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Thread: Clip On Ferrite Cores & Coax

  1. #1

    Join Date
    May 2014
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    Bridlington, East Yorkshire
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    Default Clip On Ferrite Cores & Coax

    Hi all.

    In the not too distant future (when the jet stream decides not to send us another storm) I intend to put up a diamond X50N VHF/UHF colinear on the chimney.
    It will be sat on top of a four foot aluminium pole, the coax will enter the bottom of the pole and be terminated on the bottom of the antenna with the normal N type connector using Ultraflex 7 coax.
    It was my intention also to wind the top of the coax into a several turn choke to control any common mode currents until I saw a video where someone had used four small ferrite cores slid over the coax just under the feed point, secured in place with small cable ties.
    I wondered what peoples thoughts were on using cores over coiled coax. I am attracted to using cores for two reasons, firstly I have found some that will clip on and not require any additional ties and also they will slide up the inside of the mounting pole along with the N type and be hidden from the elements.

    If anyone has any first hand objective experience that would be great.

    Cheers, Steve.

  2. #2

    Join Date
    Nov 2019
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    Ash Fork, Arizona
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    Why are you worried about common mode currents?
    Have you experienced a lot of RFI?
    If you have, do you know it is from common mode currents?

    It sounds like you are trying to fix a problem that you don't have yet.

    I have two VHF/UHF antennas. One is a 3-Element Yagi (2 Meter), that I bought over 50 years ago, and a Diamond X30A Dual-Band (2M &70CM). The Yagi uses a 4:1 Coaxial Balun, but that is for impedance matching. I don't use any ferrites or coils of coax on either one of them. And I don't have any common mode current problems.

    Non-symmetrical antennas, such as Off-Center-Fed antennas or End-Fed antennas, create common mode issues. The more offset the feeder, the worse the issue, with end feed being the worst possible condition for common mode. At HF, this can be a big problem. But, like most VHF/UHF operators, your using a vertical antenna. You should only have common mode problems with vertical antennas if there was very poor antenna design or very poor antenna installation.

    With my Yagi, the driven element is a split dipole. It used a Beta Match (aka Hairpin Match) to increase the feed impedance to around 200 Ohms. All of that is "balanced". It then uses 4:1 Coaxial Balun to handle the conversion from Balanced to Unbalanced (coax). The antenna has a very low SWR (< 1.2:1) across the entire 2 Meter band. As long as the design is sound, there shouldn't be any issues.

    On Eham.com there are over 100 reviews of your antenna. I didn't read all of them, but for the ones I did, there was no mention of common mode problems.
    Martin, K7MEM
    http://www.k7mem.com
    Ash Fork, AZ - 60 miles from the Grand Canyon on Rt-66. Elevation 5,300 ft.

  3. #3

    Join Date
    May 2014
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    Bridlington, East Yorkshire
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    Quote Originally Posted by k7mem View Post
    Why are you worried about common mode currents?
    Have you experienced a lot of RFI?
    If you have, do you know it is from common mode currents?

    It sounds like you are trying to fix a problem that you don't have yet.

    I have two VHF/UHF antennas. One is a 3-Element Yagi (2 Meter), that I bought over 50 years ago, and a Diamond X30A Dual-Band (2M &70CM). The Yagi uses a 4:1 Coaxial Balun, but that is for impedance matching. I don't use any ferrites or coils of coax on either one of them. And I don't have any common mode current problems.

    Non-symmetrical antennas, such as Off-Center-Fed antennas or End-Fed antennas, create common mode issues. The more offset the feeder, the worse the issue, with end feed being the worst possible condition for common mode. At HF, this can be a big problem. But, like most VHF/UHF operators, your using a vertical antenna. You should only have common mode problems with vertical antennas if there was very poor antenna design or very poor antenna installation.

    With my Yagi, the driven element is a split dipole. It used a Beta Match (aka Hairpin Match) to increase the feed impedance to around 200 Ohms. All of that is "balanced". It then uses 4:1 Coaxial Balun to handle the conversion from Balanced to Unbalanced (coax). The antenna has a very low SWR (< 1.2:1) across the entire 2 Meter band. As long as the design is sound, there shouldn't be any issues.

    On Eham.com there are over 100 reviews of your antenna. I didn't read all of them, but for the ones I did, there was no mention of common mode problems.

    Hi there.

    I'm only slightly worried about common mode currents mainly because this will be a new installation and I won't know if I will suffer them or not. I won't have any way of easily finding out if I were to get them so it's just a precautionary move. I have a 3 storey Victorian terraced house and I want this to be the last occasion I have to go on the roof at the age of 63. I didn't want to put this antenna up only to find I had a problem with it so once up it won't be coming down, even if I move. I know that clipping 3 or 4 ferrites to the coax won't be detrimental so at least it's a no loose situation. Thanks for your info, that will be helpful in the future with the erection of some of my lower height antennae.

  4. #4

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    In a previous thread, i had asked a question about ugly baluns and I was given an interesting link on the subject written by G3TXQ. I still need to do my ugly balun tests with the VNA, just too much to do right now. Anyhow.....

    One thing I took from that article is the understanding that the path common-mode currents take is no different than an antenna (that, in some cases, has its tip touching ground). Long story short, this path will have some type of reactance to it, either inductive or capacitive.

    If with every quarter wavelength of common-mode path we go from an inductive to a capacitive reactance, adding ferrites or turns of coax to increase inductance should be done only when the path is already inductive. If the path is capacitive, you are (at least partially) canceling out the reactance of the CM path and making the situation worse. Maybe take a GDO to the common-mode path to see where it resonates at and figure out if the path is inductive or capacitive at the operating frequency?

    I would guess that a specific length of coax for a given grounding situation would provide a high impedance to stop common-mode currents and eliminate the need for a choke all together.

  5. #5

    Join Date
    Nov 2019
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    Ash Fork, Arizona
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    I know what it's like going on the roof. About 20 years ago, at my previous house (shingle roof), I fell off the roof as I was transitioning from the ladder to the roof. I wound up breaking my heel. It was sheered in half. Because it was a clean break, they wouldn't put a cast on it. So I spent the next six months on crutches, with a Ace Bandage on my foot. Every time it came in contact with the ground, it sent me right through the roof, so to speak. I still feel it when the weather gets bad.

    My current house has a metal roof. While the roof will last a very long time, there is no way I can get on the roof. There is no traction on a metal roof.

    However, you can test everything before putting it on the roof. If you have a 10 foot pole handy, set it up in the middle of your yard. A expandable painters pole will work fine. You just need to get it out in an open area, so it isn't affected by nearby objects. Use the same coax that you will be using on the roof. If you don't have any problems, put it on the roof. Check everything out before you go on the roof and you can save time and money.

    BTW, I am 71 and run a Horse Boarding Business with the XYL. I have to wrestle 80-100 pound bales of hay every day. I would have no problem going on the barn roof. It's still metal, but the angle is smaller. But I won't get on the house roof.

    Good luck with your antenna.
    Martin, K7MEM
    http://www.k7mem.com
    Ash Fork, AZ - 60 miles from the Grand Canyon on Rt-66. Elevation 5,300 ft.

  6. #6
    Super Moderator 2E0FVL's Avatar
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    Tangmere, Chichester, West Sussex, United Kingdom
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    I am using an X50 on the chimney which started life with a few coils of coax, but the pole bent so was replaced and the coils removed to get a foot extra height.
    Result = No difference, still no problems even with the Freeview TV aerial six foot below it!
    Pete - 2EFVL
    The QRM Formally Known As M6PAP!!
    www.selseylifeboats.co.uk
    www.amberleymuseum.co.uk

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