A bias tee is a circuit (often sold with connectors) for sending power and signal along a single coaxial cable, this is often used with active antennas and power over Ethernet among other applications, the inductor is chosen to have an impedance below the transmission line characteristic impedance Z0 while the capacitor is chosen above, values like 1mH and 0.1uF are typical and impact the minimum frequency and signal leakage.
The biggest drawback of this simple design is self-resonance of the inductor which can cause significant signal loss, more exotic multi-inductor designs can extend the range from MHz to several GHz.
