Electromagnetic emission at the second harmonic of the plasma frequency produced by nonlinear interaction of counterpropagating laser-driven potential plasma waves are studied using particle-in-cell simulations. This process has been recently proposed as a method for generating high-power tunable THz radiation with a narrow spectral line-width (Timofeev et al 2017 Phys. Plasmas 24 103106). In the present paper, we find the optimal conditions for demonstrating this phenomenon in a laboratory experiment that implies excitation of colliding wakefields by axially symmetric 830 nm Gaussian laser pulses with the total energy 0.2 J in a supersonic gas jet. It is shown that the emission mechanism based on the collision of different-size wakes is always accompanied with the mechanism of plasma antenna which begins to radiate electromagnetic waves after the build-up of periodic ion density modulation. Such additional emission makes hydrogen more attractive for this generating scheme than gases with heavier atoms.