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Also, that TH unit won’t auto-tune with it’s internal ATU without the simple wire antenna connected. It is designed to only function and tune correctly with that wire and nothing else. You can loosely couple to sample that signal from the wire antenna by placing the scope probe nearby or clamped to the wire itself, making sure it is insulated so that there is no electrical contact between the probe and wire antenna. That should be more than adequate to measure for that envelope.
Without the external ATU to properly load and tune in the external antenna mode, you won’t get a proper measurement either. But you might try a 50 or 75 ohm non-inductive resistor on the output connector and switch to external antenna mode and loosely couple the scope probe to that load resistor, again more than adequate to measure and see the envelope.