OK I am back and wife is now quite well thank you. Getting back to the questions at hand, here is my interpretation of Verpies' measurement for the response of the two pancake coils in reply 510 using the connections shown with his S21 fixture and TEE. Note that the outer casings of the Tee and the coax cables connecting to each coil are not at ground potential, so they contribute leakage capacitance to ground shunted across the output. At low frequencies the phase delay along those coax cables is insignificant, but the cables do add capacitance across each coil that adds to the intraturn capacitance. The connection diagram simplifies to the one shown below. Thus one would expect a dip in the response at the LC resonant frequency as indeed occurs at about 5.7MHz in the measurement.
At somewhat higher frequencies the method of winding and feeding the pancake coils each using 3.2m of wire causes them to appear as a length of shorted transmission line of length 1.6m, and that feature has to be taken into consideration. That would tend towards an open circuit at the first resonant frequency where the line is a quarter wavelength, a short circuit at the next frequency where the line is a half wavelength and another open circuit when the line is threequarters wavelength and so on. That half wavelength case is seen in the measurement as a peak at 18.7MHz, followed by the threequarters wavelength dip at 29.5MHz. The broad quarter wavelength dip at around 9MHz is overshadowed by the 5.7MHz lumped constant dip but does cause that to be assymmetric. At even higher frequencies the coax cables lengths begin to take effect, but that is way outside our expected NMR freqencies.
Looking at the measurements using the Hfield detector I have added the typical amplitude response for the LC resonance of the pancake coils at just above 5MHz. Clearly this peak does not appear, we merely get an inflection of the rising curve expected for the half wavelength transmission line peaking near 18MHz. That tells us that at the 5MHz resonance the coil current sloshing back and forth between the L and the C is there but not enough to show a peak. However note that here the Q of the LC circuit is low as the 50 Ohm input resistance is across it. The coil current (hence the detected H field) can be increased if an appropriate input system that allows higher Q is applied. The other detail at higher frequencies is really only of academic interest. I have added detail along the top of the chart to indicate roughly regions where the LC approach, the transmission line effect and the coax cable lengths overlap.
My conclusion is that the lumped constant LC approach is satisfactory at the expected NMR frequency, but my suggested input circuit is not suitable due to the high self capacitance of the special pancake coils. It should be relatively easy to create an input circuit that allows much higher Q and hence greater coil current. The transmission line effect in the special pancake coils, while seriously affecting the coil current at high frequencies that are not of interest, has a small effect at our expected NMR frequency that should be even smaller when the LC circuit obtains higher Q.
Smudge
