Low-Impedance Flexible Archimedean-Equiangular Spiral Antenna

A flexible low-impedance wideband antenna with a hybrid structure of Archimedean spiral and equiangular spiral is presented in this letter. The input impedance of the antenna is reduced to about 75 Omega by the attenuation effect of the equiangular spiral connected to the Archimedean spiral. The prototypes of the designed flexible antennas with and without polydimethylsiloxane encapsulation were fabricated with the copper-clad polyimide film and the transfer-printing technique. The simulation results of impedance and voltage standing-wave ratio show the fabricated antenna can be operated over the frequency range of 2.5-4.5 GHz. The S-parameters of the antennas were measured before and after applying uniaxially tensile strain and complex strain induced by balloon. The results show that S-11 parameter of the antennas maintains less than -10 dB in the operating frequency band with deformation. The mechanical reliability of the antennas was also confirmed by the finite-element analysis of the strain in the copper layer.