For my loop preamp measurements, I use a jig between the VNA and the preamp, shown here:
- unbalanced to balanced transformer is now different, on T50-6 core, quadrifillar wound. Primary is 10+10T (center connection floating), secondary is 10T and 10T
- jumpers to change the output between in-phase (for common mode suppression measurement) and differential.
- Jumper outputs are connected to 24Ohm to 0.5Ohm dividers, serving as an attenuator. It mimics the loop impedance (somewhat, at least on the lower frequencies). This also results in a good S11 on the VNA side.
No additional attenuator is used. LZ1AQ gives about +5dBm at the peak of the curve, no problem for NanoVNA.
73, Mike AF7KR
