Figure 1. A Typical Conical Log-Spiral Antenna includes two spiral arms that must be kept in alignment.

An improved design for a conical log-spiral antenna (see Figure 1) simplifies construction and improves alignment. The radiating-element substructure of such an antenna must be properly aligned with the signal-feed substructure to obtain the correct impedance match for efficient coupling of the signal into or out of the antenna. This design provides for mating parts, the faying surfaces of which enforce alignment initially during construction and maintain alignment subsequently during use.

Heretofore, the fabrication of a conical log-spiral antenna typically involved either (1) etching a sheet of metal to form the required spiral strip, then wrapping the strip around a mold or (2) manually aligning a thin, injection-molded part with a feed wire. In either case, initial alignment depends on the subjective judgement and skill of the technician, and the parts can become misaligned subsequently.

Figure 2. A Machined Polytetrafluoroethylene Body defines the spatial relationships among the spiral arms and feed wires, thereby enforcing alignment.

In the present improved design, the outer conical surface is a machined surface on a single-piece polytetrafluoroethylene body (see Figure 2). The use of a machined solid piece guarantees consistency of the cone angle. The inside of the body is machined to provide space for a board that holds the feed circuit, plus key slots that accept opposite edges of the board, thus aligning the feed wires with the log-spiral pattern. Instead of wrapping an etched metal spiral onto the cone, the log-spiral metal pattern is etched onto the cone before final machining.

This work was done by Roland W. Shaw of Shason Microwave Corp. for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at under the Electronic & Computers category.