There has been tremendous interest in the VHF and UHF spectrum for commercial and military applications. The antennas currently in use at these frequencies are typically large. Hence, there is a need to investigate broadband antennas that are electrically small in physical size with broadband characteristics. Among the broadband antennas, the conical antenna has characteristics of omni-directional radiation pattern, vertical polarization, broad bandwidth and electrically small in size.
(1) Conical type antenna
An omnidirectional pattern with widened bandwidth conical antenna was investigated over a finite size ground plane. The Ansoft-HFSS software is used for modeling the entire structure of the conical shape design which is excited by a 50Ω coaxial probe. The ground plane used for conical antenna design in simulating the radiation pattern and the input impedance was finite and it is chosen as one wavelength by one wavelength square shape at the lowest frequency. The design considerations in reducing the size of top loaded conical antenna by using posts and lumped resistive loading are presented. The top-capped conical antenna with posts and 40Ω resistive loading has a total height of 16cm which is to be one fifth of the length of the antenna in the free space and achieved a VSWR of better than 2 over 100 - 500MHz. The flare angle (i.e. half-angle of the cone) of 45ْ gives the optimum bandwidth for the small conical antenna design. The prototype of proposed conical antenna with inductive posts and resistive loading fed by a 50Ω N-type connector is fabricated and measured with 50cm in square size small ground plane. There is good agreement between the measured and the numerical result of proposed conical antenna.
(2) Wire conical type antenna
The top capacitive loading wire conical antenna with 2 inductive posts and resistor loading was investigated. The entire wire conical structure is simulated using the Ansoft-HFSS software to understand the behavior of the wire conical antenna model, which is excited via a 50Ω coaxial probe. The size reduction of top loading wire conical antenna was done by using posts and lumped resistor loading. The top-capped wire conical antenna with posts and 40Ω resistive loading has a total height of 16cm, which is to be one fifth of the length of the antenna in the free space and the broadband result of frequency span from 100-800MHz over of VSWR<2 was obtained. The flare angle (i.e. half-angle of the cone) of small wire conical antenna is 45ْ, which gives the optimum broad bandwidth. The prototype of proposed wire conical antenna with posts and resistive loading is fabricated and measured with 50cm square size ground plane in order to verify the small size and broadband performance of the proposed structure. There is good agreement between the measured and the numerical result of proposed wire conical antenna.