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Thanks for your input , Jay

Discussion in 'CB Antennas' started by Jay in the Mojave, Dec 18, 2003.

  1. Jay in the Mojave

    Jay in the Mojave Active Member

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    Hello 53 Bayous:



    No thats not it either.



    The LMR 400 Coax probably has a lower loss than the Radio Shack RG8 Coax. As better coax is used, more energy gets to the antenna, and so does the reflected energy back to the radio. But not a hole here in the short lengths.



    But becuse the ground radials are not working properly, and the coax is radiating along with the mast, again different coax and mast lengths will hace a different SWR indiction.



    1/4 wavelength for all the elements!!!



    Jay in the Mojave






    </p>
     

  2. freecell

    freecell BANNED

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    53 bayous,



    you still need longer elements........



    Jay in the Mojave,



    LMR400 definitely has less loss than Radio Shack RG-8....

    www.firecommunications.co...stat.shtml



    Jonbah,



    feedlines consisting of odd 1/4 wave multiples are used as matching transformers for dual antennas, phased arrays and other applications where a 4:1 mismatch exists between the feedline and the load.



    FL Native,



    your theory only applies when the antennas input feedpoint impedance is 50 ohms. this is hardly the case in a mobile application. it isn't the case here either.



    concretemannj,



    If you want an accurate reading of what the antennas feedpoint impedance is you should measure it at the feedpoint of the antenna with the necessary equipment.



    the tuned half wave line is no help unless the feedpoint impedance is 50 ohms. if you're using the tuned half wave line by default without measuring the feedpoint input impedance then you're using it incorrectly.



    cactustech,



    in any instance where the swr is anything but flat, (1:1) changing the length of the feedline will affect the swr.



    Normally we want the generator (transmitter) to be at a voltage or current loop, depending upon what we were trying to do. As an example we would want half wave multiples between source and load for a single antenna, and quarter wave or odd multiple's of quarter waves for a dual antenna installation. keep in mind that an antenna is really 36 ohms not 50; so the 14.1 feet of half wave shows 36 ohms towards the radio, and the missing 14 ohms is added by the remaing 3.9 feet of coax in an 18 foot single. This gives us 50 ohms roughly at the radio end and is just exactly why antenna manufacturers spec out 18 feet for the coax run to their antenna's. When a transmission line has standing waves on it the DYNAMIC IMPEDANCE of the line is no longer equal to the CHARACTERISTIC IMPEDANCE of the line, unlike the example of only incident travelling waves without any reflections occuring. THE REFLECTED WAVES WILL ALTER THE INPUT IMPEDANCE that the GENERATOR (TRANSMITTER) "sees".



    www.firecommunications.com/vswr.shtml

    www.firecommunications.com/light.txt



    (acrobat reader required to view the files in the reflect

    directory)



    228


    </p>Edited by: <A HREF=http://pub86.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=freecell>freecell</A> at: 12/25/03 1:19 pm
     
  3. 53 bayous

    53 bayous Guest

    that 6 element metal ground plane of mine needed 50 feet of 52ohm coax. Today, I bought some RG-8 from Rat Shack, I went up to 30 feet with a tripod roof mount on my house. I had been using brand new LMR 400, but for some reason it would not work. It wasn't quite 50 feet, but gave me a 1:5 after I retuned. Now, with the other coax, my SWR needle will barely move, virtually flat. didn't need longer elements after all. Impedance in the coax wasn't matched up.


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