I think my mind cannot grasp this, as to me it is then pointless to try.
Oh no, no - don't you give up.
This "maximum near-field magnetic field (H-field) between two loops" surely needs some power to be build up, won't that power need to be fed in by the PA as efficiently as possible?
Your statement is correct* but only in reference to a constant level of magnetic flux or a coil supplied with DC.
The answer to your question at the end is "yes" but only for a constant level of magnetic flux or for the 1st quarter of the AC cycle.
What you are failing to consider is what happens AFTER the magnetic field has been built up to its maximum level in the 1st quarter of the AC cycle.
When you do that, it will become apparent that in order to maintain the oscillation, this magnetic energy needs to be decreased back to zero. This can be done either by converting it back to electric current and dissipating its energy as heat in some resistance or radiating it as a far-field EM wave or recycling it through a capacitor. This resistance and/or capacitance can be outside or inside the PA.
In case of energy recycling through a capacitance, the capacitance then becomes the sink of energy and for the next quarter of the AC cycle - the source of energy for the coil. And so, the energy sloshes back and forth between the capacitor and coil in the familiar parallel LC circuit manner. When this "sloshing" is lossless then the PA does not need to replenish that AC energy, so the
energy transfer between the PA and LC tank stops ...and the PA "sees" an open circuit. That apparent "open circuit" presents very poor match to the PA's output and this is what your SWR meter shows.
In realistic LC circuits, the energy "sloshing" between the coil and capacitance is being lost continuously in the resistance (as heat) and through EM radiation (because of radiation resistance), ultrasonics, ..and hopefully to proton's precession. The PA must then replenish this lost energy. The more needs to get replenished, the more energy flows form the PA to the LC circuit. Eventually this reaches an equilibrium, where the PA supplies as much energy as is being lost. Maximum energy transfer happens when the impedance of the PA is equal to the impedance of the load....albeit not the max efficiency. See:
MPTP.
reading somewhat further back you say:
So does this mean that ONLY at the moment "the H-field generated between the coils does some work etc...." this will be seen as a dip in the reflection coefficient and SWR?
Yes, otherwise no energy will flow from the PA to the ideal parallel LC circuit.
However, realistic parallel LC circuits always have resistive and radiative losses and these will need to be replenished by the PA, causing some net energy transfer and dipping of the reflection coefficient and SWR. My point here is that the maximization of the radiative losses (radio wave generation) is not synonymous with the maximization of near H-field amplitude because:
- the radiative losses (generation of far-field EM waves) are proportional to the product of the radiation resistance and the square of the current amplitude.
- the near H-field amplitude is proportional to the product of current amplitude and coil turns (ampturns),
The situation is a little different when the PA is connected in series with a series LC circuit, because then all the energy sloshing goes through the PA, so the PA's internal resistance and reactances participate in it.
If so, then there needs to be the focus on during the tuning i think.
Yes, the goal is still to obtain the maximum magnetic flux amplitude and that means the maximum ampturns.
If you think that your HF new magnetic field sensor is trustworthy (the one made out of the Litzed loop in the i-Probe), then this is the best indicator for tuning for the maximum amplitude of the magnetic flux.
*
except you should have used the word "energy" instead of "power"
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Topic: Smudge proposed NMR experiment replication. (Read 127102 times)
