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That whole coax length thing...

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

  1. hotrod

    hotrod
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    in simpler terms
    1 radio=50 ohms by design
    2coax = 50 ohms by design [coax lenth shouldnt change this]
    antenna=50 ohms [or about] once tuned
    imho coily up the coax isnt going to lower swr at all. may even make it worse


     

  2. Captain Kilowatt

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    Coiling up coax does NOTHING at all unless you have common mode currents on the outside of the shield which is an indicator of something else that is wrong. Coax is a shielded cable and as such each turn is shielded from the next. Two turns of coax in a coil do not even know the other turn exists. I wish people would learn and understand this. I keep hearing people say all the time NOT to coil up extra coax especially in mobiles and lay it out zig-zag fashion or some other such nonsense.
     
  3. trees

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    A physically shortened 27 MHz mobile antenna is never going to be 50 ohms at it's feed point when tuned to resonance, so there will be standing waves present...

    Choking off the reflected current at the feed point should eliminate alot of transmitter problems, I'm thinking...

    So, using a tuned line to match the transmitter to the antenna, and using a choke at the feed point should equal a fairly good situation from a reflected power/problem perspective. Yes?
     
    #108 trees, Apr 15, 2017
    Last edited: Apr 15, 2017
  4. trees

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    I'm wondering if routing the coax through a thin metal tube approximately 2'.5" long chokes off reflected current? Application would be coax routed through a west coast style mirror bracket on a semi truck..
     
  5. DXman

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    Sorry psycho, but the 18' for the match to be good is not true. I removed the 18' off my 1000's that I had and replaced them with 15' of LMR 240. Guess what? The match on both antennas where 1:2 on the two trucks!!
     
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  6. trees

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    That's pretty close to the electrical 1/2 wave point @27 MHz with 240 ultraflex... (15.185 ft) If you shortened the coax to 7' you would likely see a completely different swr...

    Antenna height above ground is also a factor.
     
    #111 trees, Apr 15, 2017
    Last edited: Apr 15, 2017
  7. trees

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    Out of curiosity, was the match different with the 18'? A 22' section of coax would likely give you a different reading as well...there are phase angles within a length of coax..
     
  8. trees

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    I think I just "got it"....

    The reflected wave, in vswr, is traveling back down the same center conductor that the forward wave is traveling on....

    Oh..

    Not the outer ground braid of the coax...

    Although there could also, and very likely, be current present there as well...
     
  9. trees

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    I think I get it now....

    If the antenna feed point is not 50hms pure resistance..

    There will be a reflected wave on the center conductor, (the power not absorbed by the load).

    You can find a point in the transmission line where the intersection of the forward, and reflected, wave meet....at the zero phase angle intersection... This would "appear" to be the perfect match point... But is merely the point where both waves meet at 0° phase. The vswr is what it is, and that is dependent on antenna feed point impedance.

    Am I on the right track here?
     
  10. M0GVZ

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    Getting there. And its why an antenna analyser is a better tool for tuning antennas than a SWR meter.

    If there's current on the outside of the braid its usually because of insufficient RF grounding.
     
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  11. trees

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    Gotcha, the feed line ground braid becomes the counterpoise, (antenna ground image), yes?
     
  12. bob85

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    Not exactly on the right track, first part is ok,
    whenever coax is not terminated with a pure resistance equal to its own characteristic impedance there will be reflections,

    impedance will swing up and down as you move along a mismatched coax but vswr ignoring coax loss is the same at any point along the coax,
    in practice vswr will read a little lower at the transmitter than at the antenna on longer runs due to loss in the coax,

    vswr deals with the currents on the inside surface of the braid and the center conductor,
    currents on the outside of the braid are common mode currents,

    chokes don't choke current inside the coax,
    they choke currents on the outside of the braid,

    common mode can play a part in vswr because common mode impedance is seen in parallel with the load terminating the coax,

    winding chokes & cutting coax changes common mode impedance seen in parallel with the load

    if winding a choke in the coax or altering the coax length by a few feet does change vswr significantly it indicates you have common mode on the braid,
     
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  13. trees

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    Ok, I understand that. If I have a shortened vertical @ 27 MHz, and I use a loading coil antenna like a Wilson 2000, to cancel capacitive reactance, and achieve resonance, but the feed point impedance is under 50 ohms, then I can use another inductor, like some enameled wire wound into an inductor, and shunt this wire inductor from feed point to ground, raising feed point to 50 ohms? (Using analyzer like MFJ 259). And then if I had a tuning capacitor at the feed point with the wire inductor I'd be able to tune the feed point network and have some bandwidth?
     
    #118 trees, Apr 16, 2017
    Last edited: Apr 16, 2017
  14. Clodhopper

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    I always set the SWR @ the antenna. No matter what length coax I use it never changes... So the whole coax length this is a MYTH..
    But just in case you can use this coax length calculator.
    http://www.angelfire.com/electronic/cbdoctor/coaxcalc.html
     
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  15. 928bolo

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