Friday, November 24, 2023

Mediumwave Skywave Prediction #6 - Wrapping Things Up

Let's close up by defining a few of the terms used in the formulas from the last post, and finish by talking a bit about diurnal and seasonal effects.


Polarization coupling loss, sometimes depicted as Lp, is the fraction of incident power lost on entry into the ionosphere. Further polarization coupling loss occurs when the wave which emerges from the ionosphere induces a voltage in the receiving antenna. Polarization coupling loss depends to some extent on frequency and angle of incidence at the ionosphere. Polarization coupling losses are low in higher latitudes because the Earth's magnetic field is almost vertical. At the magnetic equator, however, the Earth's field is horizontal and polarization coupling losses on east-west paths are large.

Polarization coupling loss at MF is an important factor in skywave propagation. It arises because the Earth's natural gyromagnetic frequency lies within the frequency band being considered. The gyromagnetic frequency of the Earth's ionosphere varies between 800 kHz in the equatorial regions and 1600 kHz near the magnetic poles. When a linearly polarized mediumwave frequency radio wave enters the ionosphere, it gets split into two waves known as ordinary and extraordinary. At the gyromagnetic frequency the extraordinary wave is so greatly attenuated that it makes a negligible contribution to the received signal. As a consequence, the extraordinary wave can be disregarded within the mediumwave band. The propagation is therefore by the ordinary wave.

To explain further, conventional antennas at mediumwave radiate vertically-polarized waves. At MF, the wave which is accepted by the ionosphere and which will propagate back to Earth usually differs in polarization somewhat - hence the ionosphere may not be excited efficiently by the incident wave. We have decreased coupling efficiency, or polarization coupling loss. The wave which subsequently emerges from the ionosphere is in general elliptically-polarized and in-turn may not excite the receiving antenna efficiently because antennas near the ground are most sensitive to vertical polarization, resulting in additional loss.


For long distance paths (1000 to 6000 km or greater), when the path is over the sea and at least one end of the link is located on or near the sea coast, the phenomenon of sea gain can add from 3 to 10 dB to the predicted field strength. 

Gains peak at the usual single, double, and triple hop distances of 2000 km (8 dB), 4000 km (10 dB), and 6000 km (10 dB). Only about 3 dB is gained at the 1000 km distance. A dip in gain (to about 5 dB) occurs at about the 2500 and 5000 km distances.

A knowledge of the land-sea boundary information is necessary to assess the sea gain phenomena. Generally, in the skywave calculation, the sea gain correction is normally set to 0 dB without this knowledge. To take any advantage of sea gain, one of the terminals (transmitter or receiver) must be within about 10 km from the sea coast. Even at 10 km inland, the penalty is about -4 dB. At 4 km, about -2 dB. At 3 km, only about a -1 dB penalty.


Solar Cycle 25 is well on its way now, having started its general upward trend in sunspot count by late 2020. The daily sunspot count for August 30, 2023, for example, was 104.

Do sunspots effect nighttime skywave propagation at the medium waves? Yes they do, at a small but noticeable level. Here are the details.

Concerning medium wave, sunspots and the increasing solar flux are relevant to skywave field strength and are accounted for in most modern (nighttime) skywave prediction methods. In general, mediumwave skywave field strength is slightly better during low or zero sunspot periods, at the bottom of the solar cycle. The calculation of the additional path loss in dB is dependent on location.

Greater consideration is given to paths within North America and Europe (nearer to the north geomagnetic pole), and Australia (nearer to the south geomagnetic pole). The North American loss factor is 4 times that of Europe and Australia, and rises for all as we get to the higher latitudes. Longer paths, those between North America and Europe are usually interpolated.

The ITU skywave prediction method is one such method which incorporates these added loss factors due to sunspots and solar flux. Figures below have been extracted from that prediction method.

Below are increased single hop skywave loss factors in dB as the sunspot count goes up.

Paths within North America:

Sunspot count = 0 a net added loss of zero
Sunspot count = 7 an additional loss of 0.28 dB
Sunspot count = 25 an additional loss of 1 dB
Sunspot count = 50 an additional loss of 2 dB
Sunspot count = 100 an additional loss of 4 dB

Paths within Europe:

Sunspot count = 0 a net added loss of zero
Sunspot count = 7 an additional loss of 0.07 dB
Sunspot count = 25 an additional loss of 0.25 dB
Sunspot count = 50 an additional loss of 0.5 dB
Sunspot count = 100 an additional loss of 1 dB

Paths between North America and Europe:

Sunspot count = 0 a net added loss of zero
Sunspot count = 7 an additional loss of 0.175 dB
Sunspot count = 25 an additional loss of 0.625 dB
Sunspot count = 50 an additional loss of 1.25 dB
Sunspot count = 100 an additional loss of 2.5 dB

Admittedly, these extra losses are small but important enough that they are factored in for skywave calculations. Be aware that 3 or 4 dB can make a difference logging a station or not. A single S-unit is 6 dB.


The final determination which really completes our skywave field strength calculation must include three more tweaks:

1. Diurnal hourly losses/gains
2. Sunrise and sunset enhancements
3. Seasonally-driven losses/gains

The D-layer of the ionosphere is characterized as having a strong dependence on frequency, but this is present only during the daytime. The E-layer is the dominant contributor to LF and MF propagation at night and is only mildly dependent on frequency, so the effects of frequency of this layer can be neglected for most practical purposes.

Although daytime ionospheric propagation is relatively unimportant, it cannot be entirely disregarded at the upper end of the band, since ionospheric attenuation decreases with the square of the frequency. Nor can it be entirely disregarded at the lower end of the band, where partial reflection from the lower edge of the D region may occur, especially in winter at temperate latitudes.

The critical frequency of the normal E layer is about 1500 kHz at sunset, but it then falls rapidly as a result of electron-ion recombination and will assume a value of about 500 kHz late at night. Skywaves may be reflected from the E layer, or they may penetrate the E layer and be reflected from the F layer, depending on the frequency, path length, and time of night. Simultaneous reflection by both layers is also possible in some circumstances. 

Upper MW band diurnal (or daily) morning enhancement can show effect as late as 3 hours after sunrise. The start of the pre-sunset afternoon enhancement is delayed a little to about 2 hours before sunset, gradually building to sunset. The lower part of the band shows little of this effect, morning or night.

The diurnal enhancement described in the last paragraph is not to be confused with the short sunrise and sunset enhancements on extreme DX due to what is called "greyline effect", the signal traveling along, or partly along, the sunrise/sunset terminator.

Skywave propagation does indeed exist during the daytime hours, and its strength varies greatly, seasonally.

Daytime, noon-hour skywave is generally pegged at approximately 30 dB lower than the nighttime field-strength prediction, and this will vary considerably seasonally. An ionospheric transition period occurs immediately surrounding sunset and lasts till approximately four hours after sunset, and another occurs during the period from 2 hours before sunrise until sunrise where the field strength goes through this 30 dB change with a very steep slope. The shapes of the curves are not symmetrical for the transition from day-to-night and night-to-day.


In this series I have attempted to present to you first a little history skywave propagation analysis, who developed the formulas and how they are geographically dependent, and the formulas themselves. I hope it has brought some perspective to the process and you have enjoyed it.