Sweep‐frequency CW radio signals transmitted over 1300‐km north‐south and 1900‐km east‐west temperate‐zone paths were received 24 hours per day for 21½ ‐day periods. Measurements of the incoming one‐hop F‐layer polarization, as a function of time‐of‐day and radio frequency, indicate that daytime rates of change of polarization rotation with frequency (at an instant of time) average 1 turn/MHz over the north‐south path and 0.25 turn/MHz over the east‐west path. These values are in good agreement with computer raytracing predictions of the rates of change of polarization rotation with frequency over similarly disposed paths. It was observed that nighttime polarization rotation rates are about 1 to 4 times as high as those measured during the day. It was also experimentally found that the rate of change of the incoming polarization with frequency first decreased and then increased with incoming radio frequency, the minimum rate occurring near 0.8 MOF0. Daytime rates of polarization rotation with time (at a given frequency) average 0.25 turn/min. The polarization rotation rates with time do not appear to vary either with path azimuth or transmitted radio frequency. Near‐zero rates of polarization rotation with time occur for much of the nighttime period. Large fluctuations in the polarization variations occur with both time and frequency throughout the day.Polarization measurements made with signals that were reflected from a nonblanketing, nighttime sporadic E layer of about two hours duration indicate that, for this layer, circumstances other than polarization effects, probably multipath, determine much of the observed signal strength variations with frequency and time.The results suggest that for round‐the‐clock temperate zone propagation, when linearly polarized antennas are employed, a qualitative threshold for envelope distortion of broadband signals (due to variation in the incoming polarization with frequency) occurs when signal bandwidths exceed approximately 100 kHz for north‐south paths and 400 kHz for east‐west paths. The effects of such distortion may be reduced either by operating near 0.8 MOF0or by the