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That whole coax length thing...

Discussion in 'General CB Services Discussion' started by The Jerk, May 17, 2010.

  1. The Jerk

    The Jerk
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    I reinstalled my Grant and TS350HDV today...took it out thinking we were selling the car. The antenna is a Wilson 5000, hole drilled in the roof. Used all the same components I took out to reinstall.

    The ONLY difference this time is that I changed the coax between the amp and antenna (on the roof mount) from the 18-foot Wilson coax (which was new and in perfect condition) with 12-foot of RG-8X Belden that I had in another installation (it was a fairly new jumper, in good condition).



    Before I was getting a 1.3:1 barefoot and 1.8:1 under power, now I'm getting 1.3:1 barefoot and under power!

    The footage myth lives! (well not really, I figure its simply fooling the meter)
     
    rabbiporkchop likes this.

  2. Captain Kilowatt

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    Yes the myth lives on and will contimue to live on as long as people refuse to actually learn what happens between the radio's antenna connector and the antenna connector. The SWR AS SEEN BY THE RADIO will change HOWEVER THE SWR AT THE ANTENNA WILL NOT CHANGE. The coax acts as an impedance transformer as as the length of the coax changes, the impedance transformation ratio also changes. Two VERY important words to remember and learn are " conjugate match" Impedance matching - Wikipedia, the free encyclopedia
     
    mechanic, Grogan and trees like this.
  3. JoeDirt

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    Some people say that coax lenght does not matter, other people say it does!

    The coax does not change your SWR, your antenna does!

    However the SWR AT your antenna stays the same, if you change your coax it can fool your meter into having a different SWR.

    You can figure out the correct length of coax with the manufactures velocity factor rating. Belden RG8-X is .84

    You divide 468/ by you frequency your transmitting on. So 468/27.205 is 17.203, that's you electrical half wave. Now you want to multiply your electrical half wave by your velocity factor, 17.203 X .84 = 14.450.

    14.450 is the correct resonant length for the coax your running.

    12 feet should be close enough! You might try a barrel connector with a 3 foot jumper and re-tune your antenna!
     
  4. BOOTY MONSTER

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    "The coax acts as an impedance transformer as as the length of the coax changes, the impedance transformation ratio also changes."

    so is there a certian coax length that gives the optimum ratio ?
    if so is that coax length the electrical half wavelength , 492 /frequency x VF. ?
     
  5. mackmobile43

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    So how did his meter go from seeing the impedance mis-match of his amplifier to not seeing it after he changed brands of coax?
     
    #5 mackmobile43, May 17, 2010
    Last edited: May 18, 2010
  6. JoeDirt

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    I dont get your question? To see the mismatch of his amp the meter would have to before the amp. Im assuming the meter is after the amp so the difference he is seeing is just the antenna/coax mismatch not the amp!
     
  7. mackmobile43

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    So you're saying the meter sees nothing upstream of it?

     
  8. C W Morse

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    I'm one of those who believes coax length is a bunch of ^&%#. I NEVER even consider "coax length" in an installation, and I've been doing installs since 1965! Never even HEARD of such a thing.......until I heard it from a CBer. And I :LOL:rolled in the floor when I DID hear it. Sure, there are some installs, like where there are ground issues, or where the antenna itself requires a certain length of coax as part of its design. But for the usual CB installation, FORGET about the coax length bull****, FORGET about velocity factor bull****, DONT WORRY about VF when setting up your station! IT IS ALL C R A P when it comes to the USUAL CB mobile install that STARTED on CB, it LIVES in its ignorance on CB, it continues to THRIVE on CB, and will continue (apparently) to live ON CB ad infinitum. Not to make any comparisons, but I almost NEVER hear about "coax length" in any other radio media, or outlet---not even the hams spend so much time obcessing about the length of their coax!:LOL: And at the risk of length and making folks mad, I'm gonna to burst the coax length guru's bubble right now!:tongue:

    As a ham, CBer and as user of certain other commercial and military frequencies (USAF and FEMA, etc) I run on my mobile what is known as a "screwdriver" antenna. It uses a 12V motor to move a 3" diameter coil UP and DOWN within a hollow mast (making it longer and shorter as needed) to operate HF frequencies between 3.5 and 30 MHZ. The antenna is resonant on EACH frequency of interest as it actually uses ONLY the number of turns that are OUT-side the top of the mast. By pressing a button on the dash, I can move this coil UP or DOWN to "tune to any of these frequencies.

    NOW! Radio antenna theory is the VERY SAME for ANY band or frequency. CB radio is NOT "special" or requires some unique arrangement of coax length as if all other radio is exempt from the SAME rules. SO! IF what the CB coax length gurus say is true, and it MUST have 18 feet, or 9 feet, or some other ridiculous nonsense to work (to get them 'SWR'zzzzzzz' right), then to work 3.923 MHZ, I must have........OH, about 60 FEET to work that 3 MHZ on that multi-band screwdriver! Then when I go to 7 MHZ, then I gots to have about 33 feet. Then at 14 MHZ, another at 16 feet, then another at about 14 feet. Then there's 21 MHZ at 11 feet, and then there's the 9 feet for 27-28 MHZ. IOW, according to the CB theory of coax length, I would have to have AT LEAST 10 DIFFERENT COAXES IN MY LITTLE PICKUP so as to work all those various bands, and the screwdriver antenna could NOT work!!!! WHERE would I PUT all those coaxes in a small pickup truck?

    Yet, I work ALL those bands from 3 to 30 MHZ (including CB, but actually I have a separate CB antenna for that to remain legal and use a legal CB set) with ONE antenna, and ONE coax of about 7 feet. The amp (when used--it's out of the truck right now: don't need it) has a jumper of about 10" long! The SWR is about 1.2 to 1 on ALL bands on that ONE coax and ONE "length" which is equal to that needed to REACH from the bumper to the radio!

    Coax length and Velocity Factor used to tune antennas is bull**** and a lot of hocus pocus designed to make people think they know more than the other folks about radio. Coax length is that length that is directly proportional to the distance between the radio and the antenna. And THAT's the bottom line!!!!!!:LOL: If I upset somebody, I'm sorry; it's just the way I feel about it! NObody has ever been able to answer this with a true and accurate rebuttal because they CAN'T!:laugh:

    CWM
     
    #8 C W Morse, May 17, 2010
    Last edited: May 17, 2010
    hotrod and midnight special like this.
  9. BOOTY MONSTER

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    "Coax length and Velocity used to tune antennas is bull****"

    nobody said anything about changing coax length to tune an antenna . the discussion was about if the length of coax changes what the meter reads . even the OP said he thought it was just fooling the meter .

    i personally think the coax was assembled improperly , that can happen even when its new .
    can you tell us why his vswr changed ?
    i hope this lowly chicken bander didnt disapoint you much by not calling it swr's ;)

    ive seen how you love to come down to the cb section and brag about your credentials and beat your chest and try to make us feel stupid ........... but can you at least be on topic when you feed your ego like that ?
     
  10. psycho

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    On the Wilson 1000 and 5000 coax length has to be 18' for the match to be good. On all the other antennas I have ever used the coax did not affect the swr but the base loaded Wilsons are different. Email Wilson, they'll tell you the same thing.
     
    #10 psycho, May 17, 2010
    Last edited: May 17, 2010
  11. Wire Weasel

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    Bravo ol' boy. My experience is same. Coax is UN-balanced feedline. Run the shortest amount possible to reach from point A to point B. Length only matters with balanced feedline like open wire ladder line.


    But the Great Coax Wars shall continue. It'll cost tens of thousands of lives and jillions of dollars, but it's a slow news day and some folks need something to do.

    Carry on

    :pop:
     
  12. psycho

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    What the hell?????????????????????????????
     
  13. hotrod

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    alot of truth here.i agree with ya
     
  14. mackmobile43

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    Well I think I figured out what's happening by lengthening your screwdriver antenna you are basically stripping the shield off of coax and by shortening it you are re-shielding all that coax if the cb'er theory holds true.:LOL:

    And to think all that is done with the push of a button, lazy ham guys.:pop:
     
  15. TonyV225

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    :w00t: shall we fire another missle in that direction sir?? :eek::LOL::pop:
     

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