A simplified technique has been devised for fabricating helical antennas for use in experiments on radio-frequency generation and acceleration of plasmas. These antennas are typically made of copper (for electrical conductivity) and must have a specific helical shape and precise diameter.
Such an antenna could be made by bending a single long piece of copper tubing or bending smaller pieces of copper tubing, then welding the pieces together. It could also be made by machining from a single large piece of copper. It is extremely difficult to bend copper tubing into a helix with a precise pitch and diameter. It is also difficult to create the helical shape from multiple pieces of tubing; moreover, welding separate pieces distorts the shape. Machining a hollow cylindrical helix from a block or cylinder of copper entails the use of a complex, expensive, three-dimensional-milling machine in a process that entails long setup and machining times.
In the present simplified technique, one begins by creating a two-dimensional paper template of a desired helical antenna shape. The template is pasted on the outer surface of a copper pipe that has the desired inner and outer diameters. Holes are drilled at the locations where corners are required to exist in the final helical antenna. Manually, using a hacksaw, diagonal cuts are made in the outer cylindrical surface of the pipe, following the lines on the template. Usually, after hacksawing, only a little filing is needed to smooth the edges of the resulting antenna. If the antenna must be water-cooled, then copper tubing can be brazed onto the outer surface of the antenna. This tubing is not required to follow the precisely defined shape of the antenna.
This fabrication technique would not be suitable for mass production, but it is ideal for a laboratory environment. The advantages of the this technique are the following:
- Precise antennas can be made from inexpensive, stock-size copper pipes.
- No welding of separate pieces is needed, and so there is no welding-induced distortion of antenna shapes.
- Prototype antennas can be fabricated fairly rapidly, without the need for complex three-dimensional-milling machines or computer-aided drafting tools.
- Notwithstanding the reliance on handwork, the total fabrication time (as little as a few hours) is competitive with, and probably less than, that of any automated process that could be used for this purpose.
This work was done by Andrew Petro of Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Manufacturing & Prototyping category. MSC-24076