Fw: Near and far, was Re: Tesla Coil RF Transmitter

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>




 Hi all,

 I was curious as to what waves where interfering to produce the near field
 that is different than the far fields.
 For example: If the transmitter is say two spheres connected by a short
(wrt
 wavelength) wire that some how shunts current from one sphere to the other
 then:
 There are three sources, the current between the spheres (current segment)
 and the E field of  each sphere.

 The current segment emits waves as dipole. Field decreasing with the square
 of distance.
 The spheres emit waves of opposite polarity relative to each other because
 their E field have opposite rates of change. The resultant field will fall
 at the cube of the distance i.e. the field of each sphere falls at the
 square of distance and the cancellation is proportional to the sin of the
 angle between the spheres which is approximately proportional to distance
 for small angles.

 What I think happens is something like this: Near to the antenna the
 Magnetic (M) field in the wire cancels with the M field (displacement
 current, think of the spheres as the plates of a capacitor) of the changing
 E fields of the spheres. The result is more E than M ie high impedance. In
 the far filed (several dipole lengths way) the E field of the spheres has
 decreased more rapidly along with its M field and hence the M field of the
 current dipole dominates  i.e. regular wave with the usual space impedance.

 Apparently its the current in the wire that produces the far field at least
 in this type of short (relative to wavelength) antenna with top loads.

 Note this is my first look at it and I have not rigorously checked the
phase
 and polarity of the E and M fields of three emitters but it will almost
 certainly be something along the above lines.


 Robert (R. A.) Jones
A1 Accounting, Inc., Fl
407 649 6400

> > Hi Jim,
> >
> > > 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.
> >
> > Thats correct if all the waves are going by in one direction.
> >
> > >
> > > 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
> >
> > It is similar the characteristic impedance of a transmission line or a
> Tesla
> > coil. Some times referred to as the wave impedance.
> > But if you measure the ratio of V and I along a shorted or open
> transmission
> > line it can be very different than the characteristic impedance.
> > This is caused by the summation of the forward and  reflected wave i.e.
a
> > standing wave. The ration of the voltage and current (and phase) vary
> along
> > the line.
> > Similarly the near field is a supposition of waves. Hence if you measure
> the
> > ratio you may not get the usual wave impedance.
> > There is only one set of Maxwell's equations and all space including
that
> > next to a dipole/conductor obey them.
> >
> > Robert (R. A.) Jones
> > A1 Accounting, Inc., Fl
> > 407 649 6400
> >
>