A large circular HF phased array radar has been used to study the angular spectrum of spread-F returns from an ionosphere modified by the intense radio waves from the high power Platteville transmitter. The results obtained are presented to show the dependence of angular spectrum on range delay, Doppler shift, operating frequency, and on the power of the heater transmitter. The phenomenon of wideband attenuation observed with the o-mode diagnostic wave is examined for its dependence on the angle of arrival of the received signal. The angular spectrum of the spread-F returns exhibits a systematic behavior. It is found that the angle of arrival migrates from north to south and the Doppler changes from positive to negative as the range delay examined is changed from the leading edge of the return to the trailing edge. The height dependence of the angular spectrum shows that the volume containing the spread-F irregularities is aligned along the earth's magnetic field. The dependence on the heater power is such that the angular extent of the returns is reduced by a factor of 3 when the heater power is reduced to −9 db of its maximum value of 1.8 Mw. The attenuation suffered by the diagnostic o-mode upon heater turn-on changes with the angle of arrival to reach a maximum with the signal returns arriving from the south. It is shown by means of ray-tracing simulation that the observed interdependence of angle of arrival, Doppler shift, and range delay is consistent with a model of field-aligned ducts for the heater-induced spread-F irregularities. The model, when applied to the conditions at Arecibo and the results compared to that for Platteville, reveals that the range delay spread is about the same and the angular spread is approximately in the same ratio as the heater beamwidths at the two places.