this post was submitted on 13 Mar 2024
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Amateur Radio
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I did consider a OCFD but the multiband feature didn't seem worthwhile over a monoband dipole or multiband fan dipole.
Thinking about it again, perhaps it is worthy. If the fundamental is 40m, then I should get dipole performance with an interesting radiation pattern on the second harmonic (20m). If I deploy as a sloper, then perhaps I can get an OK 20m DX setup and decent 40m NVIS out of it. Good timing of the activation right could mean I can capture VK stations without pulling out my 1/4w vertical.
I'll give it a quick model and perhaps find myself winding a 4:1 balun. :)
If you only need these two bands, then maybe you have an additional option. Consider 3:1 OCFD with 40m as fundamental: at 40m it would have something like 100-120ish ohms and closer to 50 on 20m. If you feed this with 75 ohm cable 1/4wl long on 40m, you'll get 50 ohms, and on 20m this section of cable is 1/2wl so impedance is matched there too. No balun needed, common mode choke at feedpoint could be useful tho. Maybe it would even work at 15m
I haven't built a feedline match before. That may be worthwhile just for the experience in building such.
Modelling this OCFD would be in order because these impedances will vary depending on exact placement of feedpoint and height above ground. When you have some impedances then you can match it by hand on smith chart or if you're lazy like me you can use interactive one like this https://www.will-kelsey.com/smith_chart/