First let me say that I am not an expert on Baluns or Unums (a good article defining the two terms here) but this is more about my exploration of the topic and what we need for Field Day coming up soon. For those that are not aware of what field day is, it is a friendly radio sport competition that the ARRL.org puts on each year to get us ready for disasters. About two years ago we were trying to have multiple antennas at our field day site and needed to separate the antennas to cut down on interference. The longer the transmission lines were, the less interference but the more signal we would loose. The idea to have a 450 ohm ladder line transmission feed, that has a minimum of loss, was attempted and worked well. This design however needs to convert 50 ohm load that the radio expects to match 450 ohms ladder line and then at the antenna, match it again to the impedance of the antenna. Using a 9:1 Unum is a transformer that can accomplish this task at one end of the line. It is placed at the connection of the coax coming from the radio (50 ohm) and connected to the end of the ladder line (450 ohm) as 9 x 50 = 450.
Lets build a 9:1 Unum… Well you need to start with a toroid of #61 material and get a plastic box to house it. You need a PL239 (female UHF connector, case mount) and stainless screws and bolts (6/32) with wing nuts. The wire you will need is not stranded copper wire rather than solid wire typically 14 gauge (THHN). I decided to make this capable to withstand 1000 watts so I used heaver wire and purchased it in 3 colors to help keep the windings straight. Use tape to keep the three colors in order and like a flat ribbon cable. You take 3 differently colored wire of enough length to make the turns through the toroid and then have some to make the connections. The counting of the turns can mess you up so a “turn” on a toroid is counted when the wire goes through the hole. In all the instructions you can see on YouTube, they talk about 9 turns of wire through the toroid as if that is the perfect number. In truth, the gauge of the wire, the size of the toroid can all change the characteristics.
The first time I tried to make this I wound up having 10 turns before I realized I was not counting it correctly. I rebuild this unum 5 times testing the output with a network analyzer until I got it close. I finally settled on 7 turns to be the closest to 1:1 SWR across the bands I was interested in (160 – 10 meters). When testing your unum, make sure that you have a 450 ohm resistor for the 9:1 test and a 200 ohm resistor to check the 4:1 side. I got the wrong resistor value and re-wound the toroid a few times trying to fix the incorrect results! (DUUHHH!!!)
Finally finding the right resistor helps a lot! You also need to calibrate the analyzer with the plugs seen on the photo first before testing your unum. The results improved a lot after I installed the toroid in the box with the shortest leads to the antenna side. I used eyelets that were crimped and soldered for the best connection. Two small “weep” holes (1/16″) were drilled in the bottom to allow any condensation to escape the box.
There is a lot more to this but this was the beginning of my exploration of Unum and Baluns. Next I will build a 1:1 Balun to be an RF choke and try to prevent transmitted radio frequency energy from coming back from the antenna, entering the shack and doing unintentional interference. Much thanks to my RF Mentor David HK1A who showed me how to do this. More later on this blog, 73 Peter N4PVH