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.

Click to view slideshow.

Todd Mitchell, NØIP, is a regular contributor to AmateurRadio.com and writes from Minnesota, USA. He can be contacted at [email protected].

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