QRO 1:1 Choke

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.

13 Replies to “QRO 1:1 Choke”

  1. Hi Just came across your site and details of your 1:1 line isolator. I’d be interested to know how it performed since commercially produced isolators can be around £60.00.

  2. Hi Sid.

    The 1:1 choke works great. It’s the same design as many commercial units. Hope that helps.


  3. Hi your 1-1 balun works great ,I wonder is it possible that you do a 4-1 balun ,if so could you tel” me details if it’s a kit ,great could you please let me know ,price plus any P&Pcharges ,thanks Dave

  4. Hi Dave. I don’t make kits but there are plenty of 4:1 BalUn designs on the internet for you to try. Glad you like my attempt! Matt.

  5. Hi would the line isolator work just as well on a ft240- 43. Also should I use 2 to wind the rg58 on. Thks

  6. Hi Kevin. Take a look at the chart in the post. 8 turns of RG58 on an FT240-43 is OK for the higher bands, 12 turns is better for the middle bands and 17 turns would be great for 80 Meters. It all depends on what band’s you like to work.

  7. Thks the bands I work with my wire is 80 to 17m. I use a vertical for 15m to 10m. So I’ll make one for each. Thks

  8. Hello
    Would this work from 160 to 10 meters.

  9. Hi Michael. I did use 2 toroids. Take a look at the images and you’ll see 2 stacked. Matt

  10. Hi, brilliant article. I’m just wondering how the characteristics of the choke would change if RG8X/Mini8 coax were used instead? Is this something that you have tried? 73

  11. Hi Jack. I think the characteristics would change slightly but it’s not something I have tried. Experiment and let us know how you get on!

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