R&D Report 1954-17 : Examination of the distribution with frequency and time of the field of WWV at Tatsfield during six months near the sunspot minimum. T.W. Bennington Hourly measurements of the field of WWV on 5, 10, 15 and 20 Mc/s have been made at Tatsfield over the greater part of the day for the last six months of 1953, and these have been analysed. The departure and arrival angles corresponding to the different propagation modes for the different frequencies, times of day and seasons were estimated. The measurements were then corrected for the vertical patterns of the aerials, and, having regard to the different powers fed to the aerials on different frequencies, reduced to a constant transmitted power of 1 kW, and expressed in decibels referred to 1 UV/m. The distribution of the field was then plotted for each month on a frequency/time scale, in contours of the monthly mean of the mean-peak field. These are given in Figs. 3 to 8., which also show the predicted MUF and LUF for each month, for comparison. The contours show that the field is distributed, with frequency and with time, somewhat as the MUF and LUF predictions indicate. Highest fields occur, however, where multiple modes of propagation are possible, which is on frequencies well below the actual MUF, provided that the absorption is low. Thus much stronger fields occur at night than during the day, and stronger fields during the winter day than during the summer day. Strong fields occur on much higher frequencies during the winter than during the summer day, because of the winter daytime increase in E2 layer ionisation but, at the same time, the field is also stronger on the lower frequencies during the winter than during the summer day, because of the decrease in absorption. The predicted MUF's tend to be low in frequency during certain times of day and season, though the strongest fields usually occur below them. The loss of field due to absorption occurs earlier in the day than is indicated by the predicted LUF's, but, in the evening, strong fields are obtained below the predicated LUFs.