Bias Tees and Other Inline Gadgets
December 10, 2010 2 Comments
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.