SWR Question.

Discussion in 'General Technical Questions and Answers' started by KD8GFC, Jul 17, 2008.

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  1. K5MC

    K5MC Ham Member QRZ Page

    The way W5DXP worded his question to me, I was afraid if I simply answered "yes" then somebody would take me to task for agreeing that the traveling waves propagate at speeds of about 0.66 for typical coax (0.66 or 66% of what?), or perhaps raise the issue of non-sinusoidal waveforms (group versus phase velocity, etc.). That's why I wrote the much longer response. However, I'm not surprised that my less than "pithy" answer generated several howls from the gallery!:)

    Much of the "debate" in this thread and in Cecil's earlier thread on the effect of tuners at the antenna is the result of several of us (including myself) using various terms (e.g., standing waves, pure standing waves, standing wave envelopes/patterns/graphs/plots/curves, traveling waves, pure traveling waves, etc.) in an informal manner and then being taken to task by somebody else. At this point I'm trying to be very careful in my comments.:D

    73, K5MC
     
  2. K5MC

    K5MC Ham Member QRZ Page

    Pithy response

    I agree. (That's a pretty concise comment!)

    73, K5MC
     
  3. KI6NNO

    KI6NNO Ham Member QRZ Page

    No doubt. I get regularly get hammered for giving "the simple answer". hihi! ;)
    73, KI6NNO
     
    Last edited: Jul 27, 2008
  4. KI6NNO

    KI6NNO Ham Member QRZ Page

    That is *very* clear. Thanks.
    73, KI6NNO
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    Of course, some purist will come back and say, "Detectable modulation is not steady-state - therefore, it is meaningless!":)
     
  6. AB0WR

    AB0WR Ham Member QRZ Page

    Oh, there is no doubt that the average value is the same. I even believe that Kraus says this directly. I don't have his book handy or I would give you the direct reference.

    The two big points I always remembered from this was that, first, there would always be an error in space along the transmission line in trying to find an exact impedance location if you just used the average value and, second, that this phenomenon makes it harder to find the max and min points on a slotted line. The max is broader than the min so you have to careful you are at the max since it is hard to see the result of a small movement and at the minimum just a little movement makes a big difference so it is hard to "tweak" the position to get right on the minimum.

    Whether these errors are significant enough to actually make a difference in the lab when students do slotted line experiments I can't say. They may be overwhelmed by the error in the measurement equipment or they may contribute significantly, I just don't know.

    tim ab0wr
     
  7. KB5PN

    KB5PN Ham Member QRZ Page

    Please..........would someone put a muffler on this thread???? Enough already!!!
    Question has long since been answered and the horse is thoroughly dead:eek:
     
  8. W5DXP

    W5DXP Ham Member QRZ Page

    Are you are being forced, a la Clockwork Orange style, to read this thread?:)
     
  9. AB0WR

    AB0WR Ham Member QRZ Page

    Who made you the Grand Poobah to decide if a discussion is done or not?

    What hubris!

    tim ab0wr
     
  10. K5MC

    K5MC Ham Member QRZ Page

    Yes, Kraus (on page 442 in his third edition, 1984) says that the average value is the same. I demonstrated that to myself a few days ago for the special case of a resistive load equal to twice the value of the characteristic impedance of the line. If I can find the integral in my CRC book I will try to prove it for any arbitrary load impedance.

    I certainly don't recall any errors being introduced in slotted line/Smith chart problems if one ignores the fact that the instantaneous phase velocity of the TOTAL instantaneous voltage wave isn't constant. This velocity (as pointed out by Kraus on page 442 in his third edition) is the phase velocity of the total (net) wave and this wave travels only in one direction, from the source to the load. In effect, it's a mathematical "trick" or "manipulation" that, while interesting, has no real bearing on the analysis and design of transmission line problems. (We do locate the Vmin points rather than the Vmax points when using a slotted line because the Vmin points are more sharply defined. I suppose one can argue that the instantaneous phase velocity of the total voltage wave determines, at least in part, the shape of the standing-wave envelope.)

    The important factor here in my view is the phase velocity (velocity of propagation) of the forward and reflected waves, not the phase velocity of the "total" wave traveling from the source to the load. To be honest, I don't recall ever reading about that velocity until I saw it discussed in Kraus a few days ago! (And I used to play a lot with slotted lines/Smith charts years ago in industry as well as teach about them.)

    73, K5MC
     
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