A bias tee is one of the most useful Software-defined Radio (SDR) accessories you have likely never heard of.
When receiving signals using a SDR, we have written about the benefits of using a low-noise amplifier (LNA). The closer the LNA is to the receiving antenna, the better the overall performance of the system. So if the LNA is on a tower, close to the antenna it’s a more favorable situation than having the LNA at the base of the tower. The question is: How do you power a LNA at the top of a tower?
This is where a Bias Tee comes in. So let’s first understand what a Bias Tee is. 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.
A DC voltage sufficient to power the LNA can be applied to one of the ports. Another port is used for only the RF signal 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.
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 SDR. 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 cable in-between. The DC port of the bias tee close to the LNA is used to power the LNA. This configuration with two bias tees enables powering a LNA at the top of a tower. In the absence of a bias tee, a DC supply would have to be added to the top of the tower which would be challenging to say the least!