Bias Tee

I have been working on building a remote antenna switch to switch from my 40-10 meter to my 80-40 meter antenna —more on the switch in an upcoming blog post.

My remote switch will be sitting at the base of the antennae and needs to be powered by 12-volts. I had the choice to either run a cable from my radio shack 70 feet away to the antennae or to use the existing coaxial cable and carry the radio signal, as well as the 12 volts for the switch. Running 70 feet of wire through the house and the backyard is not an easy nor fun task. That's why I have decided to build a bias tee and use the coax to carry both the RF signal and the switch's necessary power.

This requires one bias tee on one end of the coax to mix the DC and the RF signal and a second bias tee on the other end to separate the DC from the RF signal.

Think of a Bias Tee as a diplexer with an ideal capacitor that allows AC through but blocks the DC, and a perfect inductor that blocks AC but allows DC. Wideband bias tee, operating from kHz to GHz, can be complex to build. Each component has parasitic elements¹ and acts as a complex LC circuit, blocking some frequencies. For this application, I am only using HF frequencies from 1 to 30 Mhz. This makes the design of such a circuit less challenging. I still want my bias tee to have a 50Ω matching impedance, low insertion loss, and good separation between AC and DC.

This picture shows the final result. If you are interested in building your own, the schematics, PCB design, and BOM are available for download on EasyEDA

Conclusion

I have spent some time playing with the CAD software to get an excellent 50Ω matching impedance. As a result, the SWR is 1:1 to 1.1:1 on all the HF and VHF ham bands. On the HF bands, the separation between RF and DC is between +60dB and 33dB.

I have tested the insertion losses with a NanoVNA. The NanoVNA is excellent because it is cheap for experimenting and learning, but I don't trust it for precise measurements.

With the NanoVNA, the insertion losses are: