This report presents the comparison between calculated and measured values of the radiation pattern produced by the antenna array of the Arecibo HF heating facility, located 17 km and 32.6° NE of the Arecibo Observatory in Puerto Rico. Provided are a calculated map of the pattern of the Islote heater for a fixed altitude at a frequency of 5.1 MHz, and a graphical cross section of the pattern through each major axis. The calculated map P(X,Y,h) is derived in an appendix.  The calculations have been compared with antenna patterns measured in an aircraft on 28 September 1981 by Dr. Bodo Reinisch et. al. Their data consists of navigational and field strength measurements made on two receivers while the antenna array radiated a fixed power.