Connect with us

VSWR doesn't matter?

Discussion in 'Electronic Basics' started by billcalley, Mar 12, 2007.

Scroll to continue with content
  1. Jimmie D

    Jimmie D Guest


    Your mistake is that you assume the output of the tx is 50 ohms, in the case
    you stated the transmitter must be matched to the impedance it sees looking
    into the transmission line.
     
  2. Hi Jan,

    Actually, there is a transformer there in the typical Ham transmitter
    (and probably in every general purpose power source) that typically
    transforms the native Z to the output Z. This is a step up for solid
    state, and step down for tubes. In the solid state rigs, it is a
    literal transformer feeding the 1-2 Ohms through a 5:1 winding ratio
    to a switched bank of low pass filters. This stuff is mud ordinary.

    As for the reflected energy, depending upon the phase it will either
    combine destructively (heat) or constructively (cool) in the extremes.
    There are, of course, 179 degrees of variation between these extremes
    before they repeat themselves again. Cooling, of course, is something
    of a misnomer as nothing useful is happening (poor power transfer) so
    perhaps the terms should be destructive through uselessly benign.

    73's
    Richard Clark, KB7QHC
     
  3. I understand the concept, but what I don't
    In my opinion the simplest way to answer your question is that you are assuming
    that the transmitter is equivalent to a 50 ohm load, which is not true because
    the transmitter is instead equivalent to the series of a 50 ohm load and a
    voltage generator.

    A simple DC example grossly clarifies thre issue: connect a 12V battery to the
    series of a 50-ohm load and another 12V battery. How much current flows through
    the load? Naught (assuming the correct polarity). So no power is dissipated in
    it.

    73

    Tony I0JX
     
  4. Brian Howie

    Brian Howie Guest

    It matters when it changes suddenly, like mine did recently on my 70MHz
    beam, when one of the elements came off in a gale.

    Brian GM4DIJ
     
  5. As pointed out, VSWR does matter. A lot of bouncing means you heat the
    transmission line with the power instead of radiating the power.
    'Doesn't matter', really means it can be tolerated if need be.
    Here is what you are missing. In the case of the output, (real/resistive
    component of the transmitter), seeing the reflected wave, it is _not_
    reflecting that power back up the transmission line as you think it is.
    It would go back to that real impedance and heat the transmitter. Here
    is what is done with a miss match in the real world.

    trans-output -> match -> line -> antenna

    The 'match' is where the magic happens. All the energy coming down the
    line that got reflected from the antenna 'sees' the 'trans-output ->
    match' as a perfect reflector and gets bounced back[*]. On the other
    side of the match is the trans->output. There the trans->output sees a
    perfect impedance, (technically, the conjugate of the trans->output), so
    that all the power travels through the match toward the antenna.

    The magic is that when the match is tuned, both of the above conditions
    are satisfied.

    *The reflected wave sees a purely reactive reflector not just because of
    the network but also because of the output power of the transmitter.
    Without transmitter power the impedance as seen from the load will
    dramatically change.

    Best, Dan.
     
  6. Hi Jimmie,

    At the risk of yet another, non-quantitative reply I will repeat:
    THAT is true, and it brings us to the point of all this energy
    sloshing around until the antenna finally dissipates it out into the
    Æther. It is the reflection off the mismatch of the tuner (the
    mismatch seen by the antenna as source to the line going back) that
    prevents energy from presenting any destructive results to the source
    - the whole point of using a tuner in the first place.

    73's
    Richard Clark, KB7QHC
     
  7. Jimmie D

    Jimmie D Guest

    Correct but I just want to remember that the purpose of the tuner is to
    match the impedance of the transmitter to the impedance of the antenna/
    transmission line.The standing waves can be viewed as a reflect voltage, a
    reflect current or as a reflected impedance. Besides I thought there had
    been enough quanitative analysis of the question and was hoping a simple
    answer may be enough to turn on the light bulb for the OP. If he still
    wanted to know more I figure he would ask.
     
  8. Hi Tony,

    Turn the second battery over. Double the power is dissipated in it.

    Phase, you can't live with it, you can't live without it.

    73's
    Richard Clark, KB7QHC
     
  9. Of course. Mine was just a DC example to illustrate things in a simple manner.

    When the transmitter is properly tuned, the phase relationship is such that the
    reflected wave does not get dissipated at all into the 50 ohm output of the
    transmitter, and is then reflected back to the antenna

    Tony I0JX
     
  10. John Smith I

    John Smith I Guest

    Kewl, then I'll just run a tap directly to the 5000 ohm plates and start
    a long chat up ... what the heck is all those pi matching components in
    the way of the rf? Probably some loss there! ROFLOL!!!

    JS
     
  11. Jim Backus

    Jim Backus Guest

    OK ;-))
     
  12. Jimmie D

    Jimmie D Guest

    Saying that SWR doesnt matter is a rather broad statement(like saying never
    or always) but I have know of antenna systems having an SWR of 30:1 and his
    was normal. The feedline was balanced line made of 1 inch copper. Of course
    an SWR lie this on coax could be fatal to coax and equipment. A more common
    example of this is the 1/4 wl matching section on a J-pole antenna. It
    matches 50 ohms to a few Kohms so an SWR of 60: 1 or so would not be unusal
    here.S oas long as the feedline can handle the current and voltage peaks
    without much los it doesnt matter much as long as the source impedance is
    matched to the impedance at the input to the transmission line.Im sure there
    is a practical limit though.

    Jimmie
     
  13. If you want a quick lesson in high vswr find a ham with an old tube
    transmitter and see if he will hook it up to a mismatched load. The
    cherry red plates are the reflected energy being absorbed. Transistors
    will just turn to smoke under the same conditions.

    Dave WD9BDZ
     
  14. Jerry Martes

    Jerry Martes Guest

    Hi david

    Wouldnt it be OK to have a high VSWR along the transmission line if the
    "tank ckt" can be adjusted to match the load to the transmitter output
    impedance? That is, the VSWR along the transmission could concievely be
    high, yet, with proper "tank ckt" adjustment that impedance seen by the
    output circuit (plate) wouldnt result in a "cherry red plate".
    What I am asking is ? is the transmission line VSWR directly related to
    "plate reddening"?
    I'm more asking than *telling*.

    Jerry
     
  15. Roy Lewallen

    Roy Lewallen Guest

    Unfortunately, you'd be learning the wrong lesson.

    The cherry color is due to the transmitter being loaded with an
    impedance it's not designed for, causing the final to run at low
    efficiency. You can disconnect the antenna and replace it with a lumped
    RC or RL impedance of the same value and get exactly the same result.
    Alternatively, you can attach any combination of load and transmission
    line which give the same impedance, resulting in a wide variation of
    "reflected energy", and get exactly the same result. All that counts is
    the impedance seen by the transmitter, not the VSWR on the line or the
    "reflected power".

    The problem is that the idea of "reflected energy" turning the plates
    hot is so easy to understand, that people aren't willing to abandon it
    simply because it isn't true.

    See http://eznec.com/misc/Food_for_thought.pdf for more.

    Roy Lewallen, W7EL
     
  16. Roy Lewallen

    Roy Lewallen Guest

    Yes! All that matters to the transmitter is the impedance it sees. It
    doesn't know or care that you've mathematically separated the delivered
    power into "forward" and "reverse" components. It doesn't know or care
    what the SWR is on the transmission line connected to it, or even if a
    transmission line is connected at all.
    Absolutely not.
    That's the first step in learning.

    Roy Lewallen, W7EL
     
  17. Bob

    Bob Guest

    It appears that you agree with that part of my post but you are
    drawing
    an invalid conculsion from it. I never suggested that the passive
    matching
    network usually found in a transmitter output is unnecessary.

    Bob
     
  18. Owen Duffy

    Owen Duffy Guest

    (Don Klipstein) wrote in
    ....
    ....
    I thought we put one of these fires out just a few days ago!

    Owen
     
  19. Jerry;

    The point I was trying to make is that the reflected current is
    disapated as heat in the finals if the transmitter isn't matched to the
    load.
    In a tube radio the tank circuit is the equivilent of an antenna
    match/tuner and converts the 2000 or so ohms at the plate to the 50 ohms
    of the transmission line and the unknown ohms of the mis matched antenna.


    Dave WD9BDZ
     
  20. Bob

    Bob Guest

    As Tim Williams alludes, it depends on the transmitter design.
    It will often be complex rarther than resistive. Since the active
    device changes impedance during a single cycle of the RF
    signal it may not even be adequately described by a single
    value in ohms for a paticular frequency if you wish to
    analyse the case of forward and reflected power.

    Consider a class C or class E output stage with an
    output transistor that is low impedance during
    most of the positive half of a cycle of signal and mostly
    somewhere near open circuit for the negative half
    of the cycle. It seems to me that the effect of reflected
    power is going to be different depending its phase
    relative to the forward power.
    I think this also applys to a lesser extent to a class
    A PA with a nice hi-Q tank circuit.

    As usually whan this topic comes up, It don't feel
    like we have arrived at a usefull and convincing model
    of what happens, possibly because simple
    descriptions don't cover everything.

    Bob
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-