One of the more saintly antennas used on the VHF boards is the halo (Fig. 18-12). This antenna basically takes a half-wavelength dipole and bends it into a circle.
The ends of the dipole are separated by a capacitor. In some cases, a transmitting-type mica “button” capacitor is used, but in others (and perhaps more commonly), the halo capacitor consists of two 3-in disks separated by a plastic dielectric.
While air also serves as a good (and perhaps better) dielectric, the use of plastic allows mechanical rigidity to the system.
(source : Practical Antenna Handbook by Joseph J. Carr)
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