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“DUAL FREQUENCY MULTI-FUNCTION RADAR ANTENNA RESEARCH”

S.A. W. Moore, Dr. A.R. Moore Sea Systems Sector, Defence Research Agency, UK

BACKGROUND

In 1995 the MESAR (Multi-function Electronically Scanned Adaptive Radar) MFR research group at DRA Portsdown West began investigating dual frequency radar concepts under the Corporate Research Programme to overcome the current compromise in operational frequency choice. This work which is discussed herein builds upon the considerable expertise that has been gained with the highly successful MESAR MFR programme which began in 1977 (see FIGURE 1).

SEARCH REQUIREMENTS: Wide beamwidths to reduce search time

0 Low radar frequency to reduce Doppler waveform dwell time in severe clutter scenarios

TRACKING REQUIREMENTS: 0 Narrow beamwidths to increase angular

resolution and accuracy Wide bandwidth to increase range resolution and accuracy High update rate to maintain tracks Various parameter diversities to counter sea multipath effects

TABLE 1 - Search And Track Function Optimum Requirements

In a MFR with,

- a given antenna aperture; ~ a narrow operating frequency band; - a fixed height; - weight and cost limitations; and - limited radar time budget.

FIGURE 1 - MESAR Research History the performance commensurate with multiple dedicated sensors cannot be achieved.

TRQDUCTION

Multi-function radars (MFR) are currently being proposed for use in naval (e.g. SAMPSON and EMPAR for the Project Horizon frigate programme) and land (e.g. ARABEL for SAMP(T) and GBR for BMD) environments because they provide many weapon system performance advantages, e.g.:

- fast reaction times; - multiple target engagement capability; - provision of guidance messages to own

- enhanced counter jamming techniques. missiles; and

However these advantages belie the fact that basically an MFR design is a compromise that does not allow optimisation of performance against each radar task.

In particular, it is difficult to optimise the requirements of the Search and Track functions in an MFR, since they require different radar parameters to achieve best performance (see TABLE 1).

No techniques currently exist that can optimise separately the performance of MFR’s in both Search and Track.

AIM OF RESEARCH PROGRAMME

The principal aim of the dual-band MFR research programme is to design, model and evaluate a dual frequency antenna utilising appropriate radar frequencies to improve land or sea based weapon system MFR performance in both the Search and Track functions.

BENEFITS OF DUAL-BAND MFR’s

Besides the many MFR performance advantages gaining by utilising optimal frequencies for the Search and Track functions, other benefits have been highlighted.

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Improved Track Robustness

If a target that is being tracked moves out of the antenna beamwidth, an update can be scheduled using the search frequency (which has a larger beamwidth). This provides an alternative to beam broadening or undertaking a localised search routine.

Target illumination Application

If the necessary radar time budget is available (scenario dependent), the tracking frequency could be utilised to provide interrupted illumination on a target during terminal engagement.

Better Performance With Liow Flying Targets

If a target passes through a multipath null that denies the radar sufficient information to determine the targets’ location the search frequency can be exploited to supplement the tracking frequency, providing an independent look (since multipath is frequency dependent). This is likely to be more effective than single band frequency diversity due to the wider frequency separation.

Improved ECCM Performance

Since a jammer is unlikely to transmit simultaneously at both radar bands, targets close to jammer strobes can be tracked using the other available radar frequency.

The dual-band MFR will in addition exploit the many advances made during the MESAR prqpamme on digital adaptive beamforming to cancel both sidelobe and mainbeam jammers. This, coupled with the narrow tracking beamwidth, should allow targets to be resolved from jammers at much closer angular separations.

Co-Located Search And Track Antennae

Co-locating both the Search and Track antennae allows each of them to be accurately aligned. Separated antennae suffer from random movement due to mechanical strain (e.g. ship flexure) which introduces dynamically varying errors.

Co-location also has significant potential for size and weight savings since all the capabilities of

t‘wo large antennae have been combined into one. In addition, the two sensors no longer have to compete for the prime location on a ship (‘typically the top of a ships’ main mast).

Redundancy

In general, if one frequency cannot provide the necessary information because of a highly stressing environment (e.g. severe clutter, multipath or jamming effects), there is a “reserve” frequency that can be called upon to provide an additional degree of freedom.

Summary

The benefits of dual-band MFR operation are summarised in TABLE 2.

Improved track robustness Target illumination application Better performance against low flying targets 1 Improved ECCM performance

0 Co-located search and track antennae 0 Redundancy

TABLE 2 - Summary Of The Benefits Of Dual- Band Operation

CHOICE OF SEARCH FREQUENCY

An ideal search frequency for sealland based radar applications would be one that allowed search for moving targets in clutter to be undertaken in the least time whilst still meeting the required radar performance.

To this end the number of beam positions to be searched, the dwell time in each beam position and the detection false alarm rate must be minimised. Minimum beam dwell times require the use of pulse Doppler clutter waveforms with a srnall number of bursts (to reduce the requirement to resolve range and velocity ambiguities) and large beamwidths.

Search time budget minimisation ensures that a radar has sufficient available radar time to track and support threat intercepts whilst still performing its surveillance function - i.e. perform multiple functions.

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FUNCTION I FREQUENCY

CHOICE OF TRACKING FREQUENCY

BAND DESIGNATION RADAR EW

Once a target has been detected through surveillance, a weapon system radar must rapidly form a track on it. To obtain rapid track accuracy, the detected target must be quickly handed over to the Tracking function with a high degree of plot accuracy. If the position is not known sufficiently accurately then a time consuming search of multiple beam positions to acquire a target might be required.

Search

The Search function will typically pass a confirmed detection to the Track function with an angular plot accuracy of l / l O t h of the 3dB beamwidth (see FIGURE 2).

1 GHz 1 L-Band I D-Band

___.

Samh Tracking Frequency 3dB F ~ q ~ n c y 3dB

Beamwldlh Bmmwldlh

FIGURE 2 - Hand-Over Between Search And Track Functions

systems, but no existing system has yet integrated both into a single sensor.

CHOICE OF RADIATING ELEMENTS

An extensive antenna element study conducted early in the research programme concluded that a single element, capable of radiating at both frequencies, would not meet the specified MFR performance characteristics (e.g. large agile bandwidth and the capability to allow electronic scanning to +60° from array broadside using phase shifters).

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

ANTENNA INTEGRATION PROBLEMS

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

This implies that the minimum acceptable 3dB

size and separation, the optimum track frequency the 3dB beamwidth at the search frequency to (1 0 GHz) would require 100 times more elements, minimise hand-over time (NB: 3dB beamwidth spaced 10 times closer, than those at the search simply means the antenna beamwidth referenced

to the half power points). frequency (1 GHz) for a fully filled array (assuming a similar aperture).

beamwidth at the tracking frequency is 1/1 Oth of (i) Because of the frequency dependence of dement

~ U A ~ - B ~ N D MFR OPERATING FREQUENCIES

Preliminary studies have indicated the preferred frequencies to be used for the Search and Track functions in a naval or ground based weapons system MFR (see TABLE 3).

(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 3).

TABLE 3 - Preferred Dual Band MFR Frequencies

If a typical monopulse accuracy of I/IO'~ of a beamwidth is assumed, then these frequencies should allow a single beam hand-over from the Search function (at L-Band) to the Track function (at X-Band).

The chosen operating frequencies are currently used separately by conventional naval radar

U>,* - .>%*

FIGURE 3 - 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 d5 Gaussian density taper (see FIGURE 4).

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, ___- - __ - - - J 3 5 I C 1 I 2 ,

I Rand Cell N m l m

FIGURE 4 - 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 FlGlURE 5

FIGURE 5 - Pseudo-Random L-Band Element Location

There is now no longer an!! periodicity due to tho L-Band element matrix into which the X-Band elements are being integrated (see FIGURE 6).

FIGURE 6 - Segment Of Dual Frequency Antenna Showing Pseudo-Random L-Band Elemeiit Positions Interspersed With X-Band Radiating Dipoles

ANTENNA MODELLING RESULTS

Initial computer modelling of a 4 metre diameter circular array has shown that the novel antenna concept of pseudo-random perturbation of the L- Band element matrix has (only a small effect ori the overall L-Band antenna pattern (see FIGURE 7). This is because of the large number of L-Band elements (-500).

Od SCAN-OFF 60" S C A N a F F d

Jd

8"

5 0 71 l o ,s D 2s 7> 7, so is r 5 0

I > e p o i b r i n n H r c d i b nesoe.,il"lOrBOslde

FIGURE 7 - L-Band Diagonal Plane Array Patterns (5 bit phase shifters, +lo" phase error)

Due tc the large number of X-Band radiating dipoles within the dual-band array (-6000 for a 12% filled X-Band array), it has 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 8.

FULL A R M Y WIDTH HtAKiN SIDELOBES

.. .~ ,. . ,~ .

FIGURE 8 - 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.

MUTUAL COUPLING RESEARCH

To date mutual coupling between identical elements only has been included in the antenna performance modelling The next major phase of the research programme (which is currently underway) considers mutual coupling between the different L- and X-Band radiating elements

Initial results obtained using the finite difference time domain (FDTD) method are shown in FIGURE 9.

Oio,ectnc Plug 0, .?.hi* PerminMly ,r , '1 Prus", /"

WanQUidC

Ho Olclecms In Wnregulds

FIGURE 9 - X-Band Radiating Dipole Transverse Magnetic Field Near A L-Band Waveguide

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Currently the research team are quantifying the extent of heterogeneous element mutual coupling through computer modelling. In tandem with this theoretical work, practical solutions to limit the effects of mutual coupling are being investigated to be tested experimentally using a waveguide simulator.

However, all of the mutual coupling results obtained give confidence in the postulated dual- band antenna design.

The continuing theoretical research, together with the planned practical demonstration of the concepts, will 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 UKs pioneering research into ship-based weapon system multi-function radars.

ACKNOWLEDGEMENTS 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 MFR’s, leading to significant benefits for military weapons sensors (see FIGURE I O ) .

,-,NI HomM

The great help and significant technical contribution of Dr. George H. Hockham (GH Consultancy), 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.

FIGURE 10 - Dual-Band MFR Tasks

0 British Crown Copyright 1997 / D E W Published with the permission of the controller of Her Britannic Majesty’s Stationery Office.

lQth International Conference on Antennas and Propagation, 14-17 April 1997, Conference Publication No. 436 0 IEE 1997