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Coax length.. does it matter or not

Discussion in 'General CB Services Discussion' started by dxing440, Mar 29, 2012.

  1. dxing440

    dxing440 Active Member

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    I run a magnum s45hp radio ( turned down to 2Wdk and 35Wpep) with 6' rg8x to the dx500v,gives me (80Wdk and 400Wpep)and 12' of rg8x to the antenna, roadtech open coil, mounted on pass side about 8'' from cab and the bottom of the coil is about 3' below the cab.
    EVERYTHING is grounded to the frame, as short as it can be.
    The swr are at 2.1-2.7 with amp on, but just with radio @2Wdk the meter will not adjust over far enough to test swr.
    The radio at 15Wdk and 80Wpep the swr's are about 1.3



    WHAT GIVES?
     
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    Last edited: Mar 29, 2012
  2. Coax information
     
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  3. AudioShockwav

    AudioShockwav Extraterrestrial Admin Staff Member

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    It is " inches, not ' feet


    As long as you continue to run that antenna 8 inches away from the cab of your pick-up truck, i think you are going to have a problem.
    You need get that antenna away from or above the cab or it is just gonna drive you crazy.
    The Coil of that antenna is below the level of the roof it is even worse.

    This is the worst place to mount a loaded antenna , even more so with the LOAD ( the COIL) mounted below the roof


    I really recommend the Wilson 5000 Mag mount, and stick it up on top of the cab.
    You are gonna pay 85 or 90 bucks for a Predator and if you stick it in the same place i bet you will have the same problem.


    73
    Jeff
     
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  4. Robb

    Robb Yup

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    And if you keep smakin' that amp with 80 watts input - it will fry too. The Texas Star amp repair guy is a member of this forum - BTW. He said the most common reason for failure with their amps are people trying to get more watts out of the amp by putting too many watts in. Of course; ya don't have to take my word for it . . .
     
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  5. AudioShockwav

    AudioShockwav Extraterrestrial Admin Staff Member

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    :headbang
    Move the Antenna away from or above the cab
    :headbang
    Look at this Picture, if you have to mount the antenna behind the cab It SHOULD look like this:
    [​IMG]




    73
    Jeff
     
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  6. dxing440

    dxing440 Active Member

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    Hey bootymonster, good info on the coax. Although,:confused: I just cant understand how to calculate the half wave, full wave, thing for 11 meters or any meter. LOL

    And audio, ive heard the antenna suggestion more than once, wilson 5k must be a good one. I just dislike magmount antennas, but its that, or keep a huge bottle of asprin, for the coil antenna.


    Thx
     
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  7. dxing440

    dxing440 Active Member

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    hey robb if you read my 1 post again, its at 2wdk and 35pep NOT 80
     
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  8. Robb

    Robb Yup

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    ???
    Just sayin' man . . .

    First off, just run as much coax as you need to make it from point "A" to point "B". With just a couple extra feet of wiggle room for good measure. Forget about all of that BS about how long a piece of coax needs to be. Get yourself a Predator antenna, use a puck mount and cut the sheet metal in the center of the roof of the cab, run some LMR-240UltraFlex/RG-58 coax, and be done with your SWR problem. Or a mag mount Wilson 5000 too . . .

    BTW - if you run more than the Texas Star recommended power into that amp (20 watts/25 max), you will defeat the AB class amplification you bought that amp for.
     
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  9. IMD262

    IMD262 Well-Known Member

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  10. KC9Q

    KC9Q Supporting Member

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    To calculate the length use the following formulas:

    1 wavelength = 984/F(MHz) x cable Velocity Factor

    3/4 wavelength = 738/F(MHz) x cable Velocity Factor

    1/2 wavelength = 492/F(MHz) x cable Velocity Factor

    1/4 wavelength = 246/F(MHz) x cable Velocity Factor

    For F(MHz) use 27.185 (frequency of Channel 19)

    Velocity Factor varies by cable. Generally it it .66 (66%) for RG-8/U or RG-213/U. For RG-8X it can be either .79 (79%) or .82 (82%) depending on the manufacture.

    Example: 1/2 Wave of RG-213/U is 492/27.185 x 0.66 = 18.098 x.66 = 11.94 feet

    Hope this helps you understand this stuff.

    73,
    Mike
     
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  11. Robb

    Robb Yup

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    He couldn't use more than 12 or 14 ft of coax inside that cab.
    With that length, those losses are next to nil . . .
     
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  12. dxing440

    dxing440 Active Member

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    Robb, some good pointers. Ill be gettin for a wilson, as i dont want to cut holes in the roof top. thx to everyone whos replied
     
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  13. W5LZ

    W5LZ Crotchety Old Bastard

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    Unless there's some specific purpose for a particular electrical length of feed line (timing or phasing) the only certainty about feed line length is that it has to be long enough to reach between here and there. It doesn't do much good if it's too short, maybe a half inch from making a good connection. The other end of that scale is also true. If you only need to go 10 feet, using 300 feet of feed line is kind'a 'wrong' too, wasteful mostly.
    People use coax as an impedance transforming device. It works, but it certainly isn't the best way of doing it, it can harm the coax being used. Any impedance change means an increase in voltages present. If those voltages remain under the break-down voltage of the coax being used, no harm is done. But, it certainly does not remain under the voltage limits in all cases. That's when it will turn around and bite you. (No, that doesn't mean you get shocked by too much stray RF in the shack.)
    Think of that feed line like being a pipe between the fuel tank and an engine. It has to be at least a minimum size, too big doesn't harm anything, or help anything either. If it's too short, the engine quits. If it's too long, then the fuel pump has to be larger too. Not to mention the cost of filling that feed line with fuel! :)
    Lots of factors in all that, keeping things practical is a good idea...
    - 'Doc
     
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  14. vo1ks

    vo1ks Active Member

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    Quarter wave lines are very often used for impedance matching devices. Multiple quarter wave sections, each step with a different impedance is very common in broadcast antennas. One antenna I'm familiar with uses 36 dipoles. They are fed from a 6-inch main input power divider to six secondary power dividers. Each power divider has six outputs. The cables from the power dividers to the dipoles have to be phased for beam tilt and to create a circular pattern between horizontal and vertical elements. In this setup line length is important to having the antenna work properly. It is capable of handling about 72kW of RF at the FM band.

    For single antenna to radio, assuming the antenna is properly matched, the length of cable is not critical. For a mismatched antenna, sometimes adding or removing a short piece of cable will help bring SWR down. For power levels at CB even RG58 is unlikely to suffer from any breakdown.
     
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  15. Robb

    Robb Yup

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