Trifilar means that there are three wires wound simultaneously around the core. One of the most common designs is to wind nine turns of a trifilar winding around a toroid core. It’s relatively easy to build a 9:1 balun. Here’s a video that talks a little bit about autotransformers. In fact, the 9:1 unun that I built is actually an autotransformer. A 9:1 unun is a transformer that reduces the impedance at the input by a factor of 9. So, if you connect a length of wire that presents an impedance of about 450 Ω to the input, you’ll get an impedance of about 50 Ω on the output. In many cases, the impedance can be transformed with the help of a 9:1 unun (unbalanced input to unbalanced output). They are not a half-wavelength long, meaning that, if you choose the length of the radiator wisely, the impedance at the end of the wire will not be as high as the impedance of a half wavelength long wire. You can’t, for example, generally use a 40m EFHW antenna on 20m.Įnd-fed wire antennas are a different beast. Generally, they are not usable on bands for which they are not a half wavelength long. They use some kind of matching network to trasnsform the very high impedance at the end of a half-wave wire to about 50 Ω. Before I get into the nitty-gritty details, let me first make a distinction between end-fed half-wave antennas, such as the ones sold by LNR Precision and end-fed wires that use some kind of tuning to achieve a 50 Ω output impedance.Įnd-fed, half-wave antennas (EFHWs) are a half-wavelength long and are resonant antennas on the band of interest. For the past couple of weeks, I’ve been playing with end-fed wire antennas.
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