On Mon, Dec 2, 2024 at 11:26 AM, Brent DeWitt wrote:

failing that, I'm thinking the RF transformer may be the best way to go.

Brent, check out the Coilcraft SWB1010, SWB2010, and SWB3010 wideband RF transformers. They are cheap and readily available. Their response is very flat over HF. Depending on the accuracy you need, you may be able to get by without calibrating the VNA through the transformer, which would simplify use. Coilcraft does not specify the primary-to-secondary capacitance. If you get the model with a secondary center tap, when necessary you can try grounding it to create a voltage balun. This may work better than an isolated winding if the interwinding capacitive reactance is too low at higher frequencies.

Brian