Designing high-efficiency antennas on textiles is fundamental for the development of wirelessly-connected smart garments. Furthermore, large antenna arrays could be used to receive or harvest directional and ambient radio-frequency (RF) power from the environment, thus enabling battery-free e-textiles. The key challenges that are hindering the realisation of high efficiency antennas lie in the dielectric properties of fabrics, the conductivity of their traces, and their low textile thickness. This work numerically and experimentally analyses different RF transmission line structures to establish the limitations of widely utilised antenna designs, such as the microstrip patch, and proposes alternative wearable antenna design based on coplanar waveguide (CPW) structures. It is demonstrated that by using a CPW, insertion losses in a 20 mm line can be minimized by up to 40% for the same substrate, as compared to a microstrip, at 30 GHz. A CPW monopole antenna is demonstrated with more than 80% efficiency on a lossy, thin, poly-cotton substrate. Moreover, it is shown that the efficiency of the CPW monopole is independent of the substrate’s thickness and type of fabric.