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Wilson 5000 Magnet Mount Coax

Discussion in 'CB Antennas' started by comet429, Jan 22, 2010.

  1. comet429

    comet429 Member

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    :confused:Ok,this might be a dumb ?,but every place I see that sell the wilson 5000 mag mount has,that it has 17ft of coax,but if you go to their site it tells you to use 18ft. when replacing it.
    Are they getting us both ways(they short us a ft. then want us to buy and extra ft.

    Anybody know if it's going to make a difference?

    I know that a magnet mount is a different beast as far as the ground part.


     

  2. W5LZ

    W5LZ Crotchety Old Bastard

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    There are several things that are mixed together here, that 'complicate' things. The first is that the length of feed line makes a big difference to start with. It certainly can make a difference, but only because of misconceptions.
    An antenna has to have certain characteristics for it to work well. It should be resonant, have an input impedance that matches the rest of the system (typically, 50 ohms), and provide a radiation pattern that puts a signal at least 'close' to where you want it to go. Forget about the radiation pattern and resonance for a minute. The 'biggy' with feed line length is that it can be made to 'compensate' for an antenna that doesn't have the right input impedance. That's good, right? Under certain circumstances, yes it is. But it's not the 'best' way of doing things because it does have faults, it can be destructive. If done incorrectly it can cause more harm than good, it's just not very efficient. Another problem with doing that impedance matching with coax is that all coax, even off of the same spool, doesn't have the same characteristic impedance, it varies to some extent. One particular batch of coax may be exactly 50 ohms, the next may be slightly 'off' from that. It's man-made, it's never always exactly the same. What may require 18 feet in one instance might require 17 feet, or 19 feet in another instance. How do you know? You gotta measure it. Not just with a yardstick, but with something that can tell you what the impedance is. And then you have to know what is required for it to 'work' the way you want it to, and that is so mixed up with preconceived 'myths' that it's almost a nightmare. For instance, 18 feet of coax (of any kind) has no electrical characteristics that are beneficial at 27 Mhz. The only reason that length is used is because it's a nice 'average' for reaching from the radio to the antenna in most mobile installations. It does not take velocity factor into consideration (Yeah, I know, that 'dirty' word, velocity factor. Sorry, it's still important when doing impedance matching with feed lines.). Another factor in all this is that an SWR meter is a very 'dumb' meter. It can't tell you things you need to know to do it right. To do that 'right', you gotta dig down in that can of worms and find the one that 'tastes' just right (geeze, isn't that a nasty thought?). Terrible analogy, but I'll bet you remember it ;).
    - 'Doc

    There's more to all that, but I gotta get ready for work. Have fun.
     
  3. KingCobra_CDX882

    KingCobra_CDX882 W9WDX Amateur Radio Club Member

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    Long story short is..
    use the shortest length needed for you to go from radio to antenna..

    Whether you need 5 ft or 20 ft..
    shortest coax is best...Period..
    (exceptions in using dual antennas )
     
  4. Robb

    Robb Yup

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    What kind of coax are they using to be used exclusively with these antennas?
    I thought I was going to get slick and change my Wilson 1000 mag mount antenna coax with LMR-240...
    Guess not!
     
  5. comet429

    comet429 Member

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    Well,they sell the 1000/5000 with 17ft,then turn around the tell you to replace it with 18ft.

    Just thought I was missing something.
     
  6. W5LZ

    W5LZ Crotchety Old Bastard

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    :)
    The only thing I can see that you might be missing is that 'they' are too cheap to furnish that 'extra' foot of coax to start with. ... Come to think about it, I know several places like that, my bank being one of them.
    - 'Doc
     
  7. StomperX

    StomperX Member

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    It says on the website that it uses RG59. Is that correct?? I've heard that coax is 75 Ohms.. ????

    Can someone clarify??

    Thanks in advance! :)
     
  8. mackmobile43

    mackmobile43 Jock Supporter

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    Mine came with 18' of rg8x and I didn't bother shortening it because I'm lazy and that's how I roll.
     
  9. jazzsinger

    jazzsinger Bullshit Buster

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    Hmmm, on the subject of misconceptions.

    5/8 lambda antennas work reasonably well, but they ain't resonant.resonant and tuned/matched are very different.



    Wrong, 18 feet on any coax with a velocity factor of .66, ie the cable supplied with many magmounts, suffice to say that 18 ft is a 3/4 wave impedance inverter. i'll let you figure out the rest and why if you don't replace it with the exact length of cable it came with or an odd multiple of a 1/4 wave (which is electrically equivalent) you will find it very hard to tune.

    the k40 is another antenna that uses the very same setup.

    if you replace the coax with a type with a different velocity factor you have to compensate for that velocity factor and still keep it an odd multiple of a 1/4 wave.

    either way that so called "NON BENEFICIAL @27 MHZ" coax length "IS" part of the matching system.


    by the very same token any antenna/mount for cb that comes with 12 feet/3.6m of coax (an impedance repeater should be replaced with a multiple of a halfwave (velocity factor being accounted for), for exactly the opposite reason that an antenna using 18 feet uses its length, ITS NOT PART OF THE MATCHING SYSTEM, therefore you want it to repeat the impedance it sees at the antenna end,not transform it like the 18 foot section does.


    THIS LACK OF UNDERSTANDING OF HOW TRANSMISSION LINES CAN BE USED TO TRANSFORM IMPEDANCE IS EXACTLY THE REASON THERE IS SO MUCH CB BULLSHIT / MYTHOLOGY / MISUNDERSTANDING / MISCONCEPTION, call it what you like , of why some cb antennas use 12 feet (or any multiple thereof) and some use 18 feet (or any odd 6ft multiple), there IS a reason for those lengths.


    you being an amateur should know things like this DOC, especially if you intend giving out advice on antennas to cb'ers who trust you simply because you hold an amateur callsign.
     
    #9 jazzsinger, Feb 4, 2011
    Last edited: Feb 4, 2011
  10. jazzsinger

    jazzsinger Bullshit Buster

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    i haven't used this antenna before, but its most likely it IS 75 ohm, as 75 ohm is exactly what would be used to match a 50 ohm radio impedance to an antenna that has a higher feedpoint impedance.
    without going into too much technical detail the coax length supplied and the type used (75 ohm rg59/u) tells that the feedpoint impedance is approximately 100 ohms.which is why it needs transforming.

    exactly the same way the 75 ohm 1/4 wave section or multiple thereof is used to match a quad.

    in general terms a 3/4 wave section or more is used on a mobile antenna that needs transforming as a 6 ft (1/4 section) is just physically too short to be practical in use.

    much the same reason as a 3/4 wave section is generally used when co phasing, as again a 1/4 wave section is impractical to get the spacing of antennas required to get the best from co phasing.
     
  11. psycho

    psycho Running a special on our rooms!

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    If an auto manufacturer tells you in the owner's manual to use 5w-30 or 5w-20 motor oil, use it. If Wilson says to replace the coax with 18 ft, do it. I would just contact a Wilson dealer and buy the whole mount and coax assembly. The mini 8 that they use is real good quality coax.
     
  12. MrSuburban

    MrSuburban Guest

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    Jazz right on, 18' is exactly what you state it to be a impedance transformer.

    Doc I think it has been to long since you hit the books I see far to many errors in alot of your posts.
     
  13. StomperX

    StomperX Member

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    @ jazzsinger.. thank you for the knowledge. You really made sense! :)

    Ok......

    I just looked at my friends Wilson 5000 magnet mount on his car and it does come equip with an RG 58 C/U 50 Ohm (I don't know what the "C/U" means) but, it says that this can handle up to 5KW on AM. The length of the coax is 17FT. Now, this is all I'm concerned about is AM.

    Is that for reals? 5KW watts on AM on this coax??

    I'm already thinking of changing the coax to an LMR 240 (18FT) to be specific.

    Do anyone know where I can get the PL-259 for this 'skinny' type coax because I'm definately upgrading the coax on this Wilson 5KW?

    Any help is greatly appreciate it.. Thanks in advance! :D
     
  14. W5LZ

    W5LZ Crotchety Old Bastard

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    If you would hang a picture of that "5000 watt" antenna advertisement over your garden you'll never have to buy fertilizer again...
    - 'Doc
     
  15. W5LZ

    W5LZ Crotchety Old Bastard

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    If you start with the idea that using coax as a matching device is a good way of doing things, doesn't harm anything ,then what I posted is incorrect.

    I do agree that using coax as an impedance transformer is done fairly commonly. But, the problem with that is that coaxial cable was never designed for that impedance transformation job, doesn't do it very efficiently, and is almost always harmful to the cable. There are better ways of doing that impedance transformation.

    So why isn't it done that better way? Because it isn't as simple as jockeying the feed line length, or as cheap in some cases.

    Is that -my- idea, did I originate it? Nope, but I did learn about it by 'hitting those books' as was recommended. I think I would recommend that others 'hit those books' again.
    - 'Doc
     

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