

The following table reports a summary of the antenna requirements for some example frequencies. In particular, gain values are the minimum values which guarantee a NERCS (Noise Equivalent RCS) of about 0dBsm up to 15Km.

TABLE I. SUMMARY OF THE REQUIREMENTS: GAIN AND BEAMWIDTH

III. ANTENNA DESIGN AND PROTOTYPE MANUFACTURING The most suitable antenna system configuration to satisfy

the requirements was chosen after a feasibility study based on reflector antennas [2][3]. In order to ensure cost effectiveness in prototype manufacturing, the development of a semi-custom solution employing an existing reflector was chosen. Thus, a pair of identical on-set reflectors with 900 mm diameter and given F/D were chosen, one to be used for Tx operation and the other for Rx. The reflectors were supposed to be very closely spaced since the given overall width was below 2m.

The subsequent design activity focused on the development of a custom broadband feeding system to efficiently illuminate each reflector for both H and V polarizations. The design aimed to comply with both gain and beamwidth goals. The minimization of the isolation between the two adjacent antennas was also taken into account.

The feeding system had to:

• allow switching from H and V polarization

• have a reduced size (to minimize blockage)

• bear a maximum input power of some tens of Watt.

A ridged-horn seemed to be suitable for illuminating the reflector [4]-[6]. The use of quad-ridged horn to obtain the dual polarization operation was discarded as it showed inadequate to keep gain, beamwidth, and Return Loss under control across such a wide frequency band [7]. Moreover, the quad-ridged horn would not produce symmetrical E- and H-plane patterns, that is the condition to keep Azimuth and Elevation patterns unaltered when switching from one polarization to the other.

Hence, the use of a pair of double-ridged horn was preferred, one 90° rotated with respect to the other, as shown in Figure 1.

In order to reduce the computational complexity of the problem and the design time, a mixed approach was used. RTW modeled and optimized the ridged-horn was through CST MWS transient solver. Physical Optics (PO) was used to analyze the reflector system illuminated by the spherical wave expansion of the double-ridged horn radiation patterns [8].

The double-ridge horn was optimized so to provide an efficient illumination of the reflector from 4 GHz (low spillover and high isolation between Tx and Rx) up to 16 GHz (good efficiency and pencil beam).

As shown in Figure 1, the H-pol and V-pol horns are aligned symmetrically at an offset distance from the center of the reflector. The offset will produce a squint of the reflector antenna beam that is opposite in sign for the two polarizations and constant with frequency, and also the increase of asymmetrical side-lobes. Moreover, a deformation of the H-pol horn pattern in the Azimuth plane (E-plane) is due to the proximity of the V-pol horn. All the above effects have been considered in the design. The feeder position with respect to the reflector focal point was set after considering that the phase center of a double-ridge horn is not constant but shifts along the horn with frequency.

Figure 1. Pair of double-ridged horn illuminating the reflector

The PO analysis of the complete reflector antenna was performed taking into account the blocking effects due to the feeders. The main antenna parameters resulting from simulations of the optimized antenna assembly are summarized TABLE II.

TABLE II. SUMMARY OF OF THE DESIGN RESULTS

The double-ridge horn pair was manufactured by Pasquali Microwave Systems. Figure 2 shows the measured return loss of the double-ridge horn pair.

Frequency (GHz)

GAIN (dBi)

HPBW AZIMUTH

(deg)

HPBW ELEVATION

(deg) H-POL V-POL H-POL V-POL H-POL V-POL

4 >27 >27 >5 >5 >5 >5 8 >32 >32 >3 >3 >3 >3 10 >33 > 33 >2 >2 >2 >2 12 > 34 > 34 >2 >2 >2 >2 16 > 35 > 35 >1.5 >1.5 >1.5 >1.5

Frequency (GHz)

GAIN (dBi)

HPBW AZIMUTH

(deg)

HPBW ELEVATION

(deg) H-POL V-POL H-POL V-POL H-POL V-POL

4 29.4 29.4 5.7 5.7 5.7 5.7 8 34.0 34.0 2.9 2.9 3.3 3.3 10 35.7 35.7 2.3 2.3 2.9 2.9 12 36.8 36.8 2 2 2.5 2.5 16 38.7 38.7 1.6 1.6 2.1 2.1

Figure 2. Measured return loss of the double-ridge horn pair

The double-ridge horn pair was then assembled to the parabolic reflector by using glass-fiber struts plus a customized back plate for precise feeder positioning. The radiation patterns and gain were measured in Selex Galileo cylindrical Near Field facility located in Campi Bisenzio, Italy. Figure 3 shows the antenna prototype under test.

Figure 3. Antenna prototype under test at Selex Galileo.

IV. MESUREMENTS RESULTS This section presents the results of the experimental

characterization of the prototype performed in Selex Galileo near field test range.

Even if measured data show a substantial respect of the requirement, some discrepancy between experimental data and simulation results was observed at certain frequencies. In particular, Figure 4 reports the comparison between the measurement and the simulation (with the simplified model described in the previous section), of the azimuth radiation pattern at 16GHz. It’s apparent a discrepancy in the side lobes of the radiation pattern, while the main lobe, and consequently its beamwidth, is correctly predicted.

TABLE III reports the comparison between the predicted gain and the measured gain, even if there are some discrepancy between the values, the results are in good agreement with the requirements.

Figure 4. Prototype azimuth radiation pattern at 16GHz, comparison

between measurement and simulation with simplified model

TABLE III. MEASUREMENT VS SIMULATION: GAIN

Frequency [GHZ]

Measurement vs Simulation – Simplified Model Meaured Gain [dBi] Simulated Gain [dBi]

4 27.8 29.4

8 33.2 34

12 34.7 36.8

16 34.6 38.7

V. DESIGN ASSESSMENT In order to understand the differences between measured

and simulated data, a further design loop was carried out through full wave methods using the IDS ADF framework. In particular, a full wave approach on the whole model of the antenna was used by means of the IDSMMMP [10] code, that implements a Multi-Level Fast Multipole Method (MLFMA) [11], [12] with A Multi Resolution (MR) preconditioning [9]. The combined use of these two techniques allows the simulation of very large models, up to millions of unknowns, overcoming the limits typically imposed by the matrix size and condition number involved in “high-fidelity” electromagnetic problems when characterized by very different scales in the mesh size. In fact, the MR is proved to be able of drastically decreasing the condition number of the MoM matrix, allowing the generation of a very efficient preconditioner for MLFMA method. A preconditioner suitable for a fast method has to satisfy two different constraints: to be efficient (i.e. so to strongly reduce the number of iterations needed by the iterative solver to reach the desired precision), and to require a low computational cost for its construction and application.

The simulation were carried out on a detailed model of the antenna. This model contains the reflector with the real shape of the edge, the two horns, the four struts and the plate to mount feeders. In Figure 4 the e.m. model is reported. The model is formed by 700.000 unknowns and it takes computation times of about 3 hours.

Figure 5. Detailed e.m. model of reflector antenna

The capability to manage very detailed model with reasonable computational effort enabled to perform a sensitivity analysis on several elements of the model, in order to investigate their contribution to blockage phenomena at high operating frequencies [13]. These analyses allowed to ascribe to the horn mounting plate the major contribution to the gain degradation for blockage phenomena. A new design of the plate was developed to minimize this drawback. Figure 6 reports the comparison between the measured and simulated radiation pattern at 16 GHz.

The simulation results with the detailed model of the antenna are in better agreement with the measured data respect to the ones obtained with the simplified approach.

Figure 6. Prototype azimuth radiation pattern at 16GHz, comparison between measurement and simulation with detailed model

TABLE IV. MEASUREMENT VS SIMULATION: GAIN

Frequency [GHZ]

Measurement vs Simulation - Detailed Model Meaured Gain [dBi] Simulated Gain [dBi]

4 27.8 27.9

8 33.2 33.6

12 34.7 34.5

16 34.6 35.0

TABLE IV reports the comparison between the measured and the detailed model simulation gain results. The new data show a better agreement respect to the results obtained with the simplified model.

VI. CONCLUSIONS An “incremental approach” for the development of a

broadband radar reflector antenna is reported. The design was based on initial fast simulations of simplified e.m. models and a final analysis refinement on a complete model using a full wave “state of art” tool. The latter proved to be very accurate and suitable to reproduce some secondary effects, such as fixtures blockage, that are observable on the measured radiation patterns of antenna prototype. The proposed approach proved to be very effective representing a satisfactory compromise between electromagnetic, physical and cost requirement achievement.

ACKNOWLEDGMENT The authors would like to acknowledge Dr. Giuseppe

Mauriello for offering the chance to perform measurements in Selex Galileo cylindrical near field facility in Campi Bisenzio, Italy, along with his colleagues Mr. Massimo Beverini and Dr. Luca Lachi for their valuable support during all test campaign.

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