[Elecraft] OT: SGC Coupler / Small Wire Loops

[Ron D'Eau Claire]

Yes, Tony,  the voltage and current are always 180 degrees when a =
resonant
circuit is "excited" at the right frequency. =20

We have a parallel inductance/capacitance... well, to start somewhere =
let's
say the capacitor has a full charge - so there's a very high voltage =
across
the plates - a positive charge on one plate and negative on the other. =
The
coil across the capacitor looks like a "dead short" at that moment, so =
the
current starts flowing very rapidly through the coil. But, as the =
current
flows through the coil, it creates an expanding magnetic field.

Now, as you know, whenever a wire is within a changing magnetic field =
there
will be a current induced in the wire. In this case, as the current =
flowing
causes the magnetic field to build up, that building field creates =
another
current in the wire that is in opposition to the first current. This is
often called "back emf" (emf =3D electromotive force). Now, the back emf =
is
NOT strong enough to stop the current flow, but it does slow it down. So =
now
the inductor no longer looks like a "dead short" but instead has
"resistance" or to be accurate, has "reactance" which is the resistance =
to
an alternating current.  Unlike a resistor, which would convert the =
energy
to heat, this reactance converts the energy into a magnetic field, so it =
is
not lost.

So the effect of the back emf is to require a finite amount of time for =
the
capacitor to fully discharge, rather than the capacitor discharging in =
one
"BANG" as would have happened if there was a short solid conductor
connecting the plates.=20

As the capacitor runs out of charge, the field around the coil stops =
growing
and begins to collapses. As the field collapses the action reverses and =
the
current starts flowing in the reverse direction through the wire and =
into
the capacitor again, where it gets stored as an electrostatic charge =
between
the plates.=20

Eventually, the magnetic field will have collapsed to the point where =
very
little current is flowing into the capacitor, and the capacitor can  =
again
discharge through the coil.=20

This would keep up indefinitely except for the losses in the coil and
capacitor, inductive losses to any conductors near the coil, and, of =
course,
some of the energy get radiated as an "electromagnetic" or "radio" wave. =


Every combination of inductance with capacitance across it has a =
"natural"
resonance at the frequency where the inductive reactance equals the
capacitive reactance. At that point the currents and voltages reach =
their
highest possible levels. That's the basis of a tuned circuit showing
"selectivity", or emphasizing one frequency of oscillation over all =
others.

And, again, you are absolutely right, the current and voltage are 180
degrees out of phase and MUST be in order for energy to flow efficiently =
in
the circuit. Indeed, in any resonant circuit (including any antenna) =
that is
essential for power transfer. Indeed, why an antenna or any resonant =
circuit
does not transfer power efficiently at frequencies other than its =
"resonant"
frequency is because the voltage and current are no longer exactly 180
degrees out of phase at other frequencies.=20

There's a company claiming to have "invented" the idea of having the =
voltage
and current 180 degrees out of phase in an antenna and talks about a
"patented"  phase-shift network that they use to excite a very small
radiator. What they say is perfectly true. What they DON'T say is that =
EVERY
"antenna tuner" is just such a phase shift network that adds capacitive =
or
inductive reactance to the antenna circuit as needed to put the voltage =
and
current exactly 180 degrees out of phase at the desired frequency... =
only
Hams and engineers since Marconi was tinkering with antennas have called
this process "resonating" the antenna. Of course, "self resonant" =
antennas,
like a 1/2 wave dipole, are simply inherently resonant at one frequency
without the aid of an external "tuner". That is, they show an exact =
balance
between their inductive and capacitive reactance at the desired =
frequency so
the current can flow 180 degrees out of phase. They function exactly =
like
any other resonant circuit.

Does that make sense Tony?

Ron AC7AC
K2 # 1289

-----Original Message-----
Ron wrote:

>is, will have very high peak voltages followed by very high peak=20
>currents. Remember, the circuit is oscillating! All the energy is=20
>stored up as a

Hi Ron,

Can you expand on that a bit - are we talking high voltage and high =
current
90% out of phase here?

Regards,

Tony