Hanyang University 1/18 Antennas & RF Devices Lab. MODERN ANTENNA HANDBOOK by CONSTANTINE A.BALANIS ch – Kim Sung Peel

Hanyang University 2/18 Contents Machining Operations Special Tools Filleted Internal Corners Swarf Control and Removal Chamfering and Deburring Surface Finish Component Handling Welding, Brazing, and Soldering Bolted Joints Special Conditions Pressurization Passive Intermodulation Spaceflight Hardware Examples Machined Stepped or Corrugated Horns Fabricated Horns Transitions

Hanyang University 3/ Machining Operations There is not much information about the important mechanical aspects of microwave structures available to a machinist not experienced. A number of relatively simple but important aspects of microwave component fabrication that are sometimes overlooked between antenna and RF designers are worth discussing. Many parts for aperture antennas can be machined using turning and milling tools with carbide inserts. The machining of corrugated horns is an example. →Commercial carbide grooving inserts work extremely well, but these inserts do not always allow to fit into confined areas at the input end of a horn. →In these cases, special purpose tools can be made from steel bar and brazed-on carbide inserts, with cutting edges and angles Special Tools

Hanyang University 4/ Machining Operations Special Tools <Special purpose tools for corrugated horns>

Hanyang University 5/ Machining Operations Filleted Internal Corners The machining of a pocket or cavity on a milling machine naturally produces a cavity with rounded corners. Deep cavities will take longer to machine if small milling cutter radius are specified. Long milling cutters are prone to chatter when they enter filleted corners. Fillet: rounded inside corner CNC(Computer Numerical Control) machines can reduce the tendency to chatter in internal corners.

Hanyang University 6/ Machining Operations Swarf Control and Removal Swarf can be a significant cause of problems in the machining of antenna components. Swarf(Chips): debris or waste resulting from machining. The chips from aluminum and copper alloys are known as “long chip”, and these can become wrapped around cutting tools, damaging machined surfaces,occasionally causing tool breakage. Many modern carbide tooling inserts are provided with “chip breaker” geometries to break long chips into short chips. Corrugated horns are especially adept at trapping swarf in the corrugations. →Much care should be taken in the finishing of corrugated horns to detect and remove trapped swarf.

Hanyang University 7/ Machining Operations Chamfering and Duburring Making all sharp edges to round edges for decreasing handling injuries. O7fMEhttps:// pU_8SO7fMEhttps:// pU_8SO7fME →This can be detrimental to the performance of some microwave devices Surface Finish The finish on machined surfaces can be important to the functioning of an antenna component, especially at higher frequency. A surface finish adequate to the task should be specified before manufacture Component Handling Due to their relative softness, components made from aluminum and copper alloys require careful handling to avoid damage by scratching and denting. Clean areas should be set aside for storage of components, and care should be taken with part handling both in and out of machine tools.

Hanyang University 8/ Welding, Brazing, and Soldering Some aperture antenna components require fabrication by welding, brazing, or soldering. In choosing and applying a joining process, a number of issues should be considered. ① Suitability of process for the materials to be joined. →Aluminum alloys: easy to welding or brazing, but soldering is difficult. →Copper alloys: very easy to brazing soldering, but welding can cause porosity problems. ② Consider the heating effect of the process on the parts being joined. →Welding requires intense localized heating, which can lead to distortion of the parts being joined through thermal expansion stresses. →Brazing can be performed with either localized or generalized heating. Generalized heating can minimize thermal stresses during brazing, but can be difficult when parts are very large. ③ Consider the strength required in the assembly. →Welding & Brazing joints: result in joints approaching the strength of the parent metal. →Soldered joints: much weaker than the parent metal.

Hanyang University 9/ Bolted Joints Any bolted connection in a microwave device must ensure adequate electrical contact at the internal edges to present an uninterrupted current path for the signal. “Crush” flange joint: inside walls are contacted, while the remainder of the joint has a small definite gap(0.1~0.2mm). →Drawback: Water can penetrating at open edge, possibly leading to corrosion. “Contact” or “relieved” flange joint: two contact faces at inner and outer edges with the bolts in between. →Contact pressure is shared by both faces, so contact pressure at inner wall is reduced. →Outer contact provides improved protection from corrosion.

Hanyang University 10/ Special Conditions Pressurization Antenna systems designed for use in outdoor environments are often lightly pressurized with some kind of gas(dehumidified air, dry nitrogen, etc.) to ensure water cannot enter. Sealing such systems to hold internal pressure requires radome covering the aperture and seals at each joint to prevent leakage of air or gas. O-ring: specially molded silicone rubber seals to protect the system against weather and to retain air or nitrogen. →In designing the proportions of O-ring grooves for sealing, the grooves’ diameter should be larger than that of O-ring.

Hanyang University 11/ Special Conditions Passive Intermodulation(PIM) PIM is the generation of interfering signals caused by nonlinearities in the mechanical components of a wireless system, especially anywhere that two different metals come toge ther. Junctions of dissimilar materials are a prime cause. →low PIM is an antenna system is as much a result of good hardware design and fabrication. For reducing PIM, the number of parts and there for joints must be kept minimum, and the same material should be used throughout the assembly Spaceflight Hardware The extreme environment of space imposes some stringent requirements on hardware design and fabrication. Materials used in flight hardware are subject to large temperature variations, solar radiation, extremely high vacuum, severe shock and vibration during launch. Extremes of temperature can lead to problems between differing materials. →The effects of differential expansion must be assessed. Must be assessed for effects of radiation and high vacuum. →Many materials degrade, loss mass under these conditions.

Hanyang University 12/ Examples Machined Stepped or Corrugated Horns →milling & turning

Hanyang University 13/ Examples Machined Stepped or Corrugated Horns Boring bar on the left has a smoothly curved top face on the carbide to promote chip flow out of the corrugation, while the other has a chip breaker geometry. Boring bar’s maximum possible size: it can clear both the front and rear walls during turning. It will probably be found that any grooving boring bar will tend to chatter at the bottom of the corrugation. →This can be prevented by reducing the spindle speed at the end of each cut.

Hanyang University 14/ Examples Fabricated Horns Rectangular horns, especially for large sizes, are often fabricated from flat sheet sections, joined by welding, brazing, or soldering, depending on the strength and accuracy required. The typical joint configurations for these alternate joining methods are quite different. ① Corner joint of a welded fabrication: →Two flat sheets do not overlap: to ensure complete penetration of the weld pool toward the inner surfaces during welding to minimize gaps along the inside of the joint. →Backup bars may be placed on the inside of the joint to prevent burn-through of the weld pool and can be used to support the parts in correct alignment during welding and help minimize distortion.

Hanyang University 15/ Examples Fabricated Horns ② Corner joint of a brazed fabrication: →One of the sections to overlap the other: overlapping section is extended past the outside surface of the joint to create a lip to which filler metal can be applied by hand. →Filler metal flows through the joint to form a small fillet on both sides.

Hanyang University 16/ Examples Fabricated Horns ③ Corner joint of a soldered fabrication: →Low strength of the filler metal usually requires additional reinforcement of the joint in the form of cover strips. →Use of a reinforcement along the outside of the joint to increase the surface of the solder joint.

Hanyang University 17/ Examples Transitions Waveguide transitions are often needed to connect waveguides to the inputs of aperture antennas. Where the transition provides a change in diameter of a circular waveguide, a turned component can be used. A method that is often used to make such devices is wire electric discharge machining. (wire EDM) →Electrical discharge between a tightly stretched wire and workpiece is used to burn a slot in the workpiece, and so to saw out a hole or shape. Occasionally, a transition must also provide a change in waveguide shape.

Hanyang University 18/18 Thank you for your attention