This study focuses on achieving wide tunability of a compact reflex triode virtual cathode oscillator (vircator). The cathode is of a bimodal carbon fiber (CF) material paired with a pyrolytic graphite anode. These materials display ideal operating characteristics which include but are not limited to, long lifetime > 106 shots, high operating temperatures > 1000 K, and large current densities 200 A/cm2. A 12 stage, 158 J pulse forming network (PFN) based Marx generator serves to drive the vircator at 350 kV, 4 kA with 100 ns pulsewidth. The operating frequency of interest is in the range of 1-6 GHz, where tunability is achieved by varying the length of the anode-cathode (A-K) gap, the length from the back wall to the A-K gap, and/or the distance from the bottom of the cavity to the A-K gap. The primary focus in this research was to increase the achievable frequencies by placing a square waveguide within a sealed vacuum tube. This allows the bottom part of the waveguide to be easily adjusted while still maintaining the waveguide integrity. The resulting microwave frequencies are shown along with the system performance.