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.
  11. OA4AJP

    OA4AJP Ham Member QRZ Page

    Thank you for your help

    Lima, Peru
  12. KI6J

    KI6J Ham Member QRZ Page


    Thanks for the useful tip. "View Frame Source" in my browser did the trick for me.

  13. K7MEM

    K7MEM Ham Member QRZ Page

    Yes, you can pick it out of the source code. However, if you are using Firefox you can just right click in the frame and left click to select "This Frame -> Show only this frame" and it will pull up the page without any of the navigation frames. The URL will then be in the URL box.

    Harry has a small script in the pages to try and prevent a right click. It's a simple matter to render the script useless in Firefox, by disallowing the context menu from being disabled. But it's a little more complicated with IE, where you actually disable Javascript and the restart IE. Since it's the only Javascript in his page, disabling Javascript doesn't matter. But you will have to re-enable it when you go to other sites. There are also a couple of hacks for this, but they don't always work. Overall, it's just something that slows you down a little bit.
  14. G0HZU

    G0HZU QRZ Member

    I suppose that if the (2 channel?) scope had enough bandwidth and was terminated in 50R then the scope could replace each detector at the coupled ports. Then the complex impedance could also be measured (using the scope) if channel 1 of the scope went to the FWD port and channel 2 went to the REV port of a decent (homebrew?) coupler.

    I think this would work quite well down on the lower HF bands with a fast scope. Not as good as a real VNA but it should give useful results :)
  15. K0RGR

    K0RGR Premium Subscriber QRZ Page

    We used to use scopes to check SWR on microwave components using directional couplers as Steve WIK suggests. In the microwave setup, the directional coupler separates the reflected power from the forward power, and feeds it to a detector diode through a 'DC block' capacitor. First, a reference voltage is set on the scope with an open circuit and again with a short circuit - both representing 'infinite' SWR - usually the result will be close to the same for both open and short. Then, the load you want to check is placed on the forward output of the directional coupler, and the reflected voltage is read. Using the scope, you calculate the dB difference between the 'infinite SWR' and the SWR of the load to be tested. The difference is the return loss. Then, you can use a calculator or a convenient table to calculate the SWR from the return loss in dB. 6 Db of return loss is approx. 3:1 SWR. 9.5 dB is 2:1.
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