Can I use a 20 cm safe RF separation distance or a body-worn product with any antenna?

Sometimes explicitly, always implicitly. An FCC grant may or may not list specific approved antennas, antenna types, or maximum antenna gain values. When those details are specified in the grant, the limitations are explicit and binding, and only the approved antennas or the stated gain and configuration limits may be used. When those details are not listed, as is the case for many cellular and RF modules, this does not mean antenna choice is unrestricted. It means the FCC approved the transmitter at the conducted output level and placed radiated compliance responsibility on the host product integrator.

In practice, antenna selection is constrained by RF exposure, spurious emissions, and overall system compliance in the final product. Changing antenna type, gain, placement, grounding, or matching network can materially change the device’s radiated characteristics. If those changes cause the final product to exceed RF exposure limits or alter emission behavior outside the compliance basis of the original filing, the module’s FCC certification no longer stands on its own. The integrator is then responsible for additional evaluation and, if necessary, regulatory action to maintain compliance.

Even when an FCC grant does not specify antennas, it still constrains antenna selection in practice. The limits show up in exposure calculations and emissions performance, not on the face of the grant.