Re: [IRCA] Max. Groundwave Propagation Distance?--Thanks!

["Jeff T. Casey" <ne2j@xxxxxxxx>]

Wow, the answers to my question about the maximum distance medium wave signals can travel via groundwave turned out to be far more complex, variable, and fascinating than I ever expected!  Thanks very much to all who responded -- I've learned a great deal from your comments and the invaluable online resources to which you directed me.  I hadn't thought about the effects on groundwaves of variables such as soil conductivity or differences in propagation between the lower and higher frequency segments of the AM broadcast band (I'm used to thinking in terms of the ham bands, which are narrow enough that propagation doesn't differ appreciably between the two ends of a band).
   
  One thing that is clear is that, unlike on VHF and UHF, groundwave bears little relation to line-of-sight.  The maximum line-of-sight distance (distance to the horizon) over flat terrain from the top of a 300 ft. tower is only about 37 miles.  A station 300 miles distant is nearly half a mile below the horizon, and a station 1,000 miles away is more than 1.5 miles below the horizon.  So, the earth, especially in areas covered by saltwater (which, of course, has extremely high conductivity), apparently acts like an RF-attracting magnet, which causes medium wave signals to "adhere" to it as they travel as far as the surface conductivity and the station's effective radiated power (ERP) will take them.
   
  Maybe it's a bit more accurate to say that groundwaves actually travel *in* the ground, near the surface, as if the earth's surface is a wire that has a certain amount of resistance, where the number of ohms is an inverse function of the soil's (or the water's) conductivity.  This idea helps to explain the reception at high noon in July of stations 1,000+ miles of ocean away, because it implies that there is no limit to how far groundwaves can travel, if the station's ERP is sufficient to overcome the losses caused by the surface resistance of the earth along the path to the receiver.  However, since surface conductivity (and resistance) presumably do not change significantly over time (except when it rains or floods?), it would seem that a signal arriving only via groundwave should come in either 24/7 or never (assuming no changes at the transmitter site).  So, I would think that a signal that is heard at noon on some days but not others must be arriving via skywave
 only, but maybe I'm neglecting some important factor.
   
  Someone with a deeper technical understanding please correct me if I'm on the wrong track here, as I may be way off or presenting a grossly incomplete picture.  One thing in particular that I don't fully understand is why high surface conductivity (including the radial system beneath a vertical antenna) enhances skywave, as well as groundwave, propagation.
   
  Thanks again to everyone who has chimed in on this very interesting subject.
   
  73,
  Jeff
  NE2J (formerly WB5GWB)
  Long Island, NY
   
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