Details are in the image, see the signal around 433 MHz which I don't understand. Is this possible?

Someone operating outside the NB transponder range?
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What is your transponder downlink QRG ? Your IF does not matter.
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433.500 to 434.000 MHz is the NB transponder tuning range, the anomaly shown is 500 kHz below the lower beacon, so at 433 MHz.
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Proposal for testing: Install Teamviewer on your cell phone and PC, then you can monitor the signal while standing next to the dish. I always point my dish that way when I change it from parking position to operating position