To find a series resonance you are looking for the frequency where the impedance is a minimum.
All three techniques you suggest will work.
1. Series between two ports, look for the frequency of maximum S21
2. Shunt, you only need 1 port. Look for the frequency of a dip in S11. Don't connect Port 2
3. With two coupling coils the result will be like #1, The frequency for maximum S21 is the self resonance
4. With one coupling loop the series resonance will be the minimum in S11, like #2. This is pretty much what you are doing with the grid dip meter, looking for maximum energy absorption,
Try all of these techniques to see the difference. That's one of the great thing about the nanoVNA it's a great educational tool at an affordable price.
73 de K9GXC, Jim
