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Exploring Bias Tee Power Injection

bias tee power

Using bias tee power injection is an easy way to send power to a remote device at the other end of a radio cable.

A remote control receiving loop antenna requires a connection for the radio signals, using an RF coaxial cable. It also requires power for the remote electronics. A neat way to send power and radio signals together on the coaxial cable at the same time removes the need for a separate power source. This method is called bias tee power injection.

As described on Wikipedia, “A bias tee is a three-port network used for setting the DC bias point of some electronic components without disturbing other components. The bias tee is a diplexer. The low-frequency port is used to set the bias; the high-frequency port passes the radio-frequency signals but blocks the biasing levels; the combined port connects to the device, which sees both the bias and RF. It is called a tee because the 3 ports are often arranged in the shape of a T.”

Here’s how it works. Remember two things. A capacitor passes radio signals but blocks DC power. Alternatively, an inductor (coil) passes DC power but can block radio signals with proper design.

A bias tee power injector circuit is shown above in the lower right picture. At the radio end of the system, you connect your radio to the jack marked “RF”. You connect a power source at the jack marked “5 Volts”. This mixes the two together for you to send to your remote device over a coaxial cable connected to the jack marked “RF+DC”.

At the other end of your system (in my case a remote receiving antenna) you also connect the distant end of the cable to the jack marked “RF+DC” on a second bias tee power circuit. This makes five volts available to power your device, broken out at the “5V” jack. It also provides a connection for the antenna at “RF”.

Bias Tee Power Experiment

I made two bias tee power injectors on a breadboard. I connected them with a 25 foot length of coaxial cable. At the source end, I injected five volts of DC power (at 5V) and a 1 MHz square wave signal generator (at RF). At the other end, I connected Channel 1 of my oscilloscope to the RF jack (yellow trace.) Channel 2 shows the output on the 5V jack (blue trace).

It worked. The blue line shows the five volts largely without any radio signal mixed in. The yellow line shows the 1 MHz radio signal without any DC bias.

Two things to note. First, there is a slight a.c. ripple on the blue line, indicating that the five volts DC is not completely pure. More or better filtering is needed. Second, there is distortion on the 1 MHz square radio wave; this is caused by the capacitance and inductance in the circuit.

I need to do some work choosing the capacitor and inductor values for the frequencies of interest, which is 0.5 to 2.0 MHz in my case. But otherwise, I have done my first bias tee power circuit and it works.

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