My career started designing RF amps, but most of it has been in power electronics over various sorts. Various sorts means from 50 Hz to about 100 MHz fundamental frequencies, with much of it in the range of 100 kHz to a few 10s of MHz, from standard PWM to class E and F amps. So, I am somewhat familiar with VNAs of both the low frequency type (like the current Omicron Bode 100) and the more standard RF type, as well as basic RF measurement and design. I've also designed some transmission line transformers, and the CM transformer is a sort of balun. In any case, in the power electronics world, loss means watts dissipated as heat, and attenuation means reduction in level. I also spent a fair amount of time diagnosing EMI problems, and CM EMI is often a big problem in EMI.
It's the series-shunt methods that I have less experience with. In my past job I had access to nice impedance analyzers, and I also used low frequency VNAs like the HP3577 in IV mode for impedance measurements. I had good isolation transformers of the conventional type, and used Pearson transformers for current measurement. The IV mode measurement means that the transformer frequency response can be pretty poor and you still get a good measurement. With this approach, I could get sub-milliohm measurements pretty accurately and repeatably. I also had access to an HP Q-meter. All this I used to measure filters, transformers, and inductors. Even if the fundamental switching frequencies were in the 100s of kHz, ringing and harmonics meant there was a lot of content at much higher frequencies, where inductors might be past a second or third resonance.
I am interested in this topic now because I am trying to characterize low value shunts up to 100 MHz or higher. I have some designs that appear to work ok, but I have nothing to compare them to. My total inductance budget is < 1 nH, so any conventional current sense solutions blow that out of the water. I'm also interested because I am trying to measure losses in PCB layouts over a wider frequency band. Finally, I am also trying to understand high frequency "ground" currents and ground bounce issues where there are very high, very fast current spikes. I don't have the same access to high end equipment that I once did, or least not RF equipment, so I need to figure out some other ways to do things. This is all work-related. The challenge is that I don't have the luxury of 50 ohm anything. Since they are power conversion circuits, a lot of amps are flowing around at low voltage, so the impedances are generally much lower than 50 ohms.
Cheers,
John