It is advised to stay at least least 3 dB below maximum low output power to ensure the output impedance is as close as possible to 50 ohms. This should work for the complete 100kHz to 350MHz range.
Going below the specified minimum output frequency of 100kHz the output impedance will gradually increase and become more and more capacitive.
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For more info on the tinySA go to https://tinysa.org/wiki/
