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That whole coax length thing...

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

  1. AudioShockwav

    AudioShockwav
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    [​IMG]



    When you going to put a 4 speed in that little cart carl?


    (y)

    73
    Jeff
     

  2. Carl

    Carl
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    soon as i find a top-loader that isnt worth blood.
     
  3. mechanic

    mechanic
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    :bdh:

    A radio or transmitter is not complete when you buy / build it.... the antenna is the balance of the circuitry.... Since most radios are designed to make use of 50 Ω impedance with an antenna, the antenna needs to be cut, trimmed to present that load then the radio is happy and your signal gets out!
    How many radios have an antenna attached to the rig directly? :whistle: .... Not many.... So we have feedlines!
    Unbalanced coax and twin lead are the common choice for feedlines and any length or size WILL change the the balance ratio (SWR)! The addition of the feed line is unpredictable so we need to change the circuit to keep the balance so the radio will be happy.
    One can change the antenna, use a matching device, coil the feed line if coax, add capacitance or inductance to the antenna base.... all this is empirical at best! And a meter helps.... sometimes!
    Yes certain lengths of coaxial feedline mean something and there are rules of thumbs. But even a cloud passing overhead will change something!
    Take a florescent bulb and try to find "nodes" along a coaxial feedline and you may see it light up at different points.... a sure sign of a bad balance or match! A Bazooka antenna is feed with coax but has a length of twin lead to load the antenna and makes the radio "happy" and gets your signal out! The 300 or 450 Ω twin lead is an element to help balance the "equation".

    mechanic

    Now I will go have my popcorn..... :pop:
     
  4. hookedon6

    hookedon6
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    don't take this wrong, but,...............

    there is so much wrong in this post that i don't know where to start :whistle::whistle: jus sayin

    psst, i like some butter on mine pls
     
  5. Captain Kilowatt

    Captain Kilowatt
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    You mean things like "Unbalanced coax and twin lead are the common choice for feedlines and any length or size WILL change the the balance ratio (SWR)!" which is wrong IF the antenna is properly tuned. Or do you mean this;"The addition of the feed line is unpredictable so we need to change the circuit to keep the balance so the radio will be happy." which is wrong because a little math will show what will happen therefore making the addition of feedline very predictable. Perhaps you mean this: " But even a cloud passing overhead will change something!" which is wrong unless that cloud has been salted with silver iodide and has become super reflective to radio signals. Maybe he means a thunder cloud in which case the lightning produced by that cloud could destroy your antenna and feedline and THAT would change things.

    :pop:
     
    338_MtRushmore and DainBramage like this.
  6. mackmobile43

    mackmobile43
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    Amazing now much CB BS can be disguised as tech talk.:pop:
    [​IMG]
     
  7. Shockwave

    Shockwave
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    Yes, it has been known to propagate extremely well too!
     
  8. Captain Kilowatt

    Captain Kilowatt
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    Like I always heard, if you can't dazzle 'em with brilliance, baffle 'em with BS. :blink:
     
  9. mechanic

    mechanic
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    :LOL: yup! :LOL:
     
  10. Bigfoot350

    Bigfoot350
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    I don't understand all this 1/4 1/2 wavelength stuff is it possible to just tell me the easiest cheapest way to tune a antenna??? And/or tell me if I could use a blasters meter to check the ohm resistance?
     
  11. Bigfoot350

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    and the antenna is for a CB on a truck
     
  12. Captain Kilowatt

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    (1)- Put your SWR meter as close to the antenna as possible to minimize the effects the coax may have.

    (2)- No you cannot use that meter to check the antenna impedance. You can use it however to check for open circuits or shorts in the coax cable and to make sure the antenna is not grounded out.
     
  13. hotrod

    hotrod
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    best way to adjust antenna is at theantenna itself. if possible use a mfj 259b anylizer.
    if not then any swr meter will do even those small cheap ones.next get a short piece of coax called a jumper i use 3 feet. i try keep ALL coax runs as short as possible. if ya need help using te swr meter cbradiomagizine has a video of it n youtube [i believe]
     
  14. trees

    trees
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    As long as the phase angle is zero, I would think... (Electrical half wave feedline, both elements driven by same length line). Start using different phase angles and I would think this technique would not work...leading, lagging elements inducing different feed point impedances... As well as different current magnitudes.. I would think. 1/4 wave elements mounted 1/4 wave apart and fed @ 90°= mutual coupling and impedance shift at antenna feed point.. I could be wrong
     
    #104 trees, Apr 13, 2017
    Last edited: Apr 13, 2017
  15. trees

    trees
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    If I understand correctly....

    Coax length will not change feedpoint impedance, but it can transform this impedance..

    If the feedpoint impedance is different from the transmitter impedance, then standing waves will be present, even though the swr "looks" low at the transmitter end, due to the tuned line..

    Am I on the right track?

    And could we then create a current choke, by coiling coax at the feedpoint through a ferrite core, eliminating the standing waves on the line?
     

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