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Read SWR with an Oscilloscope

Discussion in 'Homebrew and Kit Projects' started by OA4AJP, Aug 10, 2013.

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  1. OA4AJP

    OA4AJP Ham Member QRZ Page

    Dear All:

    Is there a way to measure the SWR using an oscilloscope.

    I can measure the forward peak 2 peak voltage to get the power but I don't know how to measure the reflected power using an oscilloscope.

    Alfredo Mendiola Loyola
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    You can't without a directional coupler that isolates the forward from reflected wave current.

    If you add a good directional coupler, you can use the scope as the "detector" if it has enough bandwidth for the frequency you're using.

    I use an old HP 778D for that.
  3. OA4AJP

    OA4AJP Ham Member QRZ Page

    Can I use a Bird 43 Line Section with a sampler slug and connect the sampler slug to an oscilloscope?

    With this method will I get an accuracy of 5%?

    Alfredo Mendiola Loyola
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Nope. You need a real directional coupler. The Bird 43 is a one-way coupler, and the detector element can be rotated to aim in either direction (forward or reverse) but the sampler doesn't do that as far as I know. A directional coupler has two separate pickup lines, one for forward and one for reverse. Bird Thrulines don't.
  5. N8ZU

    N8ZU Ham Member QRZ Page

    Hey OM
    I'd gut an ole SWR meter connect your X and Y inputs of the Scope to the RF input of the detector diodes in that SWR meter this will produce the pattern on the scope called Lissajous then it's just a matter of calibrating the pattern using known resistance from say 300 ohms down to 50 ohms being 1:1.
    73 OT
    de n8zu
  6. KE3WD

    KE3WD Ham Member QRZ Page

    Any such scheme would merely be replacing the meter movement with the scope.

    And, really, there would be nothing to gain by doing so, the meter is quite good enough here.

  7. DL3PB

    DL3PB Ham Member QRZ Page

    Hello Alfredo,
    why not use a homemade resistive coupler, a Wheatstone brigde, and measure the voltage across the load and calculate the
    SWR directly from this voltage compared to half(!) the source voltage level - if you have one of these modern DSOs, it may even provide
    some simple math functions to further ease this calculation - if you can make sure for example that supply voltage for the brigde is 2V (peak or whatever ), the voltage across the load gives you the SWR-value directly, if your DUT has a higher impedance than 50 Ohms,
    else all you have to do is to invert this value in case of impedances lower then 50Ohms.
    This is by far the simplest method I know of, all other method require either special parts or some mathematical (or graphical) effort.
  8. DL3PB

    DL3PB Ham Member QRZ Page

    sorry Alfredo, something wrong with my approach, have to think it over again!
  9. KI6J

    KI6J Ham Member QRZ Page

    A simple bridge is described on Harry's Homebrew page. >> Projects >> Useful Circuits >> PCB VSWR Bridge (direct links don't work)
  10. K7MEM

    K7MEM Ham Member QRZ Page

    Direct links do work, if you know how to extract them. The direct link would be, but that would be without the frames that have the navigation. Harry uses frames on his site so it's not obvious where the source is coming from. Most web designers have done away with frames because it is not supported in HTML5. But at the pace they work, it will probably work for some time.
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