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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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    i agree with this completly!!


     

  2. bob85

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    yes you can raise the impedance of short loaded whips using an inductor to ground, that's how shunt fed antennas like wilson & sirio's work,
    first you must adjust the loaded antenna so its capacitive or a little shorter than resonant.
     
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  3. trees

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  4. trees

    trees
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    .

    So, common mode current travels into the srw bridge and combines with the current traveling on the center conductor... Skewing the meter reading?
     
  5. bob85

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    No, its not skewing the reading,
    it is changing vswr inside the coax by changing the impedance terminating the coax,
    currents inside coax are always equal magnitude opposite phase,
    whatever leaves the centre conductor must return on something and flow back down the inside of the braid,

    you see folk talking about coax length only fooling the meter,
    you cannot fool the meter only the guy operating the meter.
     
    #125 bob85, Apr 21, 2017
    Last edited: Apr 21, 2017
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  6. Onelasttime

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    First anyone that has built an amp and had to tune the input and output understands that impedance matching matters only a moron would say other wise. That said how much it matters is another story. Any impedance changes seen by the radio or the amplifier affects the circuits inside the radio and amplifier again only a moron would say other wise. If it does not matter then give me your radio and amplifier for 5 minutes and let me tinker inside with input and put tuning!

    Truth told if one was anal-retentive enough one would install an Arco variable cap on the output of the radio and tune the output to you antenna system for perfect match not just at 50ohm load but looking at a scope and spectrum analyzer for best performance. No one does this though.

    A lot depends on the transceiver and amplifier and how tolerant they are and what you expect.

    Obviously we understand that it will affect what the meter reads but what I am talking about is how it affects the rf circuits and their performance.

    Capacitance and impedance on input and output has a HUGE affect on rf transistors and audio transistors. This is common knowledge. So why the argument? This is why you can not just bolt toss any transistors with the right pin-out and voltages into any spot you like. This is why we do not just add some wires to a 2SC2879 and toss it on the back of a CB in place of a 2SC1969. This is why we carefully select parts and impedance for high end audio amps to shape how the transistor sounds not just it's peak power output....

    Now on to meter readings.....It is kind of important to be able to trust what a meter is telling you. I have seen base station installs read over 3 for SWR be brought down by massively shortening the coax. Since we did not have a means at the time to read the SWR at the antenna which was 65 feet up int he air we had to realy on what we saw at the meter in the shack! I have no doubt that the SWR at the antenna was not magically altered by shortening the coax but what good is a perfectly functioning antenna if you can not accurately read the SWR of the antenna system? A meter is of no value if you can not trust it to give you an accurate reading. If trimming the coax allows you to get an accurate reading than what is the problem?

    You can take different length and different types of coax and run a given unit into a spectrum analyzer and a scope and see altered results on them. If science and empirical data is to be respected and accepted than loud mouth opinions really do not matter as much as what can be seen on a scope or spectrum analyzer do they? I will clean it up but there is an old saying " Opinions are like the external opening of the rectum we all have one and they all stink." Why do coax companies bother to give us velocity factors if they not of any importance to use?

    I have myself seen issues where I could not get an accurate reading with the factory supplied SWR bridge inside a radio with some lengths of coax. What good is having an swr bridge inside a radio if you can not get it to read accurately what is going on with your antenna?

    So to the high and mighty know it all's what good is sn swr bridge is you can not get it to read accurately? I eagerly await your well reasoned answer.
     
  7. wavrider

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    Manufactures give velocity factors of coax as a specification.

    NOW how the consumer uses that coax is up to them.

    I SURE need to know what the VF is on the coax I am using when I am building a four square steerable array,,the VF and length in the coax determine phase difference.

     
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  8. bob85

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    "First anyone that has built an amp and had to tune the input and output understands that impedance matching matters"

    that only applies to some of the people that build tube amp from scratch and people that understand how transistors are matched to the load at any given vcc & drive level,

    most people that build solid state amps build solder jockey style copying other peoples work & don't understand how they work,

    "I have no doubt that the SWR at the antenna was not magically altered by shortening the coax"

    you should have doubt because its not magic its physics,
    vswr can change with coax length, if the meter said its changed significantly then it DID change with coax length,

    "but what good is a perfectly functioning antenna if you can not accurately read the SWR of the antenna system? A meter is of no value if you can not trust it to give you an accurate reading"

    you can trust the reading, that is the vswr of the system even when coax length does change vswr,
    its not fooling the meter its fooling the guy using the meter, you need to understand what the "system" is and how to minimise the detrimental effects of some of it on your vswr,

    if vswr does change with coax length and you don't like it then you need to eliminate the common mode from the braid,
    it won't give you a more accurate reading as that is set by the meters basic accuracy but it will stop vswr changing dramatically with coax length,

    when your vswr goes down significantly by cutting your coax back a lot it indicates you have cut a faulty piece of coax out OR you have cmc on the braid,
    if you had good coax & no cmc vswr should have gone up a little due to reducing cable loss,

    "Why do coax companies bother to give us velocity factors if they not of any importance to use?"

    velocity factor is important when phase is important such as arrays of antennas fed with a certain degree of lead/lag, or when using coax as an impedance transformer such as 1/4wave inverters or bodgit jumpers between radio and amplifiers that have an other than 50ohm input so the radio sees closer to 50ohm load,

    velocity factor is of no significance with regards to vswr changing with coax length unless you get your info from cb forums where that sort of nonsense is posted as if it were true.

    Velocity factor determines the electrical length of the inside of the coax and resulting transformer action when the line is mismatched,

    You have to wonder why it is that some folk love their bird or mfj but ignore the instructions needed to understand what they are reading,
    instead they make sayings up like you are getting REFLECT off the cab or your amp output is not tuned to 50ohms so you have reflect,

    just another case of observing an effect but not understanding what's causing,
    its karaoki physics, for people who enjoy sounding stupid in public.
     
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  9. Road Squawker

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    Now THATS funny[​IMG]

    Yeah, "some" people can't see the forest because all those trees are in the way[​IMG]
     
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  10. Captain Kilowatt

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    The problem is that trimming the coax length does NOT give you an accurate reading. How could it? What value do you determine is correct for the particular antenna installation? Do you simply trim until you get the lowest SWR and call that accurate? I should hope not.

    This is where the operator must learn to recognize problems in their installation and also learn how to eliminate those problems in order to obtain an accurate reading. Common mode currents on coax cable WILL affect the meter reading if the length of coax is changed. If this happens then you take steps to eliminate those common mode currents and THEN you can rely on the meter readings. If the antenna impedance at the feedpoint is NOT a pure 50 ohms then the impedance along the length of cable will vary with distance from the feedpoint however the SWR as read on a meter will remain constant at any point along that cable. One thing most people do NOT understand is that there is an infinite combination of resistance and reactance that will produce the same SWR reading.This is why using an analyzer is preferred over a simple SWR meter as it shows exactly what is going on with the antenna system.
     
  11. trees

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    I think I'm getting it now... CMC affects the vswr because the coax has become part of the antenna. The outer ground shield of the coax is operating as the antenna ground, and the antenna current is freely flowing on the outer braid because....

    A) there is no current choke at the antenna feed point to obstruct cmc

    And in the case of a mobile..

    B) there is an insufficient antenna ground plane at the base of the antenna, so the feed line necessarily becomes part of the radiating system....

    CBers, especially truckers, have been advised to use 18' for this very reason.... And not to coil, or "choke off the coax", because the the coax is acting as the opposite half of the antenna.

    The problems begin when big time rf power is applied to this antenna system....you get big time rf in the cab of the truck
     
    #131 trees, Apr 26, 2017
    Last edited: Apr 26, 2017
  12. Captain Kilowatt

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    You're starting to get it now. People do not realize that current can flow on the outside of the coax shield independent of the current flow inside the coax. There are THREE paths for current to flow in coax cable. The inner conductor, the INSIDE of the shield which is the part of the shield next to the inner insulation, and the OUTSIDE of the shield which is the part of the shield directly under the outer weatherproof covering. When current flows on the outside of the shield it is called common mode current or CMC. This is not desirable at all and because this current is not shielded from its surroundings then anything brought into proximity of the cable can affect it. Even touching the cable can alter meter readings. If all current was confined to the inner conductor and the inside of the shield it is then shielded from the surroundings and anything outside the cable has no effect.
     
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  13. unit_399

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    AND ... it only took nine pages on this thread for that to happen.

    - 399
     
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  14. trees

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    [

    So far we've discussed what happens with a single antenna element, in regards to coax length, vswr, and feed point impedance....

    What happens when we use multiple antenna elements? For the sake of discussion, 2 1/4 wave verticals spaced at a 1/4 wave, fed at 90°...

    Wouldn't coax length become a factor then? As in current phasing, constructing a matching stub, or in making a Q section?

    Let's say we had 2 elements @ 50 ohms... Does the feed point impedance change because we're using a parallel circuit?
     
  15. Captain Kilowatt

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    Yes length is a factor then but ONLY in regards to phasing or in some cases impedance transformation.

    Yes it does. It becomes 25 ohms. This is the reason that you either use stubs to transform each element impedance to 100 ohms or you use a proper RF power splitter which is actually a tuned piece of feedline that transforms the 50 ohms to 25 ohms. This is called a conjugate match. My pair of 50 ohm 13B2 yagis for 2m will be combined/split thru a home made power splitter/combiner that has a characteristic impedance of about 35 ohms. When placed in at the junction of the two yagis it presents an impedance of 50 ohms to the main feedline. The result is that each antenna is tuned perfectly to 50 ohms on it`s own and the combination still presents a perfect 50 ohm match to the main feedline.

    You are starting to confuse me. :rolleyes: The questions you ask would lead me to believe that you in fact know more than you are letting on. Talking about making Q-sections and phasing lines etc yet you don`t know about common mode currents? (n)Hmmmmmm.............


    Hopefully I don't have to use this

    [​IMG]
     
    #135 Captain Kilowatt, Apr 26, 2017
    Last edited: Apr 26, 2017
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