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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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Old 03-22-2017, 11:44 PM
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Default Maxcon OCF Dipole - Choke??

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
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Old 03-23-2017, 7:34 AM
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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

Last edited by prcguy; 03-23-2017 at 7:42 AM..
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Old 03-23-2017, 9:33 AM
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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
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Old 05-22-2017, 3:29 PM
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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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