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DUAL FREQUENCY MULTI-FUNCTION RADAR ANTENNA RESEARCH

S.A.W. Moore’; G.A. Hockham*

Sea Systems Sector, DERA, UK; GH Consuitancy, UK 1

BACKGROUND

In 1995 the Radar Research Group within Sea Systems Sector at DERA Portsdown West began investigating dual frequency radar concepts under the Corporate Research Programme to overcome the current compromise in operational frequency choice. The work discussed herein builds upon the considerable expertise that has been gained by the team through the highly successful MESAR MFR (see FIGURE 1) programme, which began in 1977 (see Moore et al (1)).

INTRODUCTION

Multi-function radars (MFRs) are currently being proposed for use in naval (e.g. SAMPSON and EMPAR for the CNGF programme) and land (e.g. THAAD Radar for BMD) environments because they provide many weapon system performance advantages.

FIGURE 1 - MESAR MFR

However existing weapon system MFR designs (performing both the Search and Track function) are a based upon a fundamental frequency compromise that does not allow optimisation of performance against each radar task. The current, most expedient approach is to choose a frequency between the preferred Search and Track frequencies.

This naturally leads to the conclusion that the performance commensurate with multiple dedicated sensors cannot be achieved with a single weapon system MFR.

The aim of the dual-band MFR research is to do away with the need to compromise over optimum Search and Track frequency choice.

DUAL-BAND MFR OPERATING FREQUENCIES

Initial studies by Moore and Moore (2) have indicated the preferred frequencies to be used for the Search and Track functions in a naval or ground based weapons system MFR that form the basis for the early dual-band MFR research (see TABLE 1).

BAND DESIGNATION

Search Track 10 GHz

TABLE 1 - Preferred Dual-Band MFR Frequencies

The chosen operating frequencies are currently used separately by conventional naval radar systems, but no existing system has yet integrated both into a single sensor to support advanced weapon system requirements.

The combination of these particular frequencies into a single aperture has the further potential to enhance MFR performance to a level suitable for ballistic missile defence. This is currently being investigated.

CHOICE OF RADIATING ELEMENTS

An extensive antenna element study conducted early in the research programme (2) concluded that a single element, capable of radiating at both frequencies, would not meet the specified MFR performance characteristics.

The study was then expanded to consider different elements to radiate at L- and X-Band. After much investigation it was decided to choose a waveguide for the L-Band radiating element and radiating dipole for the X-Band radiating element.

Radar 97, 14 - 16 October 1997, Publication No. 449

L-Band Radiating Element ANTENNA INTEGRATION PROBLEMS

The chosen double tuned di-electrically loaded L- Band waveguide element (see FIGURE 2) has dimensions of 130" wide by 30" high. This is a comprise between satisfying the desired agile bandwidth requirement, whilst achieving a low element cross section- to maximise the area available for the X-Band elements.

DIELECTRIC PLUQ . TUNING

SCREW

STANDARD CDNNECTOR a WIRE COUPLER LOW GROUND PLANE

PLAN VIEW TOP VIEW

FIGURE 2 - Double Tuned Di-electrically Loaded L-Band Waveguide Element

The advantage of this type of element for use in a phased array antenna is two fold.

Firstly the small element cross section permits X- Band radiating dipoles to be placed sufficiently close to the L-Band element aperture to eliminate grating lobes (caused by the imposed periodic effect on the X-Band element lattice by the presence of large L-Band waveguides) over wide scan angles.

Secondly, the likelihood of higher order modes becoming resonant, and thereby perturbing the antenna performance, is greatly diminished.

X-Band Radiating Element

Because of the small size of a X-Band dipole relative to a large L-Band waveguide, there is not the same drive to reduce the dipole size to prevent any array lattice periodicity (and hence grating lobes). This means that established dipole theory is readily applicable.

A design was therefore chosen (see FIGURE 3) to give an element pattern close to the ideal broad pattern for phased array applications.

TRIPLATE DIPOLE DIPOLE WITH CENTRAL BALUNSTRUCTURE

MICROSTRIP INPUT

FIGURE 3 - Two Candidate Designs For The X- Band Radiating Dipole

Antenna integration problems that arise when designing a dual frequency antenna include:

Because of the frequency dependence of element size and separation, the optimum track frequency ( I O GHz) would require 100 times more elements, spaced 10 times closer, than those at the search frequency (I GHz) for a fully filled array (assuming a similar aperture).

(ii) Due to the large area required by the L-Band elements it is not possible to fully fill the same antenna aperture with X-Band elements (see FIGURE 4).

(iii)A new problem is now introduced into MFR antenna design - that of predicting and minimising the extent of the heterogeneous element mutual coupling (the interaction between homogeneous elements is well understood). .

0 5 0 i = 1 5 0 "

X-Band Radiating L-Band Unii Oiwlo Element

cell /

1 X-Band Unit Cell

L-Band Waveguide Radiating Ehement (130mm by 30")

$ 050n=15mm f---t

0 5 7 5 ~ = 1 7 3 m m a 0575A=1725mm.

FIGURE 4 - Antenna Integration Problems

Since there could be up to 100 times the number of X-Band elements compared to those at L-Band, thinning of the X-Band elements can be tolerated.

To improve the sidelobe performance of the X- Band array it was decide to apply a -35 dB Gaussian density taper (see FIGURE 5).

25 . . 44-44 1 4 4

4 4 Number Of X- 4 4

4 1 4 1 4

0 0 5 10 15 20 25

L-Band Cell Number

FIGURE 5 - X-Band Gaussian Density Taper Across The Centre Of The Aperture

However, integrating a reduced number of X-Band elements into an antenna fully filled with L-Band elements introduces undesirable periodic effects into the X-Band array.

The research conducted so far has led to a novel antenna concept in which the L-Band radiating elements are placed in pseudo-random locations (the antenna is still fully filled with L-Band elements) as shown in FIGURE 6. In other words, the uniform element matrix is randomly perturbed by a fixed amount (determined by computer modelling). This should ensure a minimum of engineering complications for final implementation.

I %W X O iom Ixo

.m I

Horizontal Dimension (mm)

FIGURE 6 - Pseudo-Random L-Band Element Location

There is now no longer any periodicity due to the L-Band element matrix into which the X-Band elements are being integrated (see FIGURE 7).

L-Band Radlatlllll

FIGURE 7 - Segment Of Dual Frequency Antenna Showing Pseudo-Random L-Band Element Positions Interspersed With X-Band Radiating Dipoles

DUAL-BAND MFR MODELLING

Method Of Moments Modelling

Initial computer modelling of a 4 metre diameter circular array has been conducted using simple method of moments theory. The model takes into account homogeneous element mutual coupling, but not heterogeneous element mutual coupling (a limitation of the existing model). However it was

used initially to assess the basic heterogeneous element integration problems.

Modelling has shown that the novel antenna concept of pseudo-random perturbation of the L- Band element matrix has only a small effect on the overall L-Band antenna pattern (see FIGURE 8) . This is because of the large number of L-Band elements (-500).

00 SCAN -OFF 600 SCAN-OFF dB dB

-10 -10

-20 -20

-30 -30

-10 -40

-so -50 -75 -50 -25 0 25 50 1 5 -15 -SO -25 0 25 50 75

Degrees Fmm Broadside Degrees Fmm Broadside

FIGURE 8 - L-Band Diagonal Plane Array Patterns (5 bit phase shifters, + I O " phase error)

Due to the large number of X-Band radiating dipoles within the dual-band array (-6000 for a 12% filled X-Band array), it has so far only been possible to calculate the X-Band array performance for the central two rows of the antenna (containing -800 dipoles). These results are shown in FIGURE 9.

NEAR-IN SIDELOBES FULL ARRAY WIDTH

dB

-10

.,I 4 0 - 2 1 6 I J 50 1s

Degrees From Broadside Degrees From Broadside

FIGURE 9 - X-Band E-Plane Array Pattern For The Central Two Rows

The X-Band array patterns confirm that the removal of any array periodicity caused by fixed L- Band element positions leads to performance advantages for the X-Band array.

Heterogeneous Element Mutual Coupling

The previous results included only mutual coupling between identical elements.

The next major phase of the D E W dual-band MFR research programme was to consider mutual coupling between the different L- and X-Band radiating elements.

It was decided early in this phase that the best way to proceed was to exploit the many

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advantages of the finite difference time domain (FDTD) method.

A dual-band MFR antenna model has been constructed using FDTD techniques to predict the heterogeneous element mutual coupling extent on a microscopic level.

Initial results are shown in FIGURE 10.

FIGURE 10 - X-Band Radiating Dipole Transverse Magnetic Field Near A L-Band Waveguide

This computer modelling work is currently quantifying the extent of heterogeneous element mutual coupling. But all of the mutual coupling results obtained so far give confidence in the postulated dual-band antenna design.

The next step in the planned research programme is to combine the two modelling approaches (method of moments and FDTD modelling techniques) to predict the macroscopic performance of the dual-band MFR antenna. This is currently on-going.

In tandem with this theoretical work, practical solutions to limit the effects of mutual coupling are being investigated and tested experimentally using a waveguide simulator.

DUAL-BAND ANTENNA EXPERIMENTATION

It was decided early in the research programme to validate any modelling or theory predictions through experimentation. This validation of mutual coupling predictions, through practical demonstration of dual-band antenna technology, should ultimately lead to a rapid growth path towards proven and exploitable dual-band MFR antennas.

A limited experimental programme based around the established concept of a "waveguide simulator" is currently underway. It is recognised that this low cost experimental option does not allow investigation and demonstration of the perceived benefits of the pseudo-random L-Band

element matrix concept, but it does address the key heterogeneous and homogeneous element mutual coupling aspects.

WAVEGUIDE SIMULATOR CONCEPT

Waveguide simulators can demonstrate, with only a few elements, the performance of an infinite phased array (see FIGURE 11). Array simulators have evolved from the original work of Brown and Carberry (3) and Hannon and Balfour (4).

FIGURE 11 - Use Of A Waveguide Simulator To Predict Infinite Phased Array Performance

The completely reflecting metal walls of the waveguide simulator image the elements into an infinite array. Measurements made inside the simulator waveguide therefore represent the performance of the infinite phased array at a discrete set of frequencies and scan angles determined by the dimensions of the simulator waveguide.

The waveguide simulator concept means that only a small portion of the array, within the dimensions of the structure, need be constructed (see FIGURE 12).

"I,,..

WA-

PLAN VIEW TOP VIEW

FIGURE 12 - Waveguide Simulator Designed To Investigate Mutual Coupling Between L- And X-Band Elements In An Infinite Array Environment

The waveguide simulator method only gives one scan angle data point at each frequency. Therefore one simulator is not adequate to test the wide angle scan behaviour of a given dual- band array configuration. However, by comparing the results from the theoretical models with the

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simulator data, the scan angle performance as a function of frequency can be confirmed and any scan anomalies uncovered. Equally important, with a confirmed theoretical model, the dual-band array performance at all planned scan angles and frequencies can be confidently predicted,

The first stage of dual-band MFR antenna experimental programme, using the waveguide simulator approach, finishes in October 1997.

CONCLUSIONS

The results of the research indicate that a dual- band MFR based upon the currently postulated design has the potential to overcome the existing single frequency compromise of MFRs, leading to significant benefits for military weapons sensors (see FIGURE 131.

FIGURE 13 - Dual-Band MFR Tasks

The theoretical research, together with the practical demonstration of the concepts, should enhance understanding of dual-band arrays and increase confidence in the ideas detailed in this paper.

In conclusion, all of the research points towards a potentially successful candidate to continue the UK’s pioneering research into ship-based weapon system multi-function radars. In addition, the work points to applications of dual-band MFR technology in the ballistic missile defence sensor role.

ACKNOWLEDGEMENTS

The great help and significant technical contribution of Dr. Chris Sullivan (SERCo Consultancy), RE Thompson Precision Engineers and the late Eurlng Harry Spiller (HSA) to the dual-band MFR research programme is gratefully acknowledged and much appreciated.

0 British Crown Copyright 1997 / DERA. Published with the permission of the controller of Her Britannic Majesty’s Stationery Off ice.

REFERENCES

1. Moore AR, Stafford WKS and Salter DM, 1997, “MESAR (Multi-Function Electronically Scanned Adaptive Radar)”, IEE Radar 97 proc.

2. Moore SAW and Moore AR, 1997, “Dual Frequency Multi-Function Radar Antenna Research”, IEE CAP 97 Proc., 1, 522-526

3. Brown CR and Carberry TF, 1963, “A Technique To Simulate The Self And Mutual Impedance Of An Array”, IEEE Trans., AP-11, 377-378

4. Hannon PW and Balfour MA, 1965, “Simulation Of A Phased Array Antenna In A Waveguide”, IEEE Trans., AP-13, 342-353