What is T-Bias/Bias Tee for Antenna Pre-Amplifier?

What is T-Bias/Bias Tee for Antenna Pre-Amplifier?

T-Bias/Bias Tee is also known as DC injector. When you have a remote antenna pre-amplifier to boost weak signal, T-Bias or Bias Tee is a method of sending the DC power through the antenna feeder coax without having an additional line along with it for DC injection. The principle is to have an inductor in the direct current input so that the radiofrequency from the antenna or radio does not get into the DC power supply. A capacitor is used to block the DC from entering the RF output section of the radio. These components are arranged in a T configuration which gives it the name. But actual construction is not that simple because the inductor will have a stray capacitance when a high value inductor is needed. This stray capacitance will leak RF coming at higher frequencies.

Typically the inductance of the inductor is such that its impedance at the operating frequency is much higher than the 50 Ohms characterisitic impendance of the transmission line. Capacitance of the capacitor is chosen to have a much lower impedance at operating frequency than the 50 Ohms of the transmission line. From this it is easy to understand that to have a higher bandwidth for operation, much more complex circuitry is needed than the simplified principle explained in the diagram. The actual circuit will not be just a combination of a capacitor and an inductor. A string of inductors are used so that each will have its own high resonant frequency in addition to a lower composite resonance frequency. In addition, resistors and capacitors are inserted to prevent resonances. A series diode is used to protect against reverse voltage. Protection from potential damage due to short circuit is given by a fuse.