RF Choke

The New Carolina Windom uses part of the feedline as a vertical radiator — the voltage balun used to feed the off-center fed dipole allows common mode current to travel down the feedline. How do you determine how much of the feedline radiates? From what I’ve read, you do this by placing an RF choke (sometimes called a “line isolator”) at some point in the feedline to clamp off the common mode current. For an 80 meter version, you put the RF choke 20 feet below the feedpoint. For a 40 meter version (like I’m building), you put it 10 feet below the feedpoint.

To build my RF choke I followed the instructions provided by Len Carlson, K4IWL. In an addendum published here, Mr. Carlson wrote:

A note about the choke [between the balun and the feedline]: The choke [line-isolator] is simply a straight piece of coax with ferrite cores strung on it. Just use the same coax that you are using for the field line from the Xceiver to the choke. I have made a mod to the choke also. Instead of bending it back inside of the CPVC tube, make it a straight piece of coax about 0.3 meter. The length is not critical but should be no shorter than about 12 inches. Use as many ferrite tubes that will fit in-line on that length.

I built my RF choke using 12 FB-56 ferrite beads (mix 43) from Palomar Engineers strung on a piece of RG-58 coax and secured on each end with wire-ties. While Mr. Carlson chose CPVC for a lighter enclosure, I used PVC. In order for the SO-239 bulkhead-mount coax connectors to fit in the endcaps, I had to go with 1 1/4″ pipe. This did make for a pretty large enclosure — the beaded coax in the pipe does slop around a bit in there when shaken. If I have to do this all over again I’ll figure out some way to secure the innards of this thing (maybe by injecting some expanding foam?). As it was I inserted a few inches of double-sticky foam mounting tape inside the last end to be sealed, which did help somewhat.

I attached one end of the beaded coax directly to an SO-239 connector/end cap, but the other end required several inches of slack to stuff into the tube when it came time to push the final end-cap onto the pipe.

Here is a slideshow of my RF choke. After I took these photos I covered the coax connectors with cling-wrap and spray-painted the whole thing forest green.

This slideshow requires JavaScript.

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8 Comments

Filed under New Carolina Windom, RF Chokes

8 responses to “RF Choke

  1. Very interesting to see how you can make a choke yourself instead of buying it off the shelf. Thanks! Added you to my ham related links immediately.

    Hans

  2. Todd Mitchell

    You’re welcome, Hans. Nice to meet you!

    73,

    Todd
    NØIP

    • Hi Todd,

      I am the Editor of The Canadian Amateur magazine, which is the membership journal of Radio Amateurs of Canada (http://wp.rac.ca/). I have received an article and the author would like to include the photo of the Ferrite Bead Balun that appears on your Blog at the following link:

      If you grant us permission to use the photo I will indicate that the photo was provided courtesy of Todd Mitchell, NØIP from the article “RF Choke” which can be found at https://elmering.wordpress.com/2011/10/19/rf-choke/

      I look forward to hearing from you.

      Tx Alan

      Alan Griffin
      Editor
      The Canadian Amateur

  3. Pingback: Lessons Learned During Antenna Construction | Elmering

  4. Duane Kaya

    I have an interesting situation. My shack is located a distance away from the antenna, that using 300 ohm twin lead makes good sense. I am wanting to remove the 4:1 balun at the Carolina WIndom (CW) and replace it with a 300 ohm twin lead, and about 10ft down (40mtrs) install a 1:1 balanced feedline (current) isolation choke. Feeding the isolation choke will be 300 ohm leading to my shack with either a 1:1 or a 4:1 (tbd) current balun to my antenna tuner. Anyone ever having the need to do this… please chime-in 🙂 73’s Duane AH6BS

  5. Donn Hilton

    Why not run the 300 ohm twin lead down to just outside the shack. Fasten it to the balanced line terminals of a 4:1 balun (75 ohms out) or 6:1 balun (50 ohm out) and run a length of coax from there to your radio. Not sure why you would bother with a balanced feedline choke. Waste of time and money.

    • dk4ca

      A consideration to think about: The design length of the radiating feedline should be in odd multiple of a 1/4 wave referenced to 15 meters. This does, in part, two things: (1) creates a predictable antenna radiation pattern and places the antenna impedance transition close enough to minimize reflections and (2) running balanced twin lead, if done properly, requires detailed installation that should be followed as to transfer power to the antenna efficiently. In my opinion, having an antenna tuner and paying attention to impedance transition(s) will probably work but what is unknown, without extensive experiments/measurements, is the final efficiency of the antenna. But one thing for sure, is installing twin lead/ladder in your budget and do-able?

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