Coax Calculator

Discussion in 'CB Radio Forum' started by mike5511, Jul 5, 2021.

  1. mike5511

    mike5511 Road Train Member

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    Meanwhile, gaining on the mobile........
     

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  3. craig_sez

    craig_sez Road Train Member

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    According to dxengineering its ment for the 55zoom but i have seen vids with guys tuning (nulling/cutting)coax with an mfj259....
    If i see one againg ill pass on a link..
     
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  4. Night Stalker10

    Night Stalker10 Road Train Member

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    Here's the formula:

    Coax Length = 492 x VF/freq. If length is 14.7 feet, then cut coax to around14.8 1/2 inches. Take .7 x 12 = 8.4 inches.

    So 492x.66 (66% VF for 58/u coax) equals 324.72, divided by 27.205 mhz (channel 20), equals 11.9ft long. That a why some tech's say 12 feet of coax is close enough.

    I use RG213 coax with 66% Velocity Factor for my base antennas. So for a simple example, if the length for channel 20 was 12ft long, you would add another 12ft to get 1 wavelength, a total of 24ft. . Then keep adding 12ft to reach your antenna. Now you have to use every other 1/2 wavelength number. The problem is that if your 1/2 wavelength coax is 4ft short from reaching your antenna, you will have a lot of excess coax because you need to cut it at the next 1/2 wavelength. Also in this example, you would need to use a 1/2 wavelength 12ft jumper cable between the radio and swr meter, instead of the usual 3 ft jumper.

    I just found this website that will explain it better.

    RadioMods coax length page
     
    Last edited: Jul 27, 2021
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  5. craig_sez

    craig_sez Road Train Member

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    Yeap..All the cab's metal skin helps
     
  6. craig_sez

    craig_sez Road Train Member

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    Found this one while surfin youtube..
     
  7. craig_sez

    craig_sez Road Train Member

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    Another about le
    Another about length..
     
  8. mike5511

    mike5511 Road Train Member

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    I should have bought the 55 zoom. The 35 zoom is all I got.
     
  9. mike5511

    mike5511 Road Train Member

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    I saw it last night. I'm not that familiar with my 35 zoom yet. But you are saying it won't do what Mfj was doing in this post?
     
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  10. craig_sez

    craig_sez Road Train Member

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    No im sayin that a mfj was used in the vid but useing the 35 would be able to do the same as mfj..You would have to use the screen that shows all the swr/resistances and resonance screen..
    The 35 will not allow you to just calibrate the for the null of coax by useing an open/short/load....Not that i asked rig expert but according to the dx engineering site where i got my o/s/l its says for the 55 and up..
     
  11. mike5511

    mike5511 Road Train Member

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    Okay, I understand what you're saying, and I was thinking I should be able to do that. Still learning the Rig Master. I ran 213 from the antenna to the amp under the bunk. I've already put the interior back together and the back of the ball mount is not easy to get to to begin with. So, when the truck gets back from the shop, I'll stick a 259 on it and see what I got. I'll try out the null thing on the 8x I'm running from the amp to the radio. Anxious to see if the 4.5' SkipShooter or my old original Wilson 2000 tunes out the best.
     
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