The question is quite simple: When making a coax choke balun, does the size of the ferrite matter, and if so, why?
I have seen design notes that say to use a single FT240-43 toroid for powers up to 400W, and then to use two stacked FT240-43 toroids for powers up to 1 or 2 kW. I am specifically talking about choke baluns here, I understand that other designs operate differently. In a choke balun, the ferrite is only stopping current from travelling along the outside braid of the coax - it does not see the main transmitter power (as this is contained inside the coax).
[Image used with permission - source M0TAZ Blog]
My understanding of the theory is that these ferrites are only (ideally) effective on currents along the outside of the coax - the currents that are to be choked - and the internal currents are not affected.
Since the purpose of this is to reduce the current flowing along the coax screen outer to nothing, why does the ferrite need to be so large? A typical configuration would see a choking impedance of above 3kΩ even at the LF bands, so I am at a loss as to why such huge large masses of ferrite are needed.
I assume the basis for such "rules" is that the ferrite will saturate if there is not enough of it. But will it?
I am ignoring things like AL value, etc., since this can be achieved over a range of ferrite sizes/turns.