Total posts : 45366
[quote=radio8z]The sudden drop is probably due to moving from the near field (1/r^3 reduction) to the far field (1/r reduction). I forget the exact details, but I bet Rich could elaborate on this.[/quote]
The near field extends less than 20 feet from a 3-meter antenna on 1.65 MHz. If a sudden drop in far-field signal strength is noticed, it is most likely just related to the low radiated power, and the fact that field strength drops by at least 50% each time distance doubles.
For example, if a receiver needs a 1 mV/m field for usable performance, you can see from the numbers in my first post above that it would work OK about 0.18 miles from the tx, but moving it only a few hundred feet further away would put it in the zone of no coverage (as far as that receiver is concerned).
The same physics applies to AM broadcast stations, but with their high power level, the signal changes very slowly across any linear distance of several hundred feet, or even several miles.