G4NSJ – Bias Tee for the MiniWhip active antenna Mini Whip

Bias Tee 12 Volt Supply

Mini Whip Problems.

I wondered why the short wave bands were so quiet one evening when I was tuning around. After some investigation, I discovered that the MiniWhip active antenna had lost its 12 volt supply. The Bias Tee board, that came with the Miniwhip, had blown up! Sadly, water had found its way into the box after some heavy rain.

I was looking online for the bits and pieces to build a replacement bias tee when I found one ready-made, pictured on the left below. I like to build my own stuff but, due to the very low price, I thought I’d give this one a go. The centre photo shows the new board fitted in the old box. The original board is pictured on the right.

The cost of the new board was £4.00 which included postage. I really didn’t think that it was going to be any good for that price! However, I connected it up to the Miniwhip and it worked well! I didn’t doubt that it wouldn’t supply the necessary voltage to the antenna but, would it isolate the DC and RF properly? The set up worked well with no problems.

What is a Bias Tee?

A Mini Whip needs a low voltage supply to operate. A separate power cable could be run alongside the coax to the antenna to feed the circuitry but, there’s an easier way. Why not use the coax to carry the supply to the antenna? Connecting a power supply to the coax would ground the RF signals received by the aerial. However, if the DC power could be isolated from the RF signal… that’s where the bias tee comes in. It’s a simple device which allows the DC voltage from the power supply to be connected to the coax, but doesn’t allow the RF signals to pass back to the power supply.

In the old days!

Back in the 1960s there were some areas where the TV signal was pretty dismal. Very often, a mast-head amplifier was used to boost the signal. The power for the amplifier, usually 9 Volts, was provided by a battery housed in a plastic box at the rear of the television. The box also housed a bias tee circuit which sent 9 Volts up the coax to the mast-head amplifier.