Editing Ground wave (section)

==Overview==

Lower frequency [[Electromagnetic radiation|radio waves]], below 3&nbsp;MHz, travel efficiently as ground waves. As losses increase with frequency, [[high frequency]] transmissions between 3 and 30&nbsp;MHz have more modest groundwave range and groundwave is unimportant above 30&nbsp;MHz.<ref name=":0" /> Surface conductivity affects the propagation of ground waves, with highly conductive surfaces such as sea water providing the best propagation, and dry ground and ice performing the worst.<ref name=":0" /><ref>{{cite book |title=Introduction to Wave Propagation, Transmission Lines, and Antennas |date=September 1998 |publisher=Naval Education and Training Professional Development and Technology Center |series=Naval Electrical Engineering Training, Module&nbsp;10 |page=2.16 |chapter=Chapter&nbsp;2: Ground Waves |id=NavEdTra 14182 |chapter-url=http://www.tpub.com/content/neets/14182/css/14182_76.htm |chapter-format=PDF (archive zipped) |archive-url=https://web.archive.org/web/20180511150057/https://www.dralos.com/downloads/united_states_navy_navedtra%2014182.zip |archive-date=2018-05-11 |chapter-url-access=limited}}</ref>

As the distance increases, ground waves spread out according to the [[inverse-square law]]. The imperfect conductivity of the ground tilts the waves forward, dissipating energy into the ground.<ref>{{cite journal |last1=Ling |first1=R. T. |last2=Scholler |first2=J. D. |last3=Ufimtsev |first3=P. Ya. |year=1998 |title=Propagation and excitation of surface waves in an absorbing layer |url=http://www.jpier.org/PIER/pier19/02.970718p.Ling.SU.pdf |url-status=live |department=Northrop Grumman Corporation |journal=Progress in Electromagnetics Research |volume=19 |pages=49–91 |doi=10.2528/PIER97071800 |archive-url=https://ghostarchive.org/archive/20221009/http://www.jpier.org/PIER/pier19/02.970718p.Ling.SU.pdf |archive-date=2022-10-09 |access-date=2018-05-10 |doi-access=free}}</ref> The long wavelengths of these signals allow them to diffract over the horizon, but this leads to further losses. Signal strength tends to fall exponentially with distance once the Earth's curvature is significant. Above about 10 kHz, [[atmospheric refraction]] helps bend waves downward.<ref name=":0" /> Only vertically [[Polarization (waves)|polarized]] waves travel well; horizontally polarized signals are heavily attenuated.

Groundwave signals are relatively immune to [[fading]] but changes in the ground can cause variation in signal strength. Attenuation over land is lowest in the winter in [[Temperate climate|temperate climates]] and higher over water when seas are rough. Hills, mountains, urban areas, and forests can create areas of reduced signal strength.<ref name=":0" /> The [[Skin effect|penetration depth]] of ground waves varies, reaching tens of meters at medium frequencies over dry ground and even more at lower frequencies. Propagation predictions thus require knowing the electrical properties of subsurface layers, which are best measured from groundwave attenuation.<ref name=":0" />