RTL-SDR Bias Tee: Simplify and Amplify RF Signals

Understanding and Utilizing the Bias Tee Function in the RTL-SDR Software-defined Radio

Table of Contents

Introduction

RTL-SDR is one of the most popular SDRs on the market today. The dongle form factor and low price make it very compelling for a wide range of applications. One valuable feature available in this SDR is the Bias T function.

This article explores what Bias T is, how it works in RTL-SDR, and why it is beneficial for radio enthusiasts.

What is Bias Tee?

Bias T is a technology used in SDRs, radio receivers and other radio equipment to supply DC power to active antennas, low noise amplifiers and other external RF components directly through the coaxial cable that carries the RF signals. This eliminates the need for a separate power supply for these devices, simplifying setups and reducing cable clutter.

How Bias T Works in RTL-SDR

In the RTL-SDR, the Bias T function enables the user to activate a +4.5 Volt DC source @ 180 mA that is sent up the antenna cable.

This voltage powers pre-amplifiers, active antennas, or other inline devices that require power without needing an external power source.

The picture below shows the basic construction of a Bias T.

Essentially it consists of an inductor and a capacitor

The inductor passes DC from the RTL-SDR up to the active components and blocks RF energy
The cap passes the RF input signal. It also blocks the DC from entering the front-end of the SDR

Key Features of RTL-SDR Bias Tee

User-friendliness: RTL-SDR V4 has a small red LED that is illuminated when the Bias T is activated. This is visible through a tiny hole next to the SMA port.
Ease of Activation: In V3 and V4 the bias tee can be toggled entirely in software.
Protection: RTL-SDR Bias T has circuit protection to prevent damage to the SDR or the powered device, safeguarding your equipment during operation.

Benefits of Using Bias Tee

Simplified Setup: By powering devices directly through the coaxial cable, Bias T removes the need for additional power wires, making the setup cleaner and simpler.
Enhanced Performance: For applications like receiving weak signals from far-off satellites, using an active antenna with Bias T can significantly improve signal strength and quality.
Versatility: Bias Tee supports a variety of RF devices, making RTL-SDR extremely versatile for different applications, from amateur radio to satellite watching.

Practical Applications

Amateur Radio: Ham radio operators can use Bias T to power pre-amplifiers at the antenna, optimizing signal reception without the noise typically introduced by longer cable runs.
ADS-B Reception: For those tracking aircraft, Bias T can enhance the capabilities of active antennas, and filtered LNAs improving ADS-B signal reception quality.
Satellite Radio: Enthusiasts tracking weather satellites or receiving other satellite signals can benefit from the improved signal integrity provided by powered antennas.

Comparison of Bias Tee’s

Here is how Bias Tee’s in other SDRs compare.

The RTL-SDR is able to support the highest level of continuous current

Both RTL and SDRplay have over-current protection as a plus over the HackRF.

SDR	Bias Tee Voltage (V)	Bias Tee Current (mA)	Over-current Protection
RTL-SDR	4.5	180	Yes
HackRF	3.3	50	?
SDRPlay RSPdx	4.7	100	Yes

Notes

Versions V1 and V2 of the RTL-SDR required opening the case and manual soldering to enable the Bias T
V3 and V4 have a Bias Tee that’s enabled by software

Conclusion

Bias T is an essential tool in a radio receiver. Including this subsystem gives the user the ability to improve the sensitivity of the RTL-SDR software defined radio system and detect weaker signals.