Although some might consider it an advantage to use very low frequencies (20 kHz would be considered extremely low in the context of "RF"), it's electromagnetic wavelength, in the neighborhood of 10 miles, is so long that it's impractical to form a "beam" to direct it. So its energy goes everywhere ... very inefficient. Also consider that the field strength falls off as the square of distance ... also very inefficient for any "broadcast" of power. And any long conductor (think barbed-wire fence) becomes a receiver you can't turn off ... dangerous. More realistic proposals have been made (in IEEE journals) to put huge solar arrays in stationary earth orbit, convert the collected power to a narrow microwave beam which is received by antennas on the ground. Theoretically, it works at a reasonable efficiency ... but how do you guarantee that a misaligned or out-of-control pointing system wouldn't burn a swath across the countryside? There are lots of practical problems ... and risks. I think wireless power transmission will remain inside ordinary transformers or slightly modified ones like chargers for electric toothbrushes and cell phones (and, on a larger scale, vehicles) over short distances for the foreseeable future. I built several Tesla coils in junior high school, primarily (pardon the pun) to amaze friends by lighting up handheld fluorescent or neon bulbs or draw foot-long arcs to a handheld metal object. Most of these, and I suspect Teslas abandoned one, operated at frequencies near 1 MHz (AM broadcast band). - Bill Whitlock, chief technology manager, Jensen Transformers, Inc.