Highly divergent radiation is expected from lasers with sub-wavelength apertures due to diffraction. However the short range variations of intensity have been observed in the far field of THz quantum cascade lasers with sub-wavelength apertures. We developed an antenna model for long lasers with subwavelength transverse dimensions (wire lasers). Based on this model we show that the observed far field variations are formed due to the interference of the radiation from the longitudinal mode distribution. We predict that radiation from subwavelength wire laser can be concentrated in a narrow beam with a divergence determined by the ratio of the wavelength to the length of the laser.