Estimate key dimensions of Archimedean and equiangular spiral antennas based on operating frequency range. Designed for UWB, EMC, and direction‑finding applications.
A spiral antenna is a frequency‑independent antenna with a geometry defined by a spiral curve. It exhibits extremely wide bandwidth (often 10:1 or more), circular polarization, and a bidirectional radiation pattern. Spiral antennas are widely used in EMC testing, direction finding, and ultra‑wideband (UWB) communications.
Two common types:
The active region of a spiral antenna occurs where the circumference of a turn is approximately one wavelength. At lower frequencies, the active region moves outward; at higher frequencies, it moves inward. This property gives the antenna its extremely wide bandwidth. The two arms are fed differentially at the center, producing circular polarization.
An EMC test house requires a receive antenna covering 2‑18 GHz. Using the calculator: flow=2000 MHz, fhigh=18000 MHz → Rout≈23.9 mm, Rin≈2.65 mm, N=10 → a≈0.34 mm/rad. The fabricated spiral on 0.8 mm substrate shows gain variation ±1.5 dB over the band and axial ratio <3 dB.
| Band (GHz) | Rout (mm) | Rin (mm) | N (turns) |
|---|---|---|---|
| 0.5‑3 | 95.5 | 15.9 | 10 |
| 1‑10 | 47.7 | 4.77 | 8 |
| 2‑18 | 23.9 | 2.65 | 10 |
| 3‑10 | 15.9 | 4.77 | 6 |
The equiangular (log‑periodic) spiral was first described by John D. Dyson in the late 1950s. Archimedean spirals have been used since the 1960s for broadband applications. Both are classic examples of frequency‑independent antennas.
These links provide peer‑reviewed and standard references.