Presented by
BIRAJ MOHAPATRA

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DEVELOPMENT OF HIGH PRECISION SLOTTED ARRAY WAVE GUIDE ANTENNA FOR AEROSPACE APPLICATION
INTRODUCTION:
Missiles are guided towards the target through the seeker. Mono pulse antennas give the location of target. Thus tracking of the target depends upon the performance of the antennae.
The slotted wave guide array antenna offer various advantages like planar configuration and compact structure.
The performance of these antennae depends upon various parameters like slot position and its dimension like its length and width. These slots are to be fabricated very precisely maintaining their positions and dimensions on the wave guide.
The development of slotted wave guide array antennae is one of the challenging tasks as machining requirements are in precision range. High speed machining technology has been implemented to achieve the required dimensional and geometrical accuracies.
DESCRIPTION OF ANTENNAE SYSTEM
The assembly shown in fig. consists of antennae and comparator. The antenna is made up of radiating layer and a coupling layer which feed signal to radiating layer.
The comparator is of two layer consisting of side A and side B to generate mono pulse signal.
The antenna consist of 6 layers and they are as follows:
RADIATING LAYER
BASE PLATE WITH WAVE GUIDE CHANNELS
BASE PLATE WITH COUPLING LAYER
COMPARATOR SIDE A
COMPARATOR SIDE B
COVER PLATE
DIAGRAM OF ANTENNAE SYSTEM
Radiating layer
This layer consists of slots made up of thin metal sheet of 0.5 mm thick. The slot dimensions varies from 3.9 to 4.2 mm in length and 0.5mm width. The inter element spacing of slots is of order 4-5.0 mm.
The main criticalities are machining accurate slots on thin sheet which can be sheared if excessive cutting force applied on it. For this reason a special cutter of 0.5 mm slot drill is used.
The component is loaded on a special fixture to provide uniform clamping and machined all the slots in a special pattern to avoid distortion.
BASE PLATE WITH WAVE GUIDE CHANNEL
The wave guide channel has channels cut in them. The channel dimensions are in order of 5.68mm in length, 2.84mm in width, and 0.75mm wall between them.
These dimensions are to be maintained for complete channel length of 350mm long.
At the edge of the wave guide channel, 0.5mm wide and 2.84mm deep gap is to be cut for removing the salt deposits after brazing .
The overall accuracy required is 20 microns .
BASE PLATE WITH COUPLING LAYER
Below the wave guide channel there is coupling layer. This layer has inclined slots cut in them for feeding the radiating layer.
The tilt angle of these slots varies from 1’ to 25’. The placements of these slots is of order o f 4.00mm.
The coupling channels have 6.9mm wide and 2.84mm deep. There are three coupling channels of 350mm length. The maintenance of the length is a very critical and precision job.
COMPARATOR
It is very important part of antenna .
This enables the antenna to locate the target position.
It takes four inputs from the antenna and convert into one sum signal and two difference signals.
This conversion is done with the help of magic-t’s
COMPARATOR SIDE A
This part is below the antenna as shown in fig. the magic t’s have to be machined with utmost care to achieve required accuracy
COMPARATOR SIDE B
This part has the wave guide channels rooted so as to bring out the three outputs of the antenna
APPLICATION OF HSM FACILITY IN MACHINING OF ANTENNA SYSTEM
The following steps were taken to develop machining technology for antenna system:
Selection of machining process
Based on the type of machining and the concerned operations i.e., contour slots, plain slots, drills , and raw material condition, the number of stages can be determined. Generally roughing , semi finishing and finishing operations are common in machining these components. These semi-finished plates were processed with lapping to achieve required finish.
Selection of tool holders and cutting tools
Selection of proper cutters and tool holders plays an important role in HSM. The inaccuracy and run out of the cutting tool depends on run-out of spindle system and type of holding between cutting tool and tool holder
C. Design of chip
The chip size should be adequate to take away the heat from cutting zone to avoid tool wear and distortion of the component.
D. Cutting direction
Climb milling was adopted for rough machining thin walled ribs as this type of cutting results in less heat and distortion, finally stress free surfaces with good geometrical accuracies . Finish machining was taken with up milling to achieve high surface finish.
E. Cutter geometry
Positive rake geometry was used to limit the cutting forces as well as cutting temperatures.
ASSEMBLY
Assembly of the radiating layer with the base plate is very critical and stringent.
These parts have to be assembled in such a way that there is no leakage of electromagnetic energy from one channel to other and to the outside environment.
This assembly is done by brazing process.
The resulting assemblies usually are lighter ,stronger and less costly.
CONCLIUSION:
The machining technology for slotted antennae assembly is successfully established.
The high speed machining technology has been implemented after extensive experimentation.
Different holding techniques and fixtures were applied to reduce the distortion and deflection if thin walled components.
Finally the assembly was brazed by dip brazing process and successfully delivered to the project for testing