The 1:1 isolation choke do-over.


New 1:1 choke with RG316 coax

 I have the Hustler 4BTV vertical antenna and have a 1:1 choke at the base of the antenna made by Balun designs. Some time ago I made a second choke that was placed at the radio end and posted the build here on the blog. I had a few comments regarding the coax that I used (RG8X) and that there could be an issue with the bending radius of the coax around the core. I was informed that over time the centre conductor could migrate toward the braid.

I also did some internet searching and found that the bending radius could pose an issue and I know the "issue" if and when it happened would do so at the most inopportune time. I decided to make another 1:1 choke from a thinner coax. I did have some RG316 coax from a mobile antenna mount I was not using and it would be ideal for the tight turning radius.

 On one end of the coax, there was a mini-UHF to PL-259 adapter which meant I only had to add one PL-259 connector. I will be using again an FT240-31 core with 10 turns. I decided on 10 turns as this will afford me decent characteristics on the HF bands. Using the RG316 coax the bending radius is no longer an issue. 


Mike Weir, VE9KK, is a regular contributor to and writes from New Brunswick, Canada. Contact him at [email protected].

4 Responses to “The 1:1 isolation choke do-over.”

  • Larry Wheeler W9QR:

    Thank you for your input on the balun design. There was a two part article in Ham Radio Magazine written by George Badger, W6TC. It was published in February and March of 1980. It’s entitled “A New Class of Coaxial Line Transformers”. I am sure that it is available on one of the archive sites. It describes the design of coreless baluns and coaxial cable transformers using short lengths of cable.I have built several and tested them on all types of antennas and found them to be superb.

  • Mike VE9KK:

    Good morning Larry, very interesting and I just gave it a fast google search but came up with nothing. I will have to later today do a more detailed search and see if I can come up with it.
    Thanks for the info Larry,

  • George VE3YV:

    Suggest that if you haven’t already done so, check out K9YC’s extensive writings based on real-world measurements and practical building techniques on baluns/RF chokes:

    Also good source of information:
    towertalk reflector at

    GL 73, George VE3YV

  • Good afternoon George and thanks for the comment. I have been to this site in the past and there is a ton of information there. I am going to have to give it a revisit again and do some more reading.
    Thanks for the link and have a great weekend.

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