9.1 UN-UN

9:1 ratio unbalanced to unbalanced wideband transformers are useful in several ways. One example is with random wire antennas.  The impedance of a random wire antenna over a wide frequency range such 1.8 - 30Mhz can vary wildly to an ATU. But we can even it out quite a bit by transforming it down to hopefully a usable average, and something that an ATU can handle easily.  It's not a magic bullet, but a tool in the armoury. The same un-un can be used (without a box) for coupling things like Mitsubishi RD16 power FET's to the outside world. The higher impedance side would feed the input to a 50R LPF.  

The test of the transformer  is to place a 450R resistance across the un-un and it should transform it back to 50R. It's pretty much there if you look at the Nano VNA screen shot. Wideband transformers can vary a bit, they are not perfect. Also there's a degree of variation in mine probably brought about by the long lead lengths that connect the windings to the connectors. NOTE - these small enclosures usually have a coat of laquer on them. Be sure to remove it if you want true chassis earth.

Here's the basic circuit diagram (diagram VK6YSF). I made my QRP un-un using an FT50-43 toroid, and will handle 10 watts. Need more power handling, just use a physically larger toroid of the same material or stack an extra toroid or two together and that's it. You twist the three wires together and then wrap the turns into the toroid. I managed to get 13 turns into mine. More turns gives better LF performance. As with any transformer, you are looking to make the input or primary winding appear to be about 4 or 5 times the reactive impedance of the source impedance when measured at the lowest operating frequency. This is essential.

For more excellent info on making baluns and unun's take a look at VK6YSF's excellent website and data.