Rebonjour just returned to this topic...
a few notes
1/ study the Pearson shield, your shield configuration is not good.
The shielding you see (Al foil over 3d printed plastic) is a quick, cheap and fast method of trying different shielding geometries.
The final version will be machined from billet aluminium.
I have never seen/touched a Pearson sensor so not really a practical option.
Looking at their products on-line indicates shielding that is a close fit to the coil. There could be a resin supporting the coil/core inside the shield. Can someone confirm?
2/ in the frequency ranges mentioned and dimensions there's no " cavity" resonance. Check the wavelength vs shields dimensions
Agreed. Bad explanation on my part. I think the space between the coil and the shielding is acting as a capacitor to the coil forming an LC resonator.
3/ termination should be at the winding and distributed
If you mean that the coil winding termination should be very close to the core, agreed. I have proved that by experiment. There is resonance at 23MHz due to the length of bare wire between the core and the coax cable.
The eccentric shape of the prototype allows just enough spacing for a panel mounted SMA connector. The need to keep the coil terminations as short as, lead to the choice of SMA rather than BNC.
BNC is easier to use and entirely adequate for the job, but the lead lengths would be longer.
4/ try various types of core material, suggesting high permiability 10,000 u ferrite or thin tapewound cores.
The bare open coil/unknown junkbox core has proven to provide adequate bandwidth, good flatness and everything else I am looking for.
The enclosure has proven to be far more important than the core and windings.
Exotic ferrite materials are only available through companies like RS, Element14, Digikey etc. They all have minimum purchase values and they are all expensive.
I do have some known cores purchased specifically for what I am doing right now. The junkbox cores and wire wrap are already working beyond expectations.
5/ wire, insulation, core coatings or wrap, and winding design have significant effects on frequency response, transients response and flatness.
Experimental results do show that the choice of wire does make a difference. I am using 40 year old mil-spec wire wrap made with real silver plating and real copper on the inside. The advantage of wire wrap is that it minimises the gaps between the core and the conductor, minimizing stray magnetic fields. I would have to use something very exotic and expensive to make any further observable improvement.
6/ I have never seen filling with beads around the toroids, it should have little effect.
It definitely works. They soak up energy that would otherwise be used to resonate.
Given that I have LC resonance that is reducing the upper bandwidth, I can either reduce LC to increase the resonance frequency out of the desired measurement band
or
dampen the resonance by loading the LC resonator with ferrites acting as resistors.
Both have advantages and disadvantages. Adding ferrite beads is the option of last resort.
Just my thoughts
There is a thing called the principle of 1+1=3. When two minds work together, the outcome is more than the sum of the two. It is three.