Radio, GPS and nav equipment, transponder communication, ADS-B, ELT, and TAS/TCAS data systems usually include at least one belly antenna. You may also have a separate antenna for various other avionics equipment. Well-equipped aircraft often have several!
Belly antennae come in all shapes and sizes for different purpose and different aircraft – they could be whip-like and flexible, sharp and pointy, or more like a shark’s dorsal fin.
They are small, usually white, and often hard to see in a hangar environment.
Unless they include a big red Trisoft antenna cover!