ITU-R REC. P.1147
This model is based on Recommendation 1147 of the ITU-R. It is a model that provides a prediction procedure for the sky-wave field strength at frequencies between 150 kHz and 1700 kHz.
This method predicts values of the sky-wave field strength for a given power radiated from a vertical antenna, when measured by a loop antenna at ground level aligned in a vertical plane along the great-circle path to the transmitter. The method has been based on measurements made in the broadcasting frequency bands and applies for paths of length 50 km up to 12000 km for those LF and MF bands in particular. This method should be used with caution for latitudes greater than 65°.
The predicted sky-wave field strength is given by:
E = V + E0 – Lt = V + Gs – Lp + A – 20 log p – La – Lt - Lr
E: annual median of half-hourly median field strengths (dB(µV/m)) for a given transmitter cymomotive force, V, and at a given time, t, relative to sunset or sunrise as appropriate.
The above factors are described below:
Transmitter cymomotive force, V: The transmitter cymomotive force (dB(300 V)) consists of the radiated power dB(1kW) taking into account the gain factors due to vertical and horizontal directivities.
Sea gain, Gs: The sea gain is the additional signal gain when one or both terminals is situated near the sea, but it does not apply to propagation over fresh water. It consists of the gain when the terminal is on the coast and the path is unobstructed by further land, a first correction to take account of the distance between the terminal and the sea and a second one to take account of the width of one or more sea channels. The maximum gain at MF is 10 dB and at LF is 4.1 dB.
Polarization coupling loss, Lp: This loss factor takes into account the effect that magnetic dip and the path azimuth from the magnetic E-W direction have on the wave propagation.
Loss factor incorporating effects of ionospheric absorption and related factors, La: This loss factor incorporates effects of ionospheric absorption, focusing, terminal losses and loss between hops of multi-hop paths.
Hourly loss factor, Lt: This factor includes the loss due to the hour at which transmission occurs. The large values of hourly loss factor near midday are not defined, but it is considered a maximum value of 30 dB. During the night the hourly loss factor is approximately 0 dB so the propagation reaches further distances. Specific equations are applied when near sunrise or sunset in order to obtain the hourly loss factor. The time is taken at the ground at the mid-path position for path lengths smaller than 2000 km. For paths longer than 2000 km, the times are determined at points 750 km from each terminal, and the selected time is the one where the solar elevation angle is higher.
Loss factor incorporating effect of solar activity, Lr: This loss factor includes the twelve-month smoothed international relative sunspot number (also known as Wolf number). It is a quantity that measures the number of sunspots and groups of sunspots present on the surface of the sun. This sunspot activity is cyclical and reaches its maximum around every 9.5 to 11 years. For more information you can see the NGDC (National Geophysical Data Center) webpage of the NOAA (National Oceanic and Atmospheric Administration).
 ITU-R P.1147-4, "Prediction of sky-wave field strength at frequencies between about 150 and 1700 kHz".