Portable 4:1 UNUN

I recently came across a product on Etsy that caught my eye. It’s a 3D printed project box with an integrated antenna wire winder. I couldn’t resist, so I ordered one.

The Etsy vendor is 3dPrintedSolution, and they list the product as “EFHW End Fed Half Wave Antenna Radio Box.” While its name suggests they designed it with EFHW antennas in mind, I had a slightly different plan for it. 

The box will accommodate a T-130 toroid, and the snap-on lid comes with a gasket. There is a pre-drilled hole for a BNC-F panel mount connector, and there are marks to guide drilling for the output and ground connectors of your choice. You can choose from two colors: orange or green. The vendor states that it is “UV and weather resistant.”

I like the idea of having a box to enclose the balun or transformer. My preference for portable antennas is to avoid exposed components or circuit boards. 

I used mine to build a 4:1 unun for portable use in a Rybakov configuration. The unun consists of 19 bifilar turns of #24 solid hookup wire on a T130-2 toroid. You can find plans for winding the unun here and other places on the Internet. For the output and ground connections, I used #10-24×3/4″ stainless steel machine screws, along with some nuts, flat washers, and lock washers. 

Inside view of the 4:1 unun. I used double-sided foam mounting tape to secure the toroid. I also use a small piece of packing foam (not shown) between the lid and the toroid for added stability.
Inside view of the 4:1 unun. I used double-sided foam mounting tape to secure the toroid. I also use a small piece of packing foam (not shown) between the lid and the toroid for added stability.

To go along with the completed unun, I prepared two 26-foot wires; one for the radiator and one for a counterpoise. I finished up by attaching a length of 2.5mm bungee cord. This cord keeps everything together for travel. I have also used it to secure the box to a vertical support, e.g., fiberglass mast, fence post, etc. 

The 4:1 unun deployed in a Rybakov vertical configuration with a 26-foot radiator and a 26-foot counterpoise
The 4:1 unun deployed in a Rybakov vertical configuration with a 26-foot radiator and a 26-foot counterpoise

In the field, this Rybakov antenna worked as well as others I have built over the years. I tested it using my Penntek TR-35 and Elecraft T1 tuner with 18 feet of coax. It tuned up easily on 40, 30, 20, and 17 meters, and I made a couple of QSOs while testing. The integrated winder made it easy to deploy and take down. 

The 4:1 unun packed up for travel
The 4:1 unun packed up for travel

I have a feeling another one of these boxes is in my future. Maybe a 9:1 unun next time?

73, Craig WB3GCK

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