I’ve been meaning to build a good 1:1 choke or line isolator for a while now and the parts have been sitting on the shelf for over a year. Well today I finally got around to it.

I chose to build something that would handle the full UK power limit (400W) with no problem and used G3TXQ’s excellent chart to select the best combination of core and windings.


I opted for 11 turns of RG58 coax on 2 FT240-52 cores to give the characteristics that I wanted. I won’t bore you with words so here is the pictorial version…


11 Turns of RG58 on two stacked FT240-52 cores.

SO239 connectors fitted.

Choke soldered.

Box filled with casting epoxy.

All finished and labelled. Spot the mistake…

Next week I will get outside and use the choke to finish off a new HF multi-band vertical (with remote tuner) and see how well it performs.

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