intermodulation occurs when two frequencies are amplified / combined together. Several additional frequencies are artifically generated (called intermodulation product, or harmonic, of order 2, 3, 4, 5, 6, …).
You can google it, I’m sure you’ll find a wikipedia entry about it.
The formula to find what are the harmonics is :
f(h) = k * f1 + i * f2
with f1 and f2 the downlink frequencies in MHz
k and i : integers that vary between -3 to +3 (they can take larger values, but the higher the factors, the lower the power of the harmonic).