  Testbeam experiment 576 (T576) at the SLAC National Accelerator Laboratory
sought to make the first measurement of coherent radio reflections from the
ionization produced in the wake of a high-energy particle shower. The >10 GeV
electron beam at SLAC End Station A was directed into a large high-density
polyethylene target to produce a shower analogous to that produced by an EeV
neutrino interaction in ice. Continuous wave (CW) radio was transmitted into
the target, and receiving antennas monitored for reflection of the transmitted
signal from the ionization left in the wake of the shower. We detail the
experiment and report on preliminary hints of a signal consistent with a radio
reflection at a significance of 2.36$\sigma$. We recommend another test-beam
measurement in order to verify the signal.
