Re: Tesla Coil RF Transmitter

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 10:51 AM 9/18/2005, Tesla list wrote:
> Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Jim,
>
> Is another differentiator between near and far fields is where the 
> wave impedance becomes 377 (impedance of free space, iirc) ohms???

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)



> Gerry R.
>
> > I personally prefer the term "reactive near field" to indicate that 
> > energy is being stored in the field and might be returned to the 
> > physical object (n.b. the field around a TC is clearly in this 
> > category).  In the "radiated far field", no energy is stored, it's 
> > just propagating away.