Maxcon OCF Dipole - Choke??

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ckmcdonald

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Dec 21, 2016
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My Maxcon 3K80 OCF Dipole came a few days ago. I pre-gathered all the hardware I needed to put it up. But wait, a choke is needed?! That wasn't mentioned on the website at all. 25' of coax wound in a 6" loop at the base of the balun. What?!

This is my first OCF dipole. Do other types/brands require such a choke? This will be a very unsightly wart in an installation that is already pushing the edge of that I can get away with in my location.

How critical is it (can I omit it)? I sent an email to Maxcon but no response.

Thanks
 

prcguy

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An OFC dipole is inherently unbalanced and should have an effective choke in the feed line near the antenna. Otherwise the coax becomes a radiating part of this antenna. Coiling coax is not an effective choke and at best might reflect some common mode RF currents back toward the antenna on one band.

A good ferrite choke will absorb the RF currents and turn it into heat. You can make an effective choke by wrapping coax around and through a large ferrite ring and 10 turns of RG-58 size coax around a #31 or #43 mix FT-240 size core is good across the 80 through 10m bands.

Some of the most effective 1:1 balun/chokes on the market are made by MyAntennas with their CMC-130-3K and CMC-230-5K having the most resistive choking impedance of anything I've seen over the entire 160 through 10m range. Their CMC-330-1K is more affordable and still has a lot of choking impedance across 80 through 10m.

The chokes I mentioned have multiple cores with different ferrite mixes to cover different parts of the band. Most of the other long tubular chokes are simply a string of large ferrite beads over some coax and are only marginally effective in comparison.

BTW, I've used a few OFC dipoles and the type of balun Myantennas uses for theirs appears to be the favored design. I now use one of their 80m OFCs as my main HF antenna.

Edit: I looked on the Maxcon site and they offer a 1:1 balun/common mode choke for their antennas. Their rating is 600 ohms inductive impedance at 3.5MHz, which I assume is where they measured its best specs. Compare that to the 7,000 to 9,000 ohms of resistive choking impedance over 3.5 to 30MHz for the lowest performing MyAntenns choke I listed.
prcguy
 
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jwt873

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A few years back, I put up a Buxcomm OFC. It didn't come with a choke.

The unbalanced condition created quite a bit of RF in the shack. It was so bad that I was getting an RF tingle when I touched the base of my CW key.

I installed a ferrite choke outside the house about 20 feet from the antenna feed point. I bought a commercial one which is just a tube with connectors on each end and a bunch of ferrite donuts stacked up inside.. It did the trick. I used the MFJ offering, MFJ Enterprises Inc. .

But note, as prcguy points out, you can build your own choke with ferrite toroids of the correct size and type. I bought a commercial one because I was lazy :)
 

ke0ncp

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ckmcdonald - How is the maxcon working out for you and how do you currently have it set up?


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