A bias tee is a passive three-port device used to supply DC currents or voltages to bias RF circuits. A signal that consists of RF and DC enters on the input. Conceptually an ideal capacitor blocks all DC signals from getting through and only allows the AC-/RF-signals to pass through to a high frequency port. An ideal inductor blocks all RF signals only letting DC signals through to a DC port. A bias tee can be used to insert DC power into an AC signal to power remote antenna amplifiers. It can also be used to for DC biasing an active unit under test.