I'm planning to make a magnetic loop antenna mainly for receiving LW, MW, and SW radio broadcasting bands, as well as perhaps the 2.2km band and 630m band. (Not all of them with the same antenna, I'mm gone start with LW probably, so I can listen to foreign radio stations).
Now, I want to attach that antenna to the input of an RTL SDR, which has a 50Ω coax input. I can use the RTL SDR in direct sampling mode going from basically 1kHz to around 25MHz, so that's technically no problem, but I'm unsure how to attach the magnetic loop antenna (my first one is just gonna be something similar to this).
I'm also planning to make a ferrite core antenna, and see how that works out. But that's for a later question.
Now, I can use online calculators to get things down like the diameter, the number of turns, and the values for the tuning cap. But I'm unsure about how to attach it to the SDR. I have trouble figuring out the coupling loop which then goes to the 50Ω RG58 coax. So, I'd need a little help and/or ideas with that.
OK, now to clarify further, I'd like to build a loop antenna as described here: AM Loop Antenna Calculator - UMR EMC Lab Formula
So according to that and the build instructions I can make a box AM loop antenna for the sort of frequency range I'd like.
The pic up loop in those designes goes once around the entire loop of the antenna, and has nearly the same diameter. But how do I actually design the pick up loop? I.e. I have to match it to the 50Ω impedance of the coax cable. The build instructions just tell me to get a length of wire, and do a single turn around the perimeter of that antenna, but I don't see how that just matches the 50Ω impedance of my coax.