Hi Mike,
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I think what you are seeing is not unusual. No transformer is perfect. The parallel magnetizing and loss current across the 50 ohms is bringing it down to 46 ohms. At the lowest frequency transformer action is failing as the core can't sustain the higher flux required so it begins to look like a short circuit. It would be interesting to plot the transformer input impedance across the frequency range with both an open and short circuited secondary. That's how we were taught to measure transformer characteristics (admittedly at power frequencies but there's no reason why it can't be done at RF).?
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A VNA, nano or otherwise, would be very helpful in analysing this. Ditto a vector impedance meter. I'd love to duplicate your experiment but can't do it soon as my bench is totally occupied and will be for while with another project.
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Cheers,
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Morris
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