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 Ω 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.