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Thread: Some great information about coax lengths and swr

  1. #1
    doughboy's Avatar
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    Some great information about coax lengths and swr

    There is a misconception among CB operators that changing the length of your coax can "tune" your antenna system. This is absolutely untrue! This short discussion will detail why.
    The coax cable is a means for transferring your RF signal to the radiating portion of the radio system. The cable, in theory, is meant to be a contained, non-radiating link. Because it does not radiate and serves only to transfer RF between two components of the transmitter system, it's performance in terms of efficiency is affected by length, but only in terms of overall resistance. In other words, using a long run of coax will reduce the total amount of signal at the antenna, but only because of loss due to resistance and NOT because of standing waves.
    Ideally, you want to check the SWR of your antenna at the antenna feedpoint. In a perfect world, this is the best way. However, we all know that this is ludicrous to expect in a standard base antenna installation. Unless a remote SWR meter head is incorporated, we usually use the standard SWR meter located at the radio. The drawback is that resistance and slight impedance mismatch of the coax affects the overall SWR reading.
    Because radio waves are tuned wavelengths of energy, we have to take into account the coax cable length. A typical 11-meter signal has a basic wavelength of 36 feet/wave. "Tuning" the coax for the exact full wavelength tends to throw off the SWR meter by not allowing any standing waves to return to the meter. Excess RF on the coax has been given an ideal medium by which to "hide" electrically from your SWR meter. That is not to say that the excess RF is not returning to the radio, you just can't see it on your meter.
    What we want to do is create an environment where any excess RF (standing waves) are rendered as visible as possible to the meter. This is effectively done by using multiples of the 1/2-wavelength of the radiated signal. One half wave for the 11-meter band is 18 feet. However, this is not the length that you will cut your coax. There is another factor that affects the length. This is Velocity Factor. The velocity factor is basically a term for how fast the signal moves through the coax. This factor affects the overall electrical performance of the coax and thus needs to be accounted for when determining the true half wave length

    Here are the velocity factors of the various Belden coaxial cables:
    RG-59 .66

    RG-59/U (foam) .79

    RG-58 .66

    RG-58/U (foam) .79

    RG-8A/U .66

    RG-8/U (foam) .80 9913 .84

    RG-213/U .66

    Here is how to figure out your true 1/2-wave:
    492 x (Velocity Factor) / Frequency (MHz)
    For example, I want to figure out the true half wave coax length for RG-59/U (foam) on my home channel (ch. 33 - 27.335):
    492 x .79 / 27.335 = 14.219 feet
    Now add 14.219 to itself to determine your 1/2 wave multiples. Remember to use every other number. See the example below:
    14.219 feet 1/2-wave multiple, 28.438 feet 1-wave multiple, 42.657 feet 1/2-wave multiple, 56.876 feet 1-wave multiple, 71.095 feet 1/2-wave multiple, 85.314 feet 1-wave multiple
    and so on . . . .
    Use only the lengths that fall on the 1/2-wave multiples and you will be all set.

    Now in order to get the true SWR of the system, you have to throw away that 3-foot jumper cable for now. The SWR meter has to fall on a 1/2-wave point on the coax run. Using the example above, you need a 14.219 foot jumper from the radio to the SWR meter, and a 1/2-wave multiple length from the SWR meter to the antenna. If my antenna is 65 feet away from my radio, I need a 14.219 foot jumper from the radio to the SWR meter, and a 71.095 foot length between the SWR meter and the antenna.

    In laymans terms, the coax length fools your SWR meter into thinking that your SWR is different to what it actually is !
    The ONLY way to know what your SWR actually is, is to use a correct length of coax for the operating frequency

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    707 Lumpy Delaware (04-03-2012), BOOTY MONSTER (01-31-2012), High Voltage Mobile NJ (08-16-2012), Mistah Gravy (03-31-2012)

  3. #2
    Mr.5150cbrn is offline Banned Dx Number: 5150 central,il.
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    thanks Doughboy , thats easy enough to understand!

    but im sure someone will chime in to contradict that explanation
    as they always do.......

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    BOOTY MONSTER is offline Heavy Hitter Dx Number: 420
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    "Here is how to figure out your true 1/2-wave:
    492 x (Velocity Factor) / Frequency (MHz)
    For example, I want to figure out the true half wave coax length for RG-59/U (foam) on my home channel (ch. 33 - 27.335):
    492 x .79 / 27.335 = 14.219 feet
    Now add 14.219 to itself to determine your 1/2 wave multiples. Remember to use every other number. See the example below:
    14.219 feet 1/2-wave multiple, 28.438 feet 1-wave multiple, 42.657 feet 1/2-wave multiple, 56.876 feet 1-wave multiple, 71.095 feet 1/2-wave multiple, 85.314 feet 1-wave multiple
    and so on . . . .
    Use only the lengths that fall on the 1/2-wave multiples and you will be all set.
    "

    if im understanding what he's saying ... hes saying 1 or 2 or 3 electrical wavelengths don't work to get the effect he's talking about , but 1/2 , 1 1/2 , 2 1/2 etc. will . it's my understanding that a electrical half wavelength or any multiple of that length for that frequency will work .

    doughboy , can you supply a link to the article ?
    thanks

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    doughboy's Avatar
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    I believe that to be correct booty because you want a half cyle through the coax
    Take a look at this link might help
    http://signalengineering.com/ultimate/coax_basics.html

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    Mr.5150cbrn is offline Banned Dx Number: 5150 central,il.
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    now is that what they mean by resonating the coax ?
    to the given frequency to obtain the length , without using the math end of the equation .


    and also does that also apply to a mobile installations too?

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    BOOTY MONSTER is offline Heavy Hitter Dx Number: 420
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    i didn't have any issue with the resignating coax article itself once i figured out what they were trying to achieve , although they definitely took the long hard way to do it just to avoid two simple math equations . i took issue with the following post suggesting 3 s-units of gain from doing it .

    pure CB bullchit worthy of being posted as fact over at the mule droppings forum .
    Last edited by BOOTY MONSTER; 01-31-2012 at 07:05 PM.

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    Mr.5150cbrn is offline Banned Dx Number: 5150 central,il.
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    well if your not tx ing a full wattage to antenna and the antenna sys isnt resonant at ??>???? frequency ,
    then by resonating the coax to get everything in check , then yes one might see a 3db gain ?

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    if the antenna is not resonate absolutely no length of coax is going to change that .
    tune the antenna ...... not the coax .

    there is some loss associated with high vswr , but it would have to be high to reduce a signal by 18db .

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    Mr.5150cbrn is offline Banned Dx Number: 5150 central,il.
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    Quote Originally Posted by BOOTY MONSTER View Post
    if the antenna is not resonate absolutely no length of coax is going to change that .
    tune the antenna ...... not the coax .

    there is some loss associated with high vswr , but it would have to be high to reduce a signal by 18db .
    ok , but explain how you might do / pull that off with a 102 -108 ss whip? which is set usually @ 36 ohm

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    BOOTY MONSTER is offline Heavy Hitter Dx Number: 420
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    it's "usually @ 36 ohms" because of the shape of the ground element (which is the vehicle) so that will vary from vehicle to vehicle as well as where its placed on that vehicle . FWIU , a 1/4 wave ground plane antenna (which is basically what %99 of all mobile antennas are) needs ground elements sloped down at a certain angle to tune to 50 ohms .... along with the proper length radiator .

    here's a link that explains it without me doing a bunch of typing .

    http://www.comportco.com/~w5alt/antennas/notes/ant-notes.php?pg=20

    B
    TW a 36 ohm antenna is about a 1.3 vswr which is completely usable .
    Last edited by BOOTY MONSTER; 02-01-2012 at 11:31 AM.

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