Surface Wave propagation and coverage on the HF radio bands is well known to radio amateurs. Especially, on the Lower frequency HF bands called the Top Bands which includes the 160m band, though it’s technically an MF band. The surface wave propagation phenomena (also loosely termed as a Ground wave) plays an important role in supporting regional short-distance communication with a very high degree of reliability and robustness. Surface wave helps in filling the deaf region, if not fully at least partially that is created around the transmitter location. A region with a radius of typically several tens or hundreds of kilometers around the transmitter forms the skip-distance caused due to the ionosphere not always being dense enough to be able to return reflected (refracted) signals when they approach it at high angles of incidence. Those who might like to brush up the concept of a Skip-zone and Skip-distance could read through my post on the subject Skip Zone – HF.

Although most of us have an intuitive understanding of the basic concept of surface Wave propagation, many of us often hit against a wall and find it difficult to dig deeper into this subject due to the inherent complexity of this phenomenon. The surface wave (ground wave) propagation is dependent on a variety of physical variables which, at times, makes it confusing for a reader who might not be technically oriented. To address this gap and bring more information and clarity to the readers for understanding the surface wave propagation mode, I will try to keep this narrative simple and support it with a set of illustrative graphs. Let us first identify and list out the major factors that influence surface wave coverage on HF amateur radio bands.






Factors influencing Surface Wave Propagation behavior
We all know that the surface Wave (Ground wave) propagation method has special relevance at HF communications in the 10 KHz – 30 MHz frequency segment. When the height of the transmitter antenna operating at these frequencies is small in comparison with its wavelength (λ), Ground wave propagation appears. In the lower frequency HF bands, this propagation method can even reach several hundreds of Kilometers. It can even provide worldwide coverage on ELF and VLF bands, making it especially very useful for surface and sub-surface maritime applications.

The most important property of Ground wave propagation is that it propagates parallel to the surface of the earth. It makes the attenuation strongly dependent on the terrain constants (conductivity and permittivity), as well as other factors like polarization, antenna height, distance, and frequency. Over non-homogeneous paths, the use of vertical polarization is the only practical way of leveraging this propagation mode, because the ground quickly absorbs horizontal polarization. Even vertical polarization suffers high attenuation if the ground (soil) conductivity is poor. Paths over the sea or oceans have lesser propagation losses because of the increased conductivity of the seawater.

The surface wave propagation depends on currents that flow in the ground. The existence of the atmosphere changes the propagation characteristics but is not essential for the mode. Horizontally polarized surface waves are very heavily attenuated and have little or no practical worth. All the discussion that follows would be based on vertically polarized surface waves.

Ground Conductivity – Factors influencing conductivity include moisture content and temperature. Besides, it is necessary to consider the general geological structure of the soil, the topology of the path, as well as loss due to absorption by surface objects.

Terrain Irregularities – Shadowing which may occur in certain locations as a result of terrain and terrain irregularities may result in attenuation and phase differences for received signals. The effect on the field strength produced by terrain irregularities varies with the frequency of transmission and the specific characteristics of the irregularity. Mountainous terrain may increase signal strength through knife-edge diffraction, a phenomenon known as obstacle gain. Although it is a part of the surface wave (ground wave), diffraction is not covered in this article. To learn more about the RF diffraction phenomena, please read my article titled Terrestrial VHF Radio Signal Coverage – BLOS where I have covered this subject in fair detail.

Vegetation – Vegetation along the propagation path also influences field strength. For instance, a densely forested area will produce different propagation results than one with no vegetation, and the effect will depend on whether the forest is in leaf, wet, or covered in snow. However, below about 2 MHz, a forest environment has little effect on the ground wave.

Surface Clutter – Buildings, urban areas, steel framing, wiring, plumbing, lamp posts, and other surface objects affect propagation, and are collectively known as surface clutter.

A highlight of HF surface wave is that it is uniquely suited to situations arising out of disaster or other emergency conditions. The surface wave (ground wave) communication offers a nuclear survivable method of communication in tactical environments. Surface wave does not rely on the ionosphere for the propagation of the signal, and therefore is not susceptible to ionospheric conditions resulting from EMP produced by a nuclear explosion.





Surface wave (Ground wave) Propagation on amateur Radio HF Bands
Let us now take a look at some of the aspects of surface Wave propagation as it manifests itself on amateur radio HF bands. I will present below a set of attenuation curves representing typical behavior on all ten bands including 160-80-60-40-30-20-17-15-12-10m.

Although it should be self-explanatory, I will also explain how to interpret the attenuation graphs for those who might find them a bit confusing. Moreover, I will also draw attention to some of the other factors that one might tend to ignore at first glance.

For the sake of this presentation, let me cite below the conditions for which the surface wave propagation graphs are applicable. These attenuation curves have been derived from a simplified but fairly accurate model based on the CCIR ITU-R P_368-7 1992 recommendations for evaluating ground wave propagation over a smooth surface spherical earth. However, my model has been simplified to account for Sommerfeld-Norton Planar Earth theory since the curvature of the earth can be reasonably approximated due to the relatively short surface distances involved. The model complies to ±3 dB accuracy of the results mapped by the ITU 368-7 document.

Rather than using a long drawn set of complex equations involving tons of variables, I have derived various intermediate constants applicable to each amateur radio band in question under the specific set of conditions under consideration. This takes into account both the Sommerfeld region that defines the radio horizon as well as the Diffraction region that extends beyond the radio horizon. A normal good quality ground with Soil Conductivity of 8 mS/m and Dielectric Constant of 15 have been assumed as the basic terrain parameters. The overhead Tropospheric Diffraction Factor has been set to 0.958 to arrive at the computations.

Although the set of attenuation curves presented below have been computed taking average global terrain and atmospheric conditions into account, it may well be possible that for some regions with extreme deviations in the terrain properties or under extreme weather conditions, these attenuation figures may not remain true. There could be some deviation from the results projects below. However, with due diligence and application of mind, one should be able to apply acceptable correction on account of the altered situations. I will explain some of these points later.

Important points to remember are that the following attenuation projections have been made under the terrain and tropospheric conditions cited above and also assuming that there is no ionospheric skywave available. All curves given below have be derived assuming vertical polarization only. The attenuation on account of horizontal polarization is usually unacceptably high under most circumstances, thus rendering such a communication circuit practically useless. Therefore, I have made no attempts to perform computations for horizontal polarization. The antennas at both ends are assumed to be at or near ground level. The following results conform closely to what might be computed by ITU-R GRWAVE.




Please keep in mind that the night-time range coverage might increase in event of additional surface dew formation, ionospheric reflected component, or increase in tropospheric refractivity gradient. There could be seasonal variations in coverage range too especially after widespread rain.

In some cases, there may be seasonal variations in surface wave propagation. These may be attributed to changes in the refractivity of the troposphere, to the state of vegetative cover, to changes in the level of the water table within the ground, to freezing conditions where water becomes ice, or to a thick snow-cover, etc., where these may cause changes in the effective ground conductivity. All of these changes may affect the intensity of the surface wave field. In particular, such seasonal changes may result in a reduction in field strength in summer.

The attenuation of the surface wave propagating signals from rural transmitters into densely built-up urban areas in the courtyard of high buildings may result in a sharp decrease of the level of a signal by an excess of 15-20 dB over the nominal value projected by the attenuation curves.

Another point to be remembered is that we have not accounted for the additional attenuation that might exist on account of the non-homogeneous surface wave propagation path. In such cases, the Millington method for computing the ground-wave amplitude over non-homogeneous paths needs to be done. However, this does not imply that our simplified model is inaccurate under typical conditions because excessive non-homogeneity usually does not exist over shorter distances as in the case of HF Surface wave (Ground wave) propagation. This would have been an important factor if we had been calculating for longer distances as in the case of low MF, LF, VLF bands, etc. This is because the Surface wave coverage range would have been very long where terrain non-homogeneity would be a prominent factor.