9:1 UnUn QRO

Today’s dalliance is UnUns. An UnUn which stands for Unbalanced to Unbalanced is a transformer of sorts which in this case matches or attempts to match the impedance between an antenna and a transceiver. Today I am making a 9:1 ratio UnUn which is suppose to match a 450 ohm antenna to a 50 ohm transceiver. When the impedance is matched between a transceiver and an antenna, maximum power from the transceiver can be transmitted to the antenna. A 9:1 UnUn is used with random wires and a counterpoise. I call this UnUn QRO because it is rated at 250 watts. I would use this in the field with a radio like my IC-7300 even though I rarely go above 50 watts. I went QRO because this is my first turn at winding a toroid and I thought starting with something larger would be easier.

I will only list project specific parts. The toroid kit come from Palomar Engineers https://tinyurl.com/3tkctj2e. This gives you the toroid, wires and instructions. The other project specific part is the case which came from amazon https://tinyurl.com/2p93h9r5 Everything else is generic parts that I either had on hand, purchased locally or from amazon.

The Palomar Kit looks like this.

The kit comes with decent instructions but, with a couple of caveats. One, the wire that comes with the kit appears to be random. The colors of the wires, do not match the colors in the instructions. this is not insurmountable, just makes things a little more difficult especially for first timers like me. The second nit is the pictures provided, while of decent quality, are black and white. In today’s world, color photos should not be that big of deal. The kit can be wired as a BalUn or UnUn. The way I overcame the mentioned shortcomings was to annotate the schematic they provided.

The UnUn is the schematic on the right. According to the instructions, the 3 wires are wound 10 times or turns with each pass of the wires through the center of the toroid counting as one turn. The wound toroid looks like this.

Before I went any further, I tested the toroid. I did this by hooking up a 450 ohm 50 watt resister to the antenna and counterpoise leads and attached my Xiegu X6100 to the input of the UnUn. I then used the SWR Sweep function on the X6100. I knew this wouldn’t be an exact science since I was using a 12″ RG-316 cable and 12″ jumpers, but should be close enough to see if I am in the ball park. Here are a couple of the test.

Shown are 160 meters, 30 meters, and 20 meters with all showing an SWR of ~3:1 or less. Not too shabby. With a random antenna wire of around 49 feet and a 25 foot counterpoise, I expect the SWR to even be less. That means anything from 160- 20 are well within the capability of most tuners. I suspect that I may be able to get down to 10 meters once I hook it up to a real antenna.

The case was a guess on my part, but as it turns out, it was the right size.

Then all put together, it looks like this. The toroid is mounted on a sheet of plastic and glued into the case.

The input connector is BNC and the screws are 10-32. I have some 10-32 wingnuts on the way to make it easier to attach wires. The eyebolt was to help take some of the strain off of the antenna wire and BNC connector is on the bottom to help protect it from the weather. One final test.

This was a fun little project and gave me some experience and confidence making UnUns. My next project will be a QRP UnUn. I mostly operate QRP when out in the field (max 10 watts. I know). However, some of the stuff I bring is QRO rated. I am trying to reduce the load. Making antenna parts that are QRP rated instead of QRO rated, reduces weight and space. Right now I am doing POTA activations, but hopefully this fall when the ticks and chiggers die off, I will try my hand at SOTA. 72 de Scott

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