A chap asked the assembled experts online:
Has anybody tried stacking say a type 43 on top of a 61 on top of a 31 for a wider bandwidth? Would the losses be any greater than using one toroid for each band?
There were some very firm assertions that this will not work well (without evidential support of course). Beware of firm assertions!
Quickly the case was compared to parallel chokes… and there is no parallel (pardon the pun), they are not the same.
They are somewhat similar (but only somewhat) to two series chokes with the same number of turns, so you might expect overlap of the responses. Two series chokes carry the same current and with the same number of turns apply the same magnetomotive force (mmf) to both magnetic cores.
But rather than hypothesise, even if from experience… lets measure.
Above is the DUT. It is a stack of BN43-202 and BN73-202, 5t wound through both. This is not two chokes in series, it is just like stacking a FT240-43 and FT240-73 in concept.
Above is the measured choke impedance from 1-41MHz.
It has an impedance profile that is the sum of the contribution of both cores, it is not a low impedance result. |Z|>2kΩ from 1-34MHz.
I will leave it to readers to measure 5t on each core type and compare them.
But fact does not often get in the way of energetic discussions on social media. Do we know what fact (or truth) is anymore?
I will do some thinking here…
One of the concerns was this will increase heat loss and reduce the power handling of the combination.
My analysis is that loss in ferrite is related to the flux, which is related to the current. Deployed in an antenna system, the combination has higher impedance which will often lead to lower current, reducing flux and heating.
I would argue that appropriate combinations of core mixes is capable of a broader band higher impedance profile, and improved power handling.
Build, measure, learn.
Read widely, question everything.
… NanoVNA examination of stacked ferrite cores of different mixes – more detail