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masts, coax,CMC or anything

Discussion in 'Amateur Radio Antennas' started by davev8, Feb 3, 2018.

  1. davev8

    davev8 Gold Star/Marvin Award Member

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    i just had a thought...if i have a vertical end fed antenna (say 10-12 meters) will it affect anything in a good or bad way if the coax was feed down the inside of a long 2 inch mast as opposed to being taped to the outside of it ?..i was wondering about CMC


     

  2. fourstringburn

    fourstringburn W9WDX Amateur Radio Club Member K5KNM

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    Apparently so.

    On the A99 or Imax antennas which are end fed antennas , the first 8.5ft of coax passed the feedpoint is part of the counterpoise meaning this section of coax will radiate as part of the antenna system.

    This is the point after which a choke balun should be installed to keep the rest of the coax from radiating. That will also go a long way to tame any CMC's.

    Having said that, it's best not to have that part of the coax touch any metal.
     
    #2 fourstringburn, Feb 3, 2018
    Last edited: Feb 3, 2018
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  3. davev8

    davev8 Gold Star/Marvin Award Member

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    i had not herd of the choke a 1/4 wave below a A99 or Imax but it makes sense....the antenne i am most likely to use is a home brew ally 5/8 with ground planes or a sirio vector 4000
     
  4. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    I have not heard of the coax being part of the counter poise for an imax or a99
    there should be a rf choke placed at feed point of the imax2k or a99 and the mast pipe becomes the counterpoise NOT the coax.

    As for putting the coax inside the mast pipe.
    It can only help eliminate CMC look at the starduster install instructions, the coax is put inside the mast pipe.
     
  5. fourstringburn

    fourstringburn W9WDX Amateur Radio Club Member K5KNM

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    I never knew that either but just recently learned on this forum. It does make sense when you think about it and explains why these antennas are referred to as "bleed sticks" and I have heard many users complain about RF in the shack.

    Here is a quote from W8JI:

    ALL half wave end fed antennas need a counterpoise. Anyone saying they have one that does not, is kidding himself.

    For every single milliampere of common mode current forced into the end of any wire of any length, there has to be an exactly equal current flowing from some other conductor as common mode current. There is no possible way around this.

    If I have an end fed half wave thin wire, the common mode impedance of that wire might be a few thousand ohms. SQRT of P/R. 100 watts would require SQRT 100/5000 = .14 amperes flowing back.

    That .14 amps can only be transformed up in value along coax shields or some other form of counterpoise, unless or until it is spread out and dissipated.

    This also causes a voltage at the counterpoise.

    This is why, if you have a half wave end fed, it is almost impossible to get rid of common mode currents on the feedline or feed system without radials, or a near 1/4 wave long counterpoise and some decoupling.

    Anyone telling you it can be done some other way is blowing smoke.


    So on a end fed antennas such as the A99 without radials, the coax will be forced to make up for the lack of counterpoise and can cause RF problems all the way down to the shack. A choke balun or similar homebrew fix should be placed 1/4 wavelength down from the feed point so once passed the balun, that part of the coax is no longer part of the counterpoise.

    From what I have read and if I understand it correctly, even if multiple radials are used such as the ground plane kit, then you still need to use a choke balun but this time it's placed as close to the feed point as possible which will tame the CMC's from traveling down the coax.

    http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm
     
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  6. 543_Dallas

    543_Dallas Sr. Member

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    When using a choke at the feed point with these antennas I was always concerned with the possibility of rf coupling to the coax shield below the choke if the coax was ran parallel to the mast.

    I believe the ground plane kit would help but for that kind of money you could have bought a 5/8 aluminum ground plane.

    With my Imax I used a choke at 9 feet, ran the coax all the way down to ground level and then along the ground to the shack. I thought that if there was common mode I could minimize rfi by running the coax along the ground vs through the attic. Later that day I put up a good antenna and never looked back.
     
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  7. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    https://www.w8ji.com/end-fed_vertical.htm#IMAX

    Lets not confuse end fed antennas with other antennas like the I2K.
    Notice JI states isolation or good ground plane on end feds "Without grounds".

    https://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm
    Info on feeding that end fed antenna

    The A99 and I2K are DC grounded antennas.
    End fed antennas do not have a DC ground per say .
    Coax is split at fed point with center conductor going to one element and coax shield going to ground radials or counterpoise.

    Lots of info on WWW for I2K and A99.
     
  8. fourstringburn

    fourstringburn W9WDX Amateur Radio Club Member K5KNM

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    That's correct. I forgot those are DC mounted antennas bolted directly to typical metal mast.

    So with a choke at the feed point for CMC's, then I would think running the coax inside the mast pipe should be fine.

    The OP never said what type of antenna he was running, (wire, A99, etc.).
     
  9. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Fourstringburn

    I use my imax multiband 10 through 15 meters I put and rf choke at feed point and roughly 10 feet of mast on top of the MosleyPRO67B yagi.

    Seems to work ok and no issues with cmc.

    Then again I do not use a super whack pack., double swing kit, top gun modulated radio either.

    The homebrew 1/4 wl I use 1" sch 40 aluminum emt conduit for a mast 10', and my coax is routed through the mast pipe.

    I have no rf choke at the bottom of the mast pipe but I am sure if there was one it would not hurt anything.

    I have no issues with cmc. I use the computer speakers with cheap amplifier built in to test for any TVI RFI issues.
     
  10. fourstringburn

    fourstringburn W9WDX Amateur Radio Club Member K5KNM

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    :ROFLMAO::ROFLMAO::ROFLMAO:
     
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  11. davev8

    davev8 Gold Star/Marvin Award Member

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    it was in a few posts down ........the antenne i am most likely to use is a home brew ally 5/8 with ground planes or a sirio vector 4000
     
  12. wavrider

    wavrider W9WDX Amateur Radio Club Member

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  13. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    http://www.arrl.org/files/file/Technology/tis/info/pdf/8004019.pdf

    Wavrider, check this old article out. An old buddy of mine read this article and we used the idea to modify our Stardusters top support mast, the part inside the antenna radials...like you are suggesting.

    We used a jumper that was a resonant length of coax with connectors on both ends. We cut the mast pipe that goes into the SD'r hub a little short...so the bottom connector was exposed. We used a hose clamp to secure the connector to the mast and added our coax to this jumper. This grounded the 1/4 wavelength Bazooka balun to the top of the mast that went to ground.

    Back in the 80's, when this happened, we could not tell a difference on our radios, but my buddy Nimrod's new setup stopped his TVI problem in his area. Back then I cared less about such things, but we were starting to hear and think about interference issues during the CB "craze" of that time.

    Both Avanti and Antenna Specialists suggest this balun idea in their mounting instructions for the Sigma 4. as note below in the attached PDF file on the "Bazooka Balun."
     

    Attached Files:

    #13 Marconi, Feb 11, 2018
    Last edited: Feb 11, 2018
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  14. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Marconi what I have tried thus far is running coax through the mast pipe with an rf choke AKA ugly balun at the bottom of the mast. I have no TVI, RFI, CMC issues SWR look as to be expected using the mfj259b analyzer.

    Another install I did not put the rf choke at bottom of mast pipe, same results no RFI , TVI, CMC issues, swr same as one with choke.

    I seem to think, and as is suggested in several articles online,
    running the coax through the mast pipe presents an easy method to isolate/shield the coax within the 1st 1/4wl of the antenna effectively decoupling the coax from the antenna where the bottom radials of the starduster type antenna is radiating.

    I have built a few homebrew 1/4wl gp and use this same method of install and never any complaints of TVI/RFI or CMC. One install op is running 4kw+ through it and no bleed over, no TVI/RFI issues, Although it is a clean 4kw not a splatter box and nothing over driven.

    I like my Imax for the multiband coverage but I will never homebrew any more 5/8 wl gp, the 1/4wl does just as well if not better performance IMO.
     
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  15. bob85

    bob85 Supporting Member

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    Eddie,
    That type of balun works as a 1/4wave transmission-line shorted at the far end just like a 1/4wave piece of very stiff & fat coax,

    the relative sizes of the coax braid & inside suface of the mast & any dielectric between them determine its impedance & velocity factor and what length it needs to be cut to work correctly.
     

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