Comparison of coaxial dipole antennas for applications in the near-field and far-field regions
Coaxial antennas with properties similar to those of a two-wire dipole antenna are presented and analyzed. Since coaxial antennas extend along a straight line and can have almost any radius, they may be easily inserted into confined spaces that require an antenna. Coaxial antennas are easy to construct, naturally adapted to a coaxial input connection, thus avoiding the use of a balun (a balanced to unbalanced adapter), and avoid using a T-shaped structure (as for the two-wire dipole). By altering in specific ways the inner and outer conductors of a 50 Ω transmission line, four different coaxial antennas are formed that have characteristics similar to those of a two-wire dipole antenna. These antennas are compared and discussed, while noting the similarities and differences between them and the two-wire dipole antenna. The antenna parameters determined are the near-field and far-field radiation patterns, the input impedance and the input reflection coefficient (S 11). The antennas are modeled using a high frequency structure simulator (HFSS), then constructed and their properties measured to confirm the calculations and simulations.
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