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VSWR doesn't matter? But how about "mismatch loss"?

Discussion in 'Electronic Basics' started by billcalley, Mar 29, 2007.

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  1. billcalley

    billcalley Guest

    What I gleaned from the excellent answers for the original "VSWR
    Doesn't Matter?" thread is that high VSWR doesn't really matter in a
    lossless transmission line environment between a transmitter's antenna
    tuner and the antenna, since any reflected RF energy will simply
    continue to "bounce" back and forth between the tuner's output
    impedance and the antenna's input impedance until it is, finally,
    completely radiated from the antenna without loss.

    But then why does the concept of "mismatch loss" exist in
    reference to antennas? I have quickly calculated that if a
    transmitter outputs 100 watts, and the TX antenna has an impedance
    that will cause a VSWR of 10:1 -- using lossless transmission line --
    that the mismatch loss in this "lossless" system would be 4.81dB!
    (Reflected power 66.9 watts, RL -1.74).

    Since mismatch loss is the "amount of power lost due to
    reflection", and is as if an "attenuator with a value of the mismatch
    loss where placed in series with the transmission line", then I would
    think that VSWR would *definitely* matter, and not just for highly
    lossy lines either. But here again, I'm probably not seeing the
    entire picture here. What am I missing??

    Confused!

    -Bill
     
  2. Cecil Moore

    Cecil Moore Guest

    If the system is Z0-matched, e.g. antenna tuner, there is
    a mismatch gain at the tuner that offsets the mismatch
    loss at the load so, in a lossless system, nothing is
    lost. Wave cancellation toward the source is balanced
    by constructive interference toward the load.
     
  3. billcalley

    billcalley Guest

    Now my head *really* hurts! This is a VERY confusing subject, to
    say the least. (And I also thought antenna tuners actually had a
    *loss* due to their limited Q...I think I'm going to change careers
    now and just become a pet groomer; or perhaps simply give up
    completely and work at Radio Shack).

    -Bill
     
  4. Cecil Moore

    Cecil Moore Guest

    Real-world antenna tuners do have a loss but we previously
    specified a lossless system. Of course, real world tuners
    and transmission lines suffer losses but we all just live
    with those losses while striving to minimize them. The point
    is that an antenna tuner reflects most of the reflected
    energy back toward the load thus accomplishing a mismatch
    gain that offsets some, if not most, of the mismatch loss.
    High SWR transmission lines are indeed lossier than flat
    matched transmission lines of the same material.
     
  5. That's basically true but ignores the stress that can be placed on the
    output circuit of the transmitter. Why do you think VSWR shut-down circuits
    are so popular? One can see rather high voltages or currents that are
    potentially damaging to the transistors and capacitors.

    Now, since "lossless" is an abstraction and all materials have voltage and
    current limits, just make this easy on yourself and always strive for a VSWR
    of 2:1 or less. It simply works better and is more reliable.
     
  6. Cecil Moore

    Cecil Moore Guest

    But renders many all-HF-band dipoles useless. :) I regularly
    run up to an 18:1 SWR on my 450 ohm ladder-line. Owen's
    transmission line calculator says I'm losing about 0.8 dB
    in 100' of line under those conditions on 40m. IMO, it's
    a small price to pay for all-HF-band operation.
     
  7. How many Ham transmitters have a balanced output?

    How are you feeding a balanced line?

    If you are using an antenna tuner with unbalanced in (50 ohms) and balanced
    out (variable impedance), you should be OK in most situations.
     
  8. Jimmie D

    Jimmie D Guest

    If a tuner is placed directly after the TX and properly adjusted the TX
    will always see a 50 ohm load and the shutdown circuit will always be
    happpy. Again as long as the TX sees a match there is no unusual stress
    placed on it. Remember that before the invention of coax cable SWR was
    rarely considered. Instead the tx was tuned for proper established
    operational parametrs and all was right with the world.

    1:1 SWR CAN MEAN YOUR COAX IS FULL OF WATER.

    Jimmie
     
  9. Cecil Moore

    Cecil Moore Guest

    No tuner! I don't like tuner losses. The feedpoint impedance
    is always between 35 ohms and 85 ohms resistive. My choke
    has an impedance in the thousands of ohms.
     
  10. Tim Wescott

    Tim Wescott Guest

    Your "amount of power lost due to reflection" statement would be true if
    the line were connected to something resistive at the line's
    characteristic impedance. With a properly tuned tuner, that's not the
    case -- instead, the impedance looking into the tuner will also reflect
    power, and in a way that makes it all work out so that the power all
    ends up being radiated, which is what you wanted in the first place.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  11. Tim Wescott

    Tim Wescott Guest

    If you start considering loss in the tuner and the line then yes, a
    greater mismatch between the antenna and the line will result in more
    lost power (and more component heating in the tuner). You really want
    to leave that subject be until you understand the properties of a
    lossless system.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  12. Well, there will be tuner losses, depending on how good the tuner's
    components are.

    Yes, you're confused.

    If the lossless transmission line (obviously no such animal exists) were
    tuned with a lossless tuner, then VSWR would not matter at all.

    The loss due to mismatch in any real system will depend upon frequency,
    VSWR, type of feedline, length of feedline, and the quality of the tuning
    circuits used to match the system to the transmitter.

    Let's take your example. 100 watt transmitter into, let's say 100ft. of
    feedline at 10:1 VSWR and assume tuner losses are negligble (they often
    aren't). Here are the losses for some different kinds of 50 ohm coax at
    10mhz:

    Belden 8237 2.19db
    Belden 9913 1.63db
    Belden 9258 3.19db
    Belden 8240 3.71db
    Belden 9201 3.83db

    So, what's obvious here is that different coaxes have different losses at
    high SWR. Why is that? Because as power is reflected back and forth in
    a transmission line, the losses accumulate. So line that is very low-
    loss to begin with will be less affected by high SWR than line that has
    moderate to high losses when flat.

    If 10 percent of the power in a line is lost travelling from the
    transmitter to the antenna, and if the antenna only radiates half that
    power, sending the rest back down the line, then 45 percent of the
    transmitter power is radiated immediately, while 45 percent is reflected.
    But only 40.5 percent reaches the tuner or transmitter. If ALL of that
    is re-reflected, then only 36.45 percent of the power is available at the
    second reflection to the antenna. The antenna will radiate 18.225
    percent of the transmitter's power at this point, making the total 63.225
    percent of the transmitter's output. Another 18.225 percent will be
    reflected again and, of that 16.4025 percent of the transmit power will
    live to be re-reflected from the tuner and 14.76225 percent will arrive
    at the antenna on the next bounce. Of that we can expect 7.381125
    percent of the transmitter's total power to end up radiated while an
    equal amount starts its way back to the tuner. Anyway, it becomes a
    pretty simple bit of limit math to predict exactly how much will be
    radiated and how much lost in the coax under these conditions.
     
  13. Hi Bill,

    As a short description, that is adequate.
    "Mismatch loss" is a system description, not an antenna description.
    So your reference is wrong.
    Two things wrong here:
    1. You say nothing of a tuner inline;
    B. Your math is wrong either way.
    It does matter if you lack a tuner (in more ways that one). Most
    discussion of "mismatch loss" omits such matters as tuners as it is a
    separable issue. Combining these topics raises your chance of
    confusion.

    73's
    Richard Clark, KB7QHC
     
  14. ehsjr

    ehsjr Guest

    I think the confusion (which *always* seems to arise)
    comes from the mix of the concept with the real world.
    The concept claims that the system is lossless, so the
    power bounces around until it eventually exits the "system"
    via the antenna. Real world, the system is lossy, so
    with all the bouncing around some of the power fails to
    leave the system via RF radiation and instead leaves
    the system via IR radiation. More heat, less RF.
    Tastes great, less filling. :)

    Ed
     
  15. Joerg

    Joerg Guest

    Unless your coax decides that it had enough of all this current. I had
    that happen once. The SWR had gone up and I pressed on. Then a muffled
    boom outside and it became 100% reflective. Now I had to get on a ladder
    and painstakingly scrape all the molten gunk off the stucco.
     
  16. Rich Grise

    Rich Grise Guest

    It truly wouldn't matter if there were no such thing as resistance
    and so on. The whole circuit could be tuned, with the transmission line
    a part of it, and all of the power would go out the antenna.

    Unfortunately, that's not the way reality works, more's the pity.

    Every time those "standing waves" bounce back and forth, they warm
    up the transmission line, the connectors, the transmitter tank, the
    transmitter itself, etc, etc, and Entropy is conserved. ;-)

    Hope This Helps!
    Rich
     
  17. [/QUOTE]

    Taking this at face value, yes, the "mismatch loss" is 4.8dB.

    73's
    Richard Clark, KB7QHC
     
  18. bw

    bw Guest

    Humor noted, but Entropy is not conserved.

    Entropy increases
     
  19. Buck

    Buck Guest

    Cecil,

    Please explain your antenna and radio. I am assuming you have a solid
    state rig with an so-239 connector on it for the basic and then you do
    what?

    Thanks
     
  20. Cecil Moore

    Cecil Moore Guest

    Seems to me that evolution violates that principle.
     
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