unexplainable aliases in spectrogram
Looking at the spectrogram, the prominent artifacts go up or down in frequency synchronously to the 138 MHz signal but have larger bandwidths. That is an indication that they are its harmonics, due to a nonlinearity somewhere in the system. With the sampling frequency of 500 MHz some of the positive and negative harmonics alias to the following (bold) frequencies that match with the prominent artifact bands:


-138 MHz * 2 = -276 MHz. -276 MHz + 500 MHz = 224 MHz
-138 MHz * 3 = -414 MHz. -414 MHz + 500 MHz = 86 MHz
138 MHz * 4 = 552 MHz. 552 MHz - 500 MHz = 52 MHz


The apparent bandwidths and complex conjugacy seem to match those calculations.

You did not mention what equipment you are using, but it looks like there is no proper antialias filter.

The less prominent artifact bands can be explained similarly:


138 MHz * 5 = 690 MHz. 690 MHz - 500 MHz = 190 MHz
-138 MHz * 6 = -828 MHz. -828 MHz + 2 * 500 MHz = 172 MHz