[en] Central cell plasma in Tandem Mirror Experiment-Upgrade (TMX-U) are heated with 100 kW of ICRF transmitter power to ion temperatures of 1.5 keV at densities of 2 x 10¹² cm^-3. We have used two Faraday-shielded antennas: the first had one 90⁰ loop; and the second, in current use, has two 170⁰ loops connected in an m = 1 configuration. We are also installing a slot antenna. Optimum heating for wave launching occus below the cyclotron frequency, consistent with slow wave heating. In TMX-U, we observed a power threshold, which is consistent with computed end-loss power balance. The measured loading resistance varies with density and frequency in agreement with McVey's antenna-plasma coupling code