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Understanding SWR, my antenna..

Discussion in 'General Technical Questions and Answers' started by K1IGS, Nov 3, 2018.

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  1. K1IGS

    K1IGS Ham Member QRZ Page

    So, I want to know if I'm understanding this properly.

    My Yaesu feeds my G5RV via an auto tuner. The auto tuner shows my radio a nice 1:1ish match. That said, it's not actually making my antenna any more efficient, so even though my radio is happy my G5RV still has roughly 2.0 SWR on 20 meters.

    So, really my 100 watts is more like 50ish watts.

    Correct?

    Thanks!
     
  2. N8TGQ

    N8TGQ Ham Member QRZ Page

    Correct. All the tuner does is match your antenna to the rig.
    Not sure of the losses though.
     
    KU3X likes this.
  3. KM3F

    KM3F Ham Member QRZ Page

    Let's take this a little further.
    A 2 to 1 mismatch does not mean half the 100 watts is lost.
    Look up a chart of power vs SWR.
    You will see a 2 to 1 is only a 11% power loss.
    You can't stop at the last the last statement you made assuming a 50% loss because it's not figured that way.
    Also a 2 to 1 match can have two different values.
    For example a 100 ohm antenna presents a 2 to 1 via 100/50.
    If the antenna were to present a 25 ohm load, it would still present a 2 to 1 via 50/25.
    Your SWR can't indicate this unless you understand it's readings when you move frequency.
    Even then you can't tell values unless using an instrument such as an Antenna Analyzer.
    Good luck.
     
    Last edited: Nov 3, 2018
    K4VLF, CX3CP and AC8UN like this.
  4. KM4DYX

    KM4DYX XML Subscriber QRZ Page

  5. K1IGS

    K1IGS Ham Member QRZ Page

    Thanks folks - helpful as always!
     
  6. K4VLF

    K4VLF XML Subscriber QRZ Page

    Soap box: I think we should throw out the very confusing term "SWR" and instead focus on reflected vs forward power. Tune your rig for max forward, while minimizing reflected power.

    2:1 SWR is about 10% reflected power. And reflected power is not always wasted power. Reflections book is a good readd (not easy tho!)
     
  7. KB4QAA

    KB4QAA XML Subscriber QRZ Page

  8. N2EY

    N2EY Ham Member QRZ Page

    Yes and no.

    The first part is correct - the "auto tuner" simply transforms the impedance at the shack-end of the line to 50 ohms.

    (Hams use the terms "antenna tuner" and "ATU" and such, but those terms are misleading. A much better term is "Transmatch", from "Transmission line Matcher". Because what it really does is to match the rig to the transmission line.)

    The second part is not correct. How much power gets to the antenna is dependent entirely on line and matcher losses, not SWR.

    73 de Jim, N2EY
     
  9. KU3X

    KU3X Ham Member QRZ Page

    The more coax you add to the mix with your less than perfect antenna, the more losses you have. You can actually add enough coax to the mix that you can see a 1 to 1 swr at the end of the coax. That does not mean that 100% of the power is reaching the antenna. Bottom line: Just because you read a reasonably low swr does not mean all of you power is going to the antenna.
    Example
    An ocf40, fed 1/3 of the way in from the end and used on 15 meters will sbow an 18 to 1 swr at the feed point of the antenna, at the 4 to 1 balun. But, at the end of the coax in my shack i read about 3.5 to 1 swr. That gives one the impression that the antenna is fairly efficient on 15 meters....and it is far from efficient.
     
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    If your line loss were "zero" (say, an infinitesimal length transmission line), and your transmatch can transfer all available power from your transmitter to the load, your 100W would still be 100W at the antenna regardless of its VSWR(50).

    On HF (<30 MHz) unless you have a terribly lossy transmission line, the "increased loss" from a 2:1 termination SWR is so small you likely couldn't measure it. The loss adder becomes noticeable with gross mismatches, 10:1 and above.

    But that "10:1 and above" isn't so unusual. If you center-feed a dipole that's 1-wavelength long (as opposed to the standard 1/2-wavelength), SWR can be 50:1. That's when you see some really big line losses, unless, again, the line has zero loss (which would usually mean the antenna is connected directly to the transmatch with no line between).
     
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