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Make Your Coax Invisible?

Discussion in 'CB Antennas' started by Chicago Savage, Sep 8, 2019.

  1. Chicago Savage

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    In all seriousness, can one or more of our pro's out here please respectfully explain what this good ole boy is doing in this video. Obviously, what he has done resulted in something. You tell me what. Since the beginning of time, I've heard of people "tuning their coax". I just want to hear from our experts here on WWDX. Specifically, what is RS, and XS on the meter, and what is happening when XS goes down? Thanks, Dean.


     

  2. Highway Man

    Highway Man Active Member

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    Handy Andy likes this.
  3. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    X 2 @Highway Man (y)(y)

    In shorter words, he knew the amp had it's own tuning issues so he's trying to design a tuning "stub" to between the amp and antenna to better match the amp.

    Unfortunate - for all effort, that the MFJ - for all it is, can't just be dropped in to replace the output network so you can determine the correct impedance X = +/-j + Rohm to make a fix for the toroid's or combiners with the correct inductive winds or capacitance.

    It also doesn't help when the ferrite cores used are an unknown permeability - you have to figure those out on your own when tuning the amp. Do up too long of a "pipe" (referring to AMP combiner) and there's not enough capacitance in the word to bring it into resonance, admittance - let alone required impedance.
     
  4. Highway Man

    Highway Man Active Member

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    someone wrote that this method is in the MFJ book
     
  5. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Yes it is, but how the vid shows his interpretation - well - you get the idea that he needed to "stub" so he had to find an entry point....

    He needed more of a test jig...which once an amps layout and installed like the one above, it's not a simple drop in.
     
    357magnum likes this.
  6. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    If you’re a coyote, you’d use this.

    45A7BA80-87B6-4C17-9656-0808B49E1C32.png
     
    unit_399, 357magnum, binrat and 4 others like this.
  7. Low_Boy

    Low_Boy Well-Known Member

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    I like the video. Makes sense.
     
    Handy Andy and Chicago Savage like this.
  8. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    I'm glad he was able to make the vid to prove some points.

    My takeaway from the vid shows that each connector, for what they're worth, induces an impedance "bump" - and that is an inherited trait for each connector across brands as well as coax across it's brands exhibits this bump once the connector is installed...you can have that same connector (Silver or nickel plated - that too) will alter the "bump" it shows in this demonstration.

    But the implications for Amp building is far-reaching.

    The vid shows that no matter the method used to inject the or insert the MFJ - it is still a gamble on the jig - setup or the coax and it's connectors - all broken connections are inducing their own bump...
     
  9. nomadradio

    nomadradio Analog Retentive

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    He should take an ohmmeter to his dummy load.

    He lost me when changing the coax length made his dummy load go from 1:1 up to 1.3.

    He would seem to have the cart before the horse.

    The length of a coax feeding a perfect 50-ohm load will NOT change the SWR reading.

    The higher the SWR above one-to-one, the more difference you'll see from changing coax length.

    Somebody send him a dummy load. I suspect an accurate resistance measurement will show that his has drifted up to about 65 ohms, more or less.

    Couldn't summon the patience to watch the rest of it after the dummy-load remark. I'd like to see what his dummy reads with only a double-male barrel connector between the analyzer and his dummy. If you want an accurate reading, the closer you get to zero coax length, the closer you'll be to a true reading.


    73
     
  10. Highway Man

    Highway Man Active Member

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    good info nomad, learn something new everyday here
     
  11. Low_Boy

    Low_Boy Well-Known Member

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    By
    Nomad. When he was doing his snipping one end of the coax was hooked to his antenna analyzer the other end had nothing and was being clipped.
     
  12. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Note the display had R=0 - he had it as a short - hence the shorting stub.

    He did claim for seeing an SWR of 1:1.3 and to quote "Yor' amp needs to see nothin' but 1 to 1"
     
  13. Low_Boy

    Low_Boy Well-Known Member

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    I fo not like coax, antennas and matching.. It seems to make sense to me. Because I was told coax length really does not matter. Well I guess I dont know what to think but what he was doing was sure making a difference.
     
    Chicago Savage likes this.
  14. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    The only time coax length makes a difference is when trying to null a ground loop on the shield ..:whistle:
     
  15. 543_Dallas

    543_Dallas Sr. Member

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    This video uses balanced feedline made of copper tubing but can be applied to coaxial transmission lines also. The main difference is the amount of loss when you have high swr on a coaxial feedline. Break it up and watch it over a couple days...if you're reading this you're not too busy :)

     

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