Vircators (virtual cathode oscillators) are easily frequency tunable by simply altering the electron beam current density through either adjusting the accelerating voltage or varying the anode cathode gap (A-K gap). It is noted that vircators typically exhibit frequency hopping, an often undesired feature. Here we address the frequency hopping problem by introducing a rectangular shaped carbon fiber cathode with the width of the cathode less than one quarter of a wavelength in the direction of microwave propagation. Operating the vircator with the rectangular cathode revealed a match with a resonant condition from back wall reflections, which explains the observed dominant microwave frequency selection. That is, the vircator emitted stably during 50 ns operation at 3.89 GHz or 2.07 GHz with a 3 dB bandwidth of 37.5 MHz or 93.7 MHz, at A-K gap spacing of 8 mm, and 12 mm respectively. To further investigate frequency tuning, the A-K gap was swept from 3 mm to 17 mm while keeping a constant 5.1 cm diameter circular cathode. The resulting frequencies of operation were 1.5 GHz, 2.1 GHz, 4.1 GHz, 4.6 GHz, and 5.9 GHz at A-K gap spacing of 17mm, 14 mm, 7 mm, 5.75 mm, and 3 mm respectively. This demonstrates that a tuning range of 4.4 GHz should be achievable for a practical vircator system.