I recently got hold of a motorized roller inductor that is made PROPERLY. This is a MIL-SPEC unit that has been on stock for many years. It features a silver foil that is rolled on to a ceramic former, pretension, a shortcut cylinder that prevents eddy currents and arcing on the unused coil section under QRO operation. Due to that the unused coil is shorted and that the shorting is not a single turn, that the foil is wide, this coil has a very high Q.
I just got myself a new roller inductor. This one is designed on the principle of a silver foil that is rolled away from and onto a ceramic form with guides for the silver form. The ceramic coil forms the coil. There is a large shorting cylinder that the unused silver foil is rolled onto. The effect of this is to significantly increase the Q of the inductor. For high power QRO applications there may either be arcing from the end of the unused part of the coil or heat loss in this part of the coil. How the unused coil is completely shorted with an inner conducting cylinder and the unused part of the coil has no flux thru it.
I have now continued the investigation into a possible design for a wideband HF RF choke for QRO applications. The criteria is that the R part of the R+jX load the choke present to the common mode current on a coax should be so high that there is low risk of overheating the core even for QRO operation. It turns out that two main loops that gives sufficient resistive (and reactive load) that covers the lower frequency range combined with another loop with smaller diameter and fewer cores that takes care of the upper frequency range will give quite good results. This is similar to what GM3SEK has observed and described in his publication. Vector network analyzer measurements confirms this.
Above the three choke sections can be observed
Above, the measuring setup can be seen. On the right there is a multicore choke that was tried. It gives a big resistive and inductive peak low in the frequency range. Far more than required. The cost will be high due to the many cores and is not justified. Therefore a 3 + 3 + 2 design was found more optimal.
Above the resistive (blue) load to the common mode current, the inductive load (red) to the common mode current and the Q (green) for the choke can be studied. The choke presents around 1000 ohms resistive to HF current in the frequency range of approx 2,5-25 Mhz. Another material could probably have been added to prevent the drop above 25 Mhz. 10 Meter would be a bit marginal for QRO operation with a lot of common mode current.