Intermodulation (or IM) is a very destructive kind of interference. There are two main types of Intermodulation,
1. transmit IM and 2. receive IM.
The concept of receive IM will be discussed here.
There is a unique piece of equipment in a receiver called a mixer. Sometime two or more signals can combine in a mixer and the resultant frequency can fall right on the desired receive frequency. This is what is called an intermod product. IM products up to the 7th order can be strong enough to hurt 2.4 GHz transmissions. 3rd and 5th orders are the most common (with 3rd order being the most destructive).
To accurately determine possible IM products, perform a twelve to twenty-four hour spectrum sweep on a potential site. Run your sweep from 30 MHz to about 6,000 MHz.