I apologize, but I couldn't find specific information on a microwave model called "Link microwave 18 GB" in i am not an expert. It's possible that this model is either very new or specific to a certain region or industry.
The term "side lobes" generally refers to the radiation pattern or beam shape of an antenna. It describes the intensity of the signal in directions other than the main beam. The specific length of side lobes would depend on the design and specifications of the antenna used in the microwave system.
If you have additional information or context about the "Link microwave 18 GB" or any specific details about the antenna or system you are referring to, I may be able to provide more accurate information.