A multi-mode antenna comprising: a liquid that is electrically conductive; a nonconductive, straight tube designed to contain the conductive liquid, wherein the straight tube has a top end and a bottom end; a helical coil comprised of non-conductive tubing designed to contain the conductive liquid wherein the helical coil tubing has a top end and a bottom end; and a pump fluidically coupled to the bottom ends of the straight tube and the helical coil, wherein the pump is configured to pump the conductive liquid between the straight tube and the helical coil, such that when the conductive liquid fills the straight tube and the helical coil tubing is drained, the multi-mode antenna functions as a monopole antenna, and such that when the conductive liquid fills the helical coil tubing and the straight tube is drained, the multi-mode antenna functions as a helical antenna.