Bias Tees and Other Inline Gadgets

To feed the active antennas, you need bias tees, which feed DC to the coax near the receiver and extract it at the antenna amplifier. I normally build the amplifier-side bias tee into the amplifier, but on the receiver side I use a small box with two BNC connectors and a DC connector, the bottom one on the picture on the right. The gadget on the top is a resistive SWR bridge with an LED indicator. The design of the bridge is from Steve Weber’s low-power L tuner; it works very well.

Both are built into boxes that used to house Tektronix test-equipment inline gadgets called input RC normalizers, which consist of a resistor, a capacitor, and a trimmer capacitor. The box has a little hole with a plastic cover to let the user adjust the trimmer. To use the box as an enclosure for something else, you remove of course the original contents. In the bias tee I enlarged the hole a bit and used it for a panel DC socket. In the SWR bridge I drilled the plastic cover to make room for the LED. A third normalizer became a 50Ω terminator.

I have a few more of these boxes that I bought at a swapfest, along with two slightly different ones (a slightly bigger inline Pomona box, and one with 3 BNCs). Very handy for building little inline or 3-port devices.

Bias tees are very simple devices, consisting of just an inductor and a capacitor. The inductor lets DC pass to the antenna but prevents RF from going into the DC power supply; the capacitor prevents DC from going into the receiver but allows RF to pass.

In spite of the simplicity, it is possible to build them incorrectly. The mistake I made with the first bias tee I built for active antennas was to use an inductor with too much resistance. The series resistance caused it to heat up and burn (I think it happened when I used it to feed the active whip). In the pictures below you can see on the left the packaged bias tee (it’s huge for no good reason); on the right you can see the inside, with the burned inductor, which is now just an open circuit. I now use in my bias tees high-current inductors salvaged from broken switching power supplies.

Obviously, if the inductor has too little inductance or the capacitor too little capacitance (or a voltage rating that’s too low), that’s also not good. But there are more surprising failures to bias tees. Phil Salas described in QST in July 2004 a pair of relatively complex tees, with reverse voltage protection and with a fuse. He discovered that the 100μH inductor that he used had enough parasitic capacitance to resonate at 11.5MHz, which means that signals around that frequency will pass through the inductor and into the power supply (he modified it to remove the resonance). Another interesting article on bias tees is by Tom Cefalo Jr., who described in QEX in May/June 2002 bias tees optimized for 144MHz or 432MHz.

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2 Responses to Bias Tees and Other Inline Gadgets

  1. barbswalters says:

    I know I am little late to the game here with this post. I was curious what values you’re using for your inductors and capacitors, or how you came up with them. I have been looking at Tom Cefalo’s design, which is a bit more complicated, and has a little more hard to come by surface mount caps. If I can get my hands on the parts, I might attempt to build a couple of Tom’s, but I like how simple yours look.

    • Sivan Toledo says:

      It’s very simple. The capacitor should pass the RF signal but the inductor should block it. The RF signal is transmitted through a 50 Ohm transmission line, so the reactance of the capacitor should be a lot less than 50 Ohm at the lowest frequency you are interested in and the reactance of the inductor should be a lot more than 50 Ohm at the lowest frequency. If you use say 0.5 Ohm for the capacitor and 5000 Ohm for the inductor I think you should be okay.
      One thing to worry about is parasitic resonance. Capacitors have a bit of inductance and inductors have some capacitance. These are normally small (parasitic) but if you go high enough in frequency, they become significant and a capacitor could have a high impedance (when it should have almost none), blocking your RF, or an inductor could have a low impedance, passing the RF into your power supply etc. I did not experience this but it happens, especially if you try to use the same bias tee at very different frequencies (say from 1 to 1000MHz). Unless you are dealing with high power, high voltage, or equipement that is very sensitive to SWR, just give it a try without worrying too much about this. Or you can parallel multiple capacitors and use several inductors in series. Good luck and enjoy the experimentation.

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