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Return Loss and VSWR

Discussion in 'Beginner Electronics' started by Ernie Werbel, Mar 26, 2007.

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  1. Ernie Werbel

    Ernie Werbel Guest

    Hi all. I am an EE student currently working for a company that
    manufactures RF and Microwave components. I find this technology
    fascinating, and I beleive that RF may be something that I want to do with
    my life. Unfortunately for me, my job is in production; testing to be
    specific. Although I have learned a lot, I feel limited for the time being.
    After all, now that I have learned how to program the network analyzer,
    everything else is just numbers on a screen with a PASS/FAIL sign. But
    anyway I digress...

    I have done research on my own about what I am doing at work. I feel that I
    am finally having a grasp on what the concepts are - coupling, return loss,
    VSWR, etc - as in having a feel for what is actually going on rather than
    just comparing readings to limit lines. So I guess I will say how I
    understand it and I would greatly appreciate it if anyone could correct me
    where I am wrong (because I probably am in plenty of areas!). (-:

    VSWR Voltage STANDING WAVE Ratio tells me that the nodes and antinodes of
    the wave are stationary, so the signal varies in amplitude until it reaches
    the end of the rail, the mainline. If it is terminated with ideal impedance
    matching (50-ohms), then the signal is completely absorbed. However if the
    impadnaces are unmatched, say 50 to 49.9 ohms, some of the signal is
    reflected back, 180° out-of-phase. It is 180° because the reflected signal
    is now going in the opposite direction (like making a U turn?). This then
    is algebraically added to the original. So it follows that if there is no
    termination (or short to ground?) the entire signal will be reflected back
    180° out-of-phase and cancel out the original. This is how I understand
    reflection. I am still stuck on what the VSWR ratios actually mean. I know
    1:1 is ideal but how should I interpret 1:1.15 for example?

    I have also learned that the 1/4 wavelength is important in power dividers
    for determining trace length... is this because the voltage (or power, for
    that matter) amplitude is at its greatest (90°)? This then cooresponds to
    1/4 length of the physical wavelength? I also leanred that the trace width
    has different impedances (thinner traces are higher imp.). It is starting
    to go over my head at this point but as I go back and forth I pick up more
    and more of this.

    Lastly I will present a short gripe. I am taking classes that supposedly
    teach this stuff but we don't do like anything in the class... I guess
    that's community college for you. At any rate, I am not satisfied to just
    look at pass/fail because electronics theory is my passion and I want to
    feel it, know it, and live it.

    Thanks to anyone who reads all this for any help in advance!
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