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Coax length for wire dipole

Discussion in 'CB Antennas' started by codeman, Jan 14, 2016.

  1. codeman

    codeman Recovering Crackerhead

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    Is there any certain length for coax running to a wire dipole? I am going to mount the dipole in an inverted V from a 20' painters pole. I have tried the dipole about 8-10' feet off the ground and would use a couple of 18 footers with a barrel connection and actually had decent results. But I want to see what it will do higher off the ground. What coax type would be best?


     

  2. midnight special

    midnight special Well-Known Member

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    of the 3 dipoles i homebrewed i used just regular tv flat wire in to a tuner with wire connections,,,but that is just me experimenting,, it worked fine made a lot of contacts,,, but there are so many different ways to do dipoles,,, seen rg 58, rg8 rg 8x used then there are 300 400 ohm flat wires used with tuners or baluns,,, i think you just have to chose and experiment what works in your application
     
  3. The DB

    The DB Sr. Member

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    Is it me or has essentially the same question come up a lot lately?

    The one and only correct length is the length needed to get it from the antenna to the radio, with a little extra for wiggle room. If changing the length changes the apparent tune of the antenna then you have an antenna problem that needs to be dealt with.


    The DB
     
  4. 222DBFL

    222DBFL Sr. Member

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    Best to get a 1:1 matching balun and add wire cut to resonant freq. Then use what reaches from the feedpoint to your radio as far as coax goes!! Leave some slack for service if needed. JMHO.
     
    wavrider and midnight special like this.
  5. Needle Bender

    Needle Bender ...he thinks it's funny that I stepped in it

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    I shun superstition, especially the CB myth type and would I have NOT seen a definite performance difference (on an antenna which was showing an SWR of 1.0:1, reactance of X=0/1 & Resistance of 51Ω) when cutting down from a length of RG-213 coax cut to an odd multiple of a 1/4 wave (x VF) to an odd or even multiple of a full wave, (x VF) - I'd be inclined to agree with your theoretically correct position.
    I now always make certain I plan my coax so it is a multiple of a full wave x VF, even though many believe it to be no more than superstition or myth, only because I have attained performance improvements from doing so, even on "correctly tuned" antennas. Someday I hope to fully understand why. :)
     
  6. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Needle Bender I have researched that odd multiples of coax length for installs, always heard or read it should be 1/2 wl of course velocity factor figured in.

    Same with using ladderline, there are "certain" lengths to avoid when using an all band doublet type antenna.

    I have not "seen" any difference with any length of coax once I have the antenna tuned using the analyzer, I just stick on any old length of coax and start making contacts.

    I have an 75/40 meter fan dipole up at 45', I talk to BJ and Fire runner in the mornings on 75 meters, BJ being in Indiana and Fire Runner being in California.

    I am using 75ohm RG6 quad as coax and my impedance is 50 ohms X=0 and that old wire sure hums when I turn that 3-1000Z loose. I did not measure it, just put the connectors on the end and started using it.

    For tuning an antenna YES a 1/2 wl of coax is great. After it is tuned then whatever length is needed has worked for me.

    I really want to see a difference in connecting a certain length of coax to an antenna and it improve the rx and tx gain just by using a tape measure to measure and cut transmission line.

    Not saying you have not seen this happen, I just wished I could see it happen also as my weak ass signal can use all the help it can get(y)
     
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  7. Road Squawker

    Road Squawker Sr. Member

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    think about the voltage to current ratio as it travels along the coax (a sine wave is formed),

    they are 90 degrees out of phase along the sine wave. when one is max, the other is min.

    so,.... with an e wl (or 1/2 e wl multiples) of coax being used, the V and I are the same at the load as at the xmitter, where V is max and I is min (... and the impedance is.........?;))
     
  8. Road Squawker

    Road Squawker Sr. Member

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    go here https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjVx6zz3KrKAhUK5mMKHf8ODZYQFggeMAA&url=https://en.wikipedia.org/wiki/Lecher_lines&usg=AFQjCNFCmhK51T6H-l1s_abx4pmG0Ae6TQ

    this is useful for tuning stubs vs lumped LC circuits primarily at the higher freqs.
     
    Needle Bender likes this.
  9. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    I understand the principal of the mathematics in above quote, I have also read Walt Maxwells "Reflections". All I am saying is I have yet to see ANY length of transmission line increase RX or TX,
     
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  10. 222DBFL

    222DBFL Sr. Member

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    Let me say in my own experience I have found that if you need to have a certain length of coax to make your readings at the radio good, then you have a mismatch or other issues. You should be able to run from point A to B and be done. If the load at the end of the coax is 50ohms then the readings at the radio should be the same as that at the load connected directly to your antenna analyzer. If it's not, then either you have a coax issue or your antenna analyzer is out of calibration.
    It helps to place a 50ohm dummy load directly attached to your antenna analyzer and check what it reads. This will give you a good base line as to reference to. JMHO.
     
  11. M0GVZ

    M0GVZ Sr. Member

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    Your coax was forming part of the antenna and even radiating a signal if you did not put a RF choke at the antenna feedpoint so by choosing specific lengths you're effectively tuning the antenna system. This is even true in a balanced antenna like a dipole because the current that is flowing on the inside of the braid will split between the dipole element and the outside of the braid when it gets to the dipole leg connected to it, the amount going down the outside of the braid being determined by the impedance it sees in the dipole leg and the braid - Kirchoffs Current Law. That is why you use a RF choke so it sees the outside of the braid as high impedance and forces ideally all, but in practice most, of the RF current into the dipole leg instead. This is even more of a case in something like a vertical antenna where if there is no groundplane or a poor one it'll see the outer braid of the coax as the path of least resistance by a massive amount.

    Flow of RF current in a Dipole with no choke:
    [​IMG]

    Dipole with a decent resistive choke:
    [​IMG]

    Finally below, Dipole with a reactive (air wound) choke if you get it wrong and the inductive capacitance cancels out the reactive capacitance of the coax. Note how little flows in the dipole leg 2 which is connected to the braid. That is because the outer braid of the coax now forms a path of almost no resistance so the current flows there instead
    [​IMG]
    (images courtesy of Steve G3TXQ)
     
    #11 M0GVZ, Jan 15, 2016
    Last edited: Jan 15, 2016
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  12. codeman

    codeman Recovering Crackerhead

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    Thanks for all the answers, honestly a lot of the info is over my head. I have a wire dipole i bought that came with a balun. So I will buy 50-70 feet of RG-8X and see how it performs.
     
  13. M0GVZ

    M0GVZ Sr. Member

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    It'll work great for DX - horizontal dipoles don't do so well for local due to the whole issue of vertical and horizontal polarised antennas and the losses when one tries to talk to the other. Try to keep it at least 16ft high.
     
    codeman likes this.
  14. Needle Bender

    Needle Bender ...he thinks it's funny that I stepped in it

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    I nominate this post by Road Squawker as one of this years best. (y) Well put.

    Basically we want our antennas to get the RF the same as it would if it were attached to the coax connector on the back of the radio.

    Although, I have wondered if any difference in either efficiency or TOA would be measured if, depending on the phase of the feed point/matching network, the phase of the RF in the coax were matched to it..? :unsure:

    Regarding feeding a dipole, I prefer to use a length of odd 1/2 wave multiples using the VF of the CASING, for there to be a 1/2 wave in there somewhere to raise the impedance to help make it less interesting to potential RFI.
     
    Road Squawker likes this.
  15. w9cll

    w9cll W9WDX Amateur Radio Club Member

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    Math and formulas are wonderful but just get some wire, a balun and coax and have at it as 222 said above.
     

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