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Thread: To Choke Or Not To Choke

  1. #1

    Join Date
    May 2014
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    Bridlington, East Yorkshire
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    Default To Choke Or Not To Choke

    Hi all.
    This coming Spring I'm hoping to put up a Diamond 2m 70cm co-linear on my chimney stack and I intend to use the best quality coax I can afford rather than RG58 as it needs to be forgotten about once up there.
    I have a couple of questions that I would like peoples opinion of

    Firstly, what are your thoughts on forming an air wound choke in the coax up near the feed point and are they strictly necessary, some say vital ?
    Secondly, If I were to form a choke, how do you determine how many turns to make with bigger diameter coax compared to RG58 ? There are plenty of video's and websites explaining the building of them but I can't determine if the length and number of turns, diameter etc alters with the larger coax's.

    Your input is appreciated.
    Steve, G6JEF

  2. #2
    Super Moderator 5B4AJB's Avatar
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    Mar 2008
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    Meneou, Cyprus
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    Default

    I can't help you with a choke as I never (heard) used one on a VHF vertical.

    One thing I would suggest is slipping 2-3, 2 inch long bits of hosepipe over the coax before you put the plugs on. This will enable you to use galvanised wire to tie the cable to something.
    Outside coax has a tendency to flap about in the wind, introducing work hardening and ultimately an open or intermittent circuit.
    Zip ties are not a permanent solution, they will degrade in sunlight after a few months.

    Supporting the coax so the entire run's weight isn't tugging on the connector is often overlooked.
    Use self amalgamating tape around the connector, over the socket' thread and down to the cable itself. This will make it easier to unscrew in 10 years, even if it is all been hidden inside a mast.

    Be aware, if you do put a loop in your coax, the manufacturers often quote a minimum radius before it deforms the dielectric...

  3. #3

    Join Date
    Jun 2017
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    McAlester, Ok
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    Default

    A choke very seldom (if ever) cause any harm if not needed, so I see no absolute reason not to use one "just in case".
    A choke is an inductance used to strip unwanted RF from the outside of feed line. It affect nothing on the inside of the feed line (signal).


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    Last edited by Paul-w5lz; Fri 28th Feb 2020 at 20:25. Reason: error

  4. #4

    Join Date
    Nov 2019
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    Ash Fork, Arizona
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    Default Re: To Choke Or Not To Choke

    The reason for a choke is to decouple the antenna from the feed line and prevent feed line radiation. As long as the antenna is well matched to the coax, there shouldn't be any issue requiring a choke.

    However, Diamond does often recommend one turn of coaxial cable approximately 10" in diameter, directly beneath the antenna. This will act as an isolation choke. Of course, depending on what coax you use, a larger diameter coil may be needed to prevent the center conductor from moving.

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