Updated January 2021
A bias tee or Bias-T is one of the most useful Software-defined Radio (SDR) accessories you have likely never heard of.
Low Noise Amplifiers are used to Improve Receiver Sensitivity
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.
How is a Bias-T 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 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!
Bias-T and Software-defined Radios
Most of the SDRs on our list here have bias tees built into them at the antenna port. So they are able to power remote LNAs or active antennas. The only limitation in some of them is the amount of current that the SDR can provide to power the LNA. For instance the HackRF can only source up to +3.3 VDC @ 50 mA. That’s a very small amount of current for many LNAs. Especially LNAs that have a large dynamic range as indicated by their IP3 specification.
So how do you change the HackRF’s Bias-T voltage?
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.
A Bias T is a very useful accessory that is used to power a remote low noise amplifier. This improves the sensitivity of an SDR or any receiver. If you enjoyed this article, you might also enjoy delving a little deeper into software-defined radios and bias tees.