Propagation of 0.5 THz single cycle pulses inside an aperture-type near-field probe is studied. The E-field amplitude attenuation is experimentally measured at various distances from the aperture. Numerical simulations based on a two-dimensional model illustrate the pulse waveform transformation and the spectral blueshift, which is experimentally observed. The study shows that the sensitivity of such a near-field probe can be improved by more than a factor of 10 by decreasing the aperture-to-detector separation without reduction of the spatial resolution. © 2001 American Institute of Physics.