Actually, I've noticed that on checking up the mini circuits pointers
that the splitters they manufacture are only good for a given
frequency range. And I don't mean DC- Fx; I mean from say 30Mhz -
1000Mhz or similar. I'd have thought this type of stuff would have a
bottom end of *DC* not some relatively high radio frequency. We're
only talking about power splitters after all, not broadband filters.
So why the low frequency cut-off? Are there some capacitantances
utilized in these designs and if so, what are they doing there?

These are ferrite transformer hybrid splitters with fractional dB
insertion loss beyond the 3dB split and typically 30dB isolation between
output ports. If you want a DC splitter then they have those too- but
you're not swift enough to see them. They are simply this. What do you
think the loss and isolation is now?:
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