A bias tee or Bias-T is a three port network consisting of an inductor and capacitor as shown in the picture below.
The three ports are:
Applications in Signal Reception
When receiving wireless signals, a low-noise amplifier (LNA) can be used to improve signal reception.
The picture below shows a communication tower with an antenna on the top. A long length of cable connects to a LNA that’s placed before the receiver.
The closer the LNA is to the receiving antenna, the shorter the antenna cable (lower insertion loss) and therefore the lower the noise figure of the system. This is shown in the picture below.
There are two Bias tees placed between the Receiver and the LNA. The Bias tee is used to power the amplifier. Essentially it passes DC from the receiver to the LNA and RF signals from the antenna through the amplifier to the receiver.
Use this calculator to estimate the noise figure with and without an amplifier.
How does a Bias Tee Work?
A Bias Tee is a three-port or three-connector device that largely consists of an inductor and a capacitor as shown in the picture below.
The three ports are:
- RF + DC
(I say largely because there may be some capacitors between the DC line and ground used to filter high frequency signals).
A DC voltage sufficient to power the LNA can be applied to the DC port. RF port is used for only the RF signals in either direction. A third port allows both the RF and DC voltage to pass through.
So essentially the capacitor blocks the DC signal from appearing on the RF port while the inductor prevents the RF signal from flowing through to the DC port.
How is a Bias Tee used?
Let’s now see how this is applied in the case of our LNA example above. A bias tee is inserted before the receiver with it’s RF port connected to the receiver. Another bias tee is inserted just after the LNA with its RF port connected to the LNA.
Note that the RF+DC ports of both Bias Tees are connected to each other with the long antenna cable in-between. The DC port of the bias tee close to the LNA is used to power it. This configuration with two bias tees enables powering a remote amplifier as in this situation.
In the absence of a bias tee, a DC supply would have to be added to the top of the tower. This is difficult to do.
How to power a Bias Tee
Some radio receivers have Bias Tees built into their front end. Direct current is derived and supplied from the receiver power supply itself. In the event that the receiver does not have a BT, a separate module is required.
One such device is a USB Bias Tee.
This device makes it possible to use either a micro USB cable or a DC supply to source the voltage. The USB cable will provide +5 VDC. If you want a lower or higher voltage, then you will need to use a DC supply that can be varied.
Note that every Bias Tee has a maximum voltage and current specification. So you will need to take this into consideration when picking one for use with your LNA. If your SDR has a built-in Bias-T, then it’s good practice to disable it when using an external Bias-T.
Bias Tee Calculator
This tool calculates Bias Tee component values so you can build one yourself. As mentioned above, you will need the appropriate inductor and capacitor. At lower frequencies this is easy to construct.
At higher frequencies or for a broadband bias tee there are considerations around reflection coefficient S11, and insertion loss S21 due to the blocking cap and inductor.
Filtered Bias Tee
Sometimes a long length of cable can pick up interfering RF signals. The effect can be worse if the shielding is weak. In this situation it’s a good idea to use a filtered Bias Tee.
As the name suggests, the device includes a band pass filtered centered around the frequencies of interest.
A GNSS Bias Tee for example will pass all the Navigation bands from 1176 MHz to 1600 MHz shown in the picture below.
A Bias T is a very useful accessory that is used to power a remote low noise amplifier. This improves the sensitivity of any receiver which is a factor in a wireless communication system link margin.