What is a Bias Tee?

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?

Powering a Low Noise Amplifier 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 Bias Tee looks like a T

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.

Powering a Low Noise Amplifier with a Bias Tee

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!

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2 thoughts on “What is a Bias Tee?”

  1. First – Thanks for all of the excellent articles on LNAs!

    Your site is the only simple, straightforward resource I’ve found that answers all the SDR questions that seem so obvious to people that have been doing it for a while but nobody else explains.

    Question: Could you write an article covering how and when you can chain Bias-T powered SDR accessories?

    For example: If I have an RTL or SDRPlay RSP that has bias-T power and I attach the RTL LNA and the RTL FM Filter will they both get Bias-T power? How can I tell?

    What if I also add the No-Elec 9:1 UnUn for a total of 3 Bias-T items?


    • Thank you! Yes sure can write a post about that. Will let you know when this is up. Cheers!

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