Abstract
In this paper a dual mode helix is being proposed whereby the combined antenna acts as a single structure comprising of two quadrifilar helix antennas running in parallel and connected at top radial sections and sideways helical sections through pin diodes switches in such a way that space and land mode can be selected by the application of appropriate dc voltages at the terrestrial and space feeds. In this way, at one time, both QHA either act in space mode or in land mode, thus eliminating the problem of mutual coupling that arises when two independent antennas are used in collocation. The structure uses a triaxial cable and pin diodes with very low forward impedance and high reverse impedance. The structure by open-circuiting and short circuiting the wires at certain points shifts the effective feed point to the centre of the radial section to generate CP pattern suitable for space communication and to the centre of the helical section to obtain current distribution required to generate omnidirectional pattern suitable for terrestrial communication. The simulation results show that the QHA in the space mode has a gain of 5.4dB, 3-dB beamwidth 118° and front-to-back ratio of 12.35dB. In terrestrial mode gain and 3-dB beamwidth are 2.26dB and 93° respectively.
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