  This article proposes a concept for dynamically reconfigurable distributed
microwave circuits by leveraging the abrupt conductivity transition in
phase-change materials (PCM). Metallic surface-inclusions (<<$\lambda$) are
embedded in the PCM film$-$vanadium dioxide (VO2)$-$to provide low-loss and
reconfigurability. To validate this concept, a variety of co-planar waveguide
transmission lines are designed and fabricated with metallic surface-inclusions
in VO2 films, and the lines' performance are characterized up to 50 GHz. The
measured results are used to develop a transmission-line based model and an
equivalent circuit model of the VO2 with surface-inclusions to aid in the rapid
design of new structures. Additionally, an electromagnetic model was developed
and indicates that loss can be close to that of conventional metallic
distributed circuits with 100 to 200 nm thick VO2 films. With these thicker
films, two practical realizations of this concept were designed and simulated:
a tunable dipole from 2.13 to 9.07 GHz and a tunable triple-stub matching
network from 5 to 40 GHz with high $|\Gamma|$. Therefore, the proposed method
appears viable for the realization of arbitrary programmable distributed
circuits and antennas in the microwave and low-millimeter-wave bands.
