12 V Bias-T for the MLA-30 Loop

Swapping the USB-powered bias-T that came with my MLA-30 magnetic loop for a 12 volt one, in a shielded metal case with a Powerpole input.

The finished aluminum box seen from above, labeled ANTENNA on a gold SMA jack at the left, RECEIVER on a BNC jack at the right, 12 VOLTS above a red and black Powerpole pair, and On/Off beside a toggle switch — The finished bias-T: antenna in on the left, receiver out on the right, 12 volts in on a Powerpole

A bias-T sends DC power up the same coax that carries the signal, which is how an active antenna like the MLA-30 magnetic loop gets its power without a separate wire. The one supplied with my loop was powered from USB. I wanted the supply voltages in the ham shack to be as consistent as possible, so I replaced it with a 12 volt bias-T. The MLA-30 will run on up to 15 volts, so 12 is well within its range.

I built it into a shielded metal case. I had first 3D-printed a PETG case for the board, but I quickly decided against it and went with metal, to help keep stray RF from the transmitters around the shack out of the bias-T. In use, I noticed there is less noise on the receive signal than there was with the original.

12 Vsupply, same as the rest of the shack

15 Vthe most the MLA-30 will accept

25 kHz–100 MHzbias-T frequency range

The build

Parts. A small bias-T board, plus a die-cast aluminum box from Jameco (3.9 × 2 × 1 inches), which I built up in the spring of 2025.
Drilling. The box needs holes for the antenna jack, the receiver jack and the on/off toggle switch, and a square opening on the side for the power input, which I cut with a Dremel.
Powerpole input. The 12 volts comes in on an Anderson Powerpole pair held in a small metal bracket on the side of the box, so it plugs into the same power as the rest of my gear.
Bracket fix. Only after the holes were in the box did I find that the Powerpole bracket hung past the bottom edge. I cut one side off so the edge of the connector lines up with the edge of the box, then drilled new mounting holes in what was left.
Wiring. The antenna comes in on an SMA jack and goes out to the receiver on a BNC, since the receiver uses BNC too. The Powerpole feeds the board through the toggle switch.
Labels. ANTENNA, RECEIVER, 12 VOLTS and ON/OFF, so I never have to guess which is which.

The small green bias-T board with two gold SMA connectors, sitting in a gray 3D-printed PETG case above the empty die-cast aluminum box on a cutting mat — The bias-T board in my first 3D-printed PETG case, above the aluminum box that replaced it

The empty aluminum box lying on its side, with holes drilled in the end and side walls, next to red-handled pliers — The box with its holes drilled

A red and black Powerpole pair held in a small metal bracket on the side of the aluminum box in a bench vise, the bracket hanging past the bottom edge of the box — The Powerpole bracket test-fitted before I cut it down: it hangs past the bottom edge of the box

The cut-down Powerpole bracket screwed to a block of wood with two new holes drilled in it, the cut-off piece and a Sharpie in the background — The bracket after the cut, held to a scrap of wood while I drilled new mounting holes

The inside of the box: a BNC jack on the left, the green bias-T board on the right, the toggle switch and the Powerpole pair with red and black wires between them — Inside: the BNC, the bias-T board, the toggle switch and the Powerpole input

The finished box on a cup, showing the BNC receiver jack and the red and black Powerpole pair on their bracket, with labels on the lid — The receiver end, with the BNC and the Powerpole input

The finished box with a gold SMA antenna jack on the left and the toggle switch on the front edge, with labels on the lid — The antenna end, with the SMA jack and the toggle switch

Parts

Ciglow amplifier bias-T, 25 kHz–100 MHz, wideband coaxial feed (the original listing is gone now)
Jameco die-cast aluminum case, 3.9 × 2 × 1 in, part number 03-125A (Jameco item 11957)
Anderson Powerpole pair with a small mounting bracket
SMA and BNC jacks for the antenna and receiver connections
An on/off toggle switch

Links

My antennas Fused Powerpole Block My DIY Power Station