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(Possibly) a stupid question on a RF failsafe....

Discussion in 'General Ham Radio Discussion' started by iiiquaziii, Aug 6, 2017.

  1. KD2GOE

    KD2GOE
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    Delete


     
    #16 KD2GOE, Aug 7, 2017
    Last edited: Aug 7, 2017

  2. Captain Kilowatt

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    Please stop doing that. I restored the other post as it had pertinent info. Deleting a post because people disagree with you when they did not have all the info you had is not good form. It also screws up the thread. Thanks. (y)
     
  3. Captain Kilowatt

    Captain Kilowatt
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    Prior to this added info all we knew was that the cable was old. Nothing said about exposed ends to the weather and old enough to rust a metal spool. As long as coax cable is not laying in the mud/ground for years and the ends are sealed against the weather then previous statements including mine are true. Now with your added info I myself would be concerned about the integrity of the cable but still not worried it would simply open up and destroy my radio. The SWR may be higher and losses surely higher however.
     
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  4. Paul-W5lz

    Paul-W5lz
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    C.K.
    • You're right, those additions would certainly work. But the 'catch' to that is that the antenna's and feed line's impedance are almost never the same, so what I said still holds true. The differences in what the meter will read doesn't have to be large by any means, but that still means there is a standing wave on the feed line which varies with distance (fractions of a wave length). Huge number of variation in how an antenna system is situated and almost all of them will affect SWR. Right?
     
  5. Captain Kilowatt

    Captain Kilowatt
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    Splitting hairs aren't ya? LOL YES I will concede that you are correct about that......................this time. :whistle:
     
  6. iiiquaziii

    iiiquaziii
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    Yeah, the steel spool was rotted in half....lol. When we stripped back the ends though, the conductor wasn't all corroded. I dunno how much water actually got inside it. It works though. *shrug*
     
  7. bob85

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    W5lz, you said
    "That SWR meter is very easy to fool. It's positioning in the feed line will determine what the resulting SWR reads"

    That is untrue,
    You cannot fool a vswr meter, the meter will read the vswr at the point of insertion in the line,

    You also said

    "If the antenna's input impedance is 50 ohms, and the feed line is 50 ohms, and you measure SWR a 1/4 from the antenna, the SWR will read high. No changes at all except move that meter to a 1/2 wave from the antenna and it'll read very nicely"

    That is untrue,
    when the antenna input impedance is 50ohms resistive & the feedline is 50ohms vswr will read 1:1 anywhere along the line, unless you have CMC on the coax braid,

    even when the load is mismatched to the line lets say 50ohm coax & 25ohm resistive antenna,
    vswr will read the same 1/4wave & 1/2wave along the line unless you have VERY lossy coax or you have CMC on the braid,

    your claim that vswr when using 50ohm coax feeding a 50ohm antenna will read high 1/4wave from the antenna & low 1/2wave from the antenna is W5lz karaoki physics at its finest,

    you said

    :) " Sorry, you are mistaken, The reading of an SWR meter can certainly change because of it's placement in the feed line. Try it,"

    Yes it can but only when you have CMC on the braid,

    you said

    "I basically described common characteristics of various electrical length of feed line"

    That is untrue,
    various electrical lengths of feedline only cause impedance to swing around every 1/4wave along the line & only when the load is mismatched to the line, they do not change vswr readings unless you have CMC on the braid,

    When CMC is high it DOES effect vswr along the line and needs fixing,

    You are not fooling the meter, the meter CANNOT be fooled, it is reading the vswr at the point you insert it into the line.

    try reading "swr meters make you stupid" again & the link in my sig that TheDB posted :)
     
    tecnicoloco likes this.
  8. Paul-W5lz

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    You're right, I miss stated some things in that post. You are also wrong about a couple of your opinions of what I said. For example, where in that feed line you put that meter, -unless- there is no standing wave at all, absolutely does change what that meter will read. Have you ever seen an antenna system with no SWR? I haven't.
     
  9. Riverman71

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    Darn. Just when the conversation was getting interesting.
    Thought we were about to witness a knock-down drag-out between two long-time WWDX heavyweights. :mad::confused:

    Just kidding, fellas. Thanks for taking the time you do to help out guys like me. (y)
     
  10. Captain Kilowatt

    Captain Kilowatt
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    I think confusion surrounds the fact that impedance may change but actual SWR does not change. Yeah it took a while for me to get my head wrapped around that too but for any given value of SWR there are an infinite number of R, Xc, and Xl values that will present that SWR value. An SWR meter may read the same SWR value at various places along a transmission line however an antenna analyzer will show many different values of actual impedance values along that line when there is a mismatched condition.
     
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  11. bob85

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    "where in that feed line you put that meter, -unless- there is no standing wave at all, absolutely does change what that meter will read"

    Not in the way you described & not due to standing waves caused by a mismatched resistive load no matter how bad the match,

    to see a significant change in vswr by moving the meter 1/4wave along the line you must have CMC on the coax braid,

    in the real world with decent coax & no CMC issues you will see a small reduction in vswr the further from the load you move the meter, not detectable on a regular meter when moving it 1/4wave along the line @27mhz,

    what you will see is impedance looking into the coax to a mismatched load swinging up and down every 1/4wave along the line,

    next time you see vswr changing with coax length try winding a good choke close to feedpoint or slip several suitable ferrite beads on the coax & repeat the test ;)
     
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  12. bob85

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    CK
    it can be hard to understand, especially when you have observed different vswr readings with different coax lengths for many years and come to the wrong conclusion of why it is happening,

    it does not sink in as easy as when i was young,

    when you get older the sponge in your head gets full of crap you don't need to know but do know,
    feels like there's not enough room in there, can't go back to defrag & clean up to make space,

    i won't get everything correct or may have a brain fart and type X instead of R j instead of jX etc,

    when i get something wrong i want telling about it with links to respected sources so i can learn from my mistakes.
     
  13. Paul-W5lz

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    And the 'key' was mentioned... All those reactances don't radiate anything, only resistance 'radiates'. SWR tells you nothing about reactance, it can't even 'see' it. That's the benefit of an antenna analyzer. Then all you have to do is get rid of the reactance or neutralize it.

    ...Sometimes I wonder, "Why did I ever say that, I know better." Ever been there?
     
  14. Captain Kilowatt

    Captain Kilowatt
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    Everyday at work and most days at home with my wife. ;)
     
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  15. iiiquaziii

    iiiquaziii
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    Well, this has turned into quite the educational debate. I think I've learned something from this, even if it did take off on a tangent. I'm happy for the tangent it took off on.

    So let me ask you all this: We know I have lower grade thin coax cable with minimum shielding, that sat under a porch in a mud for long enough for the metal spool to rot away. I can tell you the ends weren't bad on the cable, when stripped back and prepared to solder the ends on. The shielding and core still looked coppery and clean once we cut off the ends and stripped it back a bit. How bad/risky is this cable? On a scale from 1 to 10, if you were pretty broke, how much would you worry about it? How much would new better shielded cable improve things? I realize this is subjective, but put yourself in the shoes of a 35 year old man, who can barely pay the mortgage with his first HAM radio for 10/11/12/15m use. It seems to work pretty well. Radio is only 1 of my hobbies.

    I am planning on better grounding, re-wiring the old 50's era 2 wire home electrical in the radio room, and getting baluns for my 20/40m fan dipole before replacing the a99 feed line. Are my priorities accurate?
     

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