Summary form only given. The coaxial virtual cathode oscillator (vircator) is an attractive microwave device due to its simplicity, but suffers from low efficiency. The coaxial vircator geometry has many physical parameters that can be changed to alter performance and possibly increase the efficiency. The typical polarity for the coaxial vircator at Texas Tech has been the cathode grounded (machine ground) with a positive potential applied to the anode. Many geometry variations on this polarity, termed the positive polarity, have been tried. An alternate polarity, the negative polarity, has been tried with the anode grounded and a negative, pulsed potential applied to the cathode. Though these polarity arrangements initially appear to be identical, subtle differences exist. The two major differences are the proximity of the virtual cathode to the overmoded output waveguide and the change of the static electric field in the diode region into a non-uniform negative polarity field which gives low energy electrons in the interaction region a push away from and out of the diode region. This, as well as decreasing the cathode width, increases the efficiency of the vircator substantially (/spl sim/ factor of 10).