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Thread: Some great information about coax lengths and swr

  1. #11
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    if anyone is interested ............

    http://www.dx-antennas.com/Coax.htm

    "WHAT LENGTH?
    As mentioned the best length is the shortest, in this way the loss due to the cable is the lowest.
    There can be one advantage by extending the length of the cable:.
    A HALF WAVE coax cable or ANY multiple (1, 1½, 2 wavelength) length of this will show you: "exactly the same input resistance at both ends of cable". In normal words:
    You can measure the exact SWR from the antenna with these lengths of coax cable.
    If your antenna is truly 1:1 it doesn't matter, each SWR measurement anywhere on the line and you will find 1:1.
    Only when your SWR isn't 1:1 but higher your measurements will change according to length, this is when it can come in handy to keep the line a half wave long (or any multiple). "



    "
    COAX AND VELOCITY FACTOR
    The shielding and the core of a coax form a capacitor. Through the use of insulation material, the value of the capacitor is bigger then it would be in air, and this has the effect of slowing down the signal. This can be of great importance in some applications, although for many purposes it does not needed this information ;-)Velocity factor
    The speed at which the signal travels is normally given the abbreviation VF and this is the fraction of the speed at which the signal travels when compared to a signal traveling in free space. So VF for a signal traveling at the speed of light would be 1.0, and for one traveling at half the speed of light it would be 0.5.

    The velocity factor of the coax cable is found by VF = 1 / SQRT (dielectric constant)
    Coax cable electrical length
    One important factor of a coax cable in some applications is the wavelength of the signals traveling in it. In the same way that the wavelength of a signal is the speed of light divided by the frequency for free space, the same is also true in any other medium. As the speed of the wave has been reduced, so too is the wavelength reduced by the same factor. So if the velocity factor of the coax cable is 0,66, then the wavelength is 0,66 times the wavelength in free space.

    In some instances lengths of coax cable are cut to a specific length to act as an impedance transformed or a resonant circuit, then this needs to be taken into consideration when determining the required length of coax cable.
    The advantage of using a coax cable with a low velocity factor is that the length of coax cable required for the resonant length is shorter than if it had a figure approaching one. For example, 1/4 lambda 75ohm coax (VF * 1/4 lambda length) for transform the impedance from 125ohm to 50ohm. "

  2. #12
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    Quote Originally Posted by BOOTY MONSTER View Post
    if the antenna is not resonate absolutely no length of coax is going to change that .
    tune the antenna ...... not the coax .

    there is some loss associated with high vswr , but it would have to be high to reduce a signal by 18db .
    well over the last weekend , i had been having issues with a 102 whip and swr readings !
    and my truck .
    when the usual antenna is mounted on cab roof adjusting swr isnt a major deal as running a half wave on the coax tunned fairly well
    but when running a 102 behind cab on bed rail and running coax in multiples , swr is way out of spec.
    pulled my hair out , but he goes; 102 ss whip 36 ohm reading taken at base of antenna @ pl-259 with a mfj259b analyzer
    now i resonated my coax (rg-8x ) vf ".78 to 27.200 giving me a coax length of 28 ft 2.250 in. for a full wave in length netting me a true 1.3;1 swr
    not tuning antenna by trimming stinger , but more by tuning the coax to the setup
    final measurements on the analyzer freq. 27.200 , swr-1.3, R=40, X=6 .( R &X readings @given freqency ) which equals a 1.1 swr even though the swr bridge reads a swr of 1.3

    so in laymens terms yes adjusting length of coax can change the caricaturization of a antenna system in a mobile installation in regards to a 102 whip,

    now with a base type of installation , and antennas having adjust-ability though gamma , beta matches , tuning rings etc and the ability to obtain a proper rf ground plane , things are some what different with regards to antenna height ground soil composition (salinity) capacitive reactance and magnetic inductance and a myriad of other variables along with coaxial length and vf's

  3. #13
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    "but when running a 102 behind cab on bed rail and running coax in multiples , swr is way out of spec. "

    so your reading probally went up because of reflection from the cab and you were already using coax that was a multiple of a electrical half wavelength ? correct ?



    "now i resonated my coax (rg-8x ) vf ".78 to 27.200 giving me a coax length of 28 ft 2.250 in. for a full wave in length netting me a true 1.3;1 swr "

    so you changed to another section of coax that was one full electrical wavelength and your reading got better ?
    is it possible one of the coax sections has been damaged or that a 259 needs some attention ?


    "so in laymens terms yes adjusting length of coax can change the caricaturization of a antenna system in a mobile installation in regards to a 102 whip, "

    i'll be sticking with coax length not changing the tuning of a antenna , just what a meter reads .


    ....

  4. #14
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    Quote Originally Posted by BOOTY MONSTER View Post
    "but when running a 102 behind cab on bed rail and running coax in multiples , swr is way out of spec. "

    so your reading probally went up because of reflection from the cab and you were already using coax that was a multiple of a electrical half wavelength ? correct ?
    well its not exactly behind cab . 10 in. to side of cab and 10 in. rearward of cab , bed is a commercial utility bed box
    original coax length 18ftwhen ant. was mounted on top of cab




    "now i resonated my coax (rg-8x ) vf ".78 to 27.200 giving me a coax length of 28 ft 2.250 in. for a full wave in length netting me a true 1.3;1 swr "

    so you changed to another section of coax that was one full electrical wavelength and your reading got better ? actually added section /s to come up with a full wave length as using 3/4 wave length gave undesirable results
    is it possible one of the coax sections has been damaged or that a 259 needs some attention ?
    no as i thoroughly inspected the coax and pl-259's


    "so in laymens terms yes adjusting length of coax can change the caricaturization of a antenna system in a mobile installation in regards to a 102 whip, "

    i'll be sticking with coax length not changing the tuning of a antenna , just what a meter reads .
    thats your opinion and like wise your entitled to your opinion , but it doesnt change the fact , that it dispels your theories or hypothesis. - using a simple swr bridge in conjunction with a analyzer and a 50 ohm dummy load shows a different side of the story


    ....
    and i did this with my own ability without using another link to disprove another's result!

  5. #15
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    18 ft of coax is a physical half wavelength , not a electrical half wavelength .
    you had 36 ohms and went to 40 ohms .

    you still didn't change any thing at the antenna , changing the length of the coax changed what the meter/259 was seeing to display it's readings .


    believe what you want ............

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    Its basically just like a ham tuner i can tune a hammer and talk on it But the hammer is still not resonant there is no doubt coax length can change Swr at the load but the antenna stays resonant where ever it was in the begining Also dont forget vf is not always 100% accurate and may vary by manufacturer even Though both may be rg-8 or whatever

  7. The Following User Says Thank You to doughboy For This Useful Post:

    BOOTY MONSTER (06-11-2012)

  8. #17
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    In the case of the steel whip they are normally about 36 to 40 ohms at resonace but some cheap equipment probably wont play well with that so yes most people adjust their coax to show the equipment the load it wants most radio stations antennas run as low as 28 ohms at resonance so they just run a large matching network they lose some power through the match but the antenna performs much better because is resonant

  9. #18
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    Quote Originally Posted by BOOTY MONSTER View Post
    18 ft of coax is a physical half wavelength , not a electrical half wavelength .
    you had 36 ohms and went to 40 ohms .

    you still didn't change any thing at the antenna , changing the length of the coax changed what the meter/259 was seeing to display it's readings .


    believe what you want ............
    oh its all understood , but booty
    # 1. ya dont have to be a dick about it!
    #2. go lick your self !

    it wasnt about believing anything , i was just stating my findings .
    in response of you getting discombobulated on every forum known on the world wide web.

    by the way , booty did you tell everyone here , how you cried on the phone to lonewolf - 4 to five times a day
    because you tried to TROLL some members and got asshurt from the flames from HELL !

    now its time to knock it off!
    keep it in the appropiate areas , or keep it to your self
    Last edited by Mr.5150cbrn; 06-11-2012 at 08:21 AM.

  10. #19
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    LMAO !!
    i just explained the cause of your findings , your assumption that you were tuning the antenna is wrong .
    changing coax length doesn't change the tuning of the antenna . but it can skew the reading on a meter .
    my post here were on topic and non flaming , so take your own advice .
    Last edited by BOOTY MONSTER; 04-04-2012 at 10:02 PM.

  11. #20
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    Quote Originally Posted by BOOTY MONSTER View Post
    LMAO !!
    i just explained the cause of your findings , your assumption that you were tuning the antenna is wrong .
    changing coax length doesn't change the tuning of the antenna . but it can skew the reading on a meter .
    my post here were on topic and non flaming , so take your own advice .
    it wasnt an assumption , you made that.
    i just made a log of my findings that coincide with the original post here- that meters can be fooled and or give false readings, i wasnt flaming untill you lit the flame ...... now its on you !

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