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Small Antenna Handbook by Robert E. Collin, Robert C. Hansen

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Chapter 4

Clever Physics, But Bad Numbers

4.1 Contrawound Toroidal Helix Antenna

The first example of an ESA concept that was clever and sound, but has unfortunate characteristics (tight tolerances, low radiation resistance, etc.), is the contrawound toroidal helix antenna (CWTHA). This was invented and patented by Corum (1986, 1988), who started with a coil of many turns bent into a ring or toroid. Inexplicably, some years later the patent office granted patents for the CWTHA to former lab assistants of Corum! Applying a voltage at the terminals produces a ring of current, like a single-turn loop. Then a second toroidal coil is added in the same volume, but wound in the opposite direction. If the second winding is excited 180° out of phase with the first, the loop currents cancel. Figure 4.1 is a sketch of a CWTHA; the loop lines represent lines of magnetic flux. The feeds are at the origin. However, the currents around the turns add, and a vertical (dipole) electric field is created. The CWTHA was heavily promoted by others without any careful measurements or calculations. Maclean and Rahman (1978) showed that the CWTHA coordinates could be exactly written in terms of a single spherical coordinate variable. A ring-bar analysis, adapted from TWT work, was performed by Hansen and Ridgley (1999); this was followed by an exact analysis (Hansen and Ridgley, 2001) using exact geometry. The exact vector potential integrals were integrated numerically. It was shown that any combination ...

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