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Re: Tesla Coil RF Transmitter



Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Jim,

I think I should have asked the question not using the term "wave impedance" since if it is a wave, it is radiating. In the near field the ratio of E and H fields will probably be something other than 377 ohms, I believe, and was wondering if this is useful in describing the difference between near and far fields.

Gerry R.


Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>


The wave impedance is everywhere 377 ohms, independent of whether you are in the reactive near field or radiating far field. All that impedance tells you is the relative strength of the E and H fields of a "propagating" EM wave. If I set up a nifty magnetic probe and an electric probe in the same location with all manner of waves rushing by, the V/m should be 377 (more properly 120 pi) times the A/m at that location.


In the reactive near field, this isn't necessarily the case (or maybe it is.. I'll have to think about it), because you can have fields that aren't radiating. Consider the field inside a charged capacitor. The V/m may be very high, but since it's DC, the A/m is zero.

just to reiterate, there's no "sudden boundary" between near and far fields. It's an arbitrary distinction. (sort of like skin depth)

Is another differentiator between near and far fields is where the wave impedance becomes 377 (impedance of free space, iirc) ohms???