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I want to connect this -- https://swling.com/blog/2021/06/jock-de ... on-issues/ -- to an MFJ 1045C active preselector, which requires coax input and does not accept a wire antenna directly.

So I need a balun?? -- maybe this? https://www.amazon.com/LDG-RBA-4-Balun- ... 810&sr=8-5

Am I correct that an unun is not the right thing?

Well, Jock, given that the loop is indoors and used on a wide range of frequencies, the impedance will be all over the place, for this reason you may consider picking (or building) a 1:1 balun or even going for no balun at all and using an adapter like this one; as for building the balun, it isn't difficult, you may just wind it following the instructions for the one used for the NCPL antenna; that one is a 1:1 (same number of turns for both windings), willing to change the ratio means doing a little math; let's say we want to have a 4:1, all we need to do is calculating the square root of 4 which will be 2, now this means that to obtain a 4:1 transformation ratio our balun will need a 2:1 windings ratio, so for example, we may wind 4 turns for the primary and 2 turns for the secondary (4/2=2) and we'll have our 4:1 balun the only thing to remember is that each time the two ends of the wire pass through the binocular core holes and exit from the opposite direction, that counts as ONE turn not a half one, look at this image



the "C" is ONE turn, the "D" is TWO turns, see it ?

As for the MFJ-1045C, had a quick look at its schematic (below) and sincerely I can't say I like it, anyhow, if it works for you...

Got it. Thanks!

I think I will try first with the adaptor.