Abstract :
When conventional monopole antennas are used for wide area broadcasting by skywave, energy is absorbed by the ground embedding the earth mat, and also in the Fresnel zone in the antenna foreground required to form the skywave. Further energy is converted to X-waves and absorbed by the ionosphere. It is shown that elliptically polarised antennas avoid these losses, increasing the amount of energy providing real coverage by a factor of 50% in mid latitudes, to up to a possible 400% in equatorial regions, with a corresponding improvement available in the carbon footprint. As crossed dipole designs can be more than five times the cost of a monopole they have a poor cost/performance trade-off. This paper presents the case for elliptical polarisation, and proposes a theoretical design for a low-cost elliptically polarised antenna using a single mast, the tri-loop utilising three conventional shunt skirt wires. These can be fed in two modes close to the base of the mast. Connected together they provide a shunt monopole feed, but fed at equal levels with phases of 0deg, 120deg, and 240deg, they would radiate circular polarisation in the vertical direction, falling to horizontal polarisation at low angles. Such an antenna would be used for groundwave coverage by day, and skywave coverage by night with maximum coupling occurring to the O-wave. A conventional ferrite rod antenna is required at the receiver.
