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Thread: Read SWR with an Oscilloscope

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  1. #1

    Question Read SWR with an Oscilloscope

    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.

    73s
    Alfredo Mendiola Loyola
    OA4AJP

  2. #2

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    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.
    What if soy milk is just regular milk introducing itself in Spanish?

  3. #3

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    Quote Originally Posted by WB2WIK View Post
    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.
    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%?

    Regards
    Alfredo Mendiola Loyola
    OA4AJP

  4. #4

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    Quote Originally Posted by OA4AJP View Post
    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%?

    Regards
    Alfredo Mendiola Loyola
    OA4AJP
    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.
    What if soy milk is just regular milk introducing itself in Spanish?

  5. #5

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    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. #6

    Default

    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.


    73

  7. #7

    Default

    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.
    73
    Peter/DL3PB

  8. #8

    Default

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

  9. #9
    Join Date
    Aug 2009
    Location
    Burbank, CA
    Posts
    833

    Default

    A simple bridge is described on Harry's Homebrew page.
    sm0vpo.com >> Projects >> Useful Circuits >> PCB VSWR Bridge (direct links don't work)

  10. #10
    Join Date
    Aug 2004
    Location
    Ash Fork, AZ - 60 miles south of the Grand Canyon, off of Route 66.
    Posts
    798

    Default

    Quote Originally Posted by KI6J View Post
    A simple bridge is described on Harry's Homebrew page.
    sm0vpo.com >> Projects >> Useful Circuits >> PCB VSWR Bridge (direct links don't work)
    Direct links do work, if you know how to extract them. The direct link would be http://sm0vpo.com/use/vswr-00.htm, 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.
    73, Martin, K7MEM
    Ash Fork, AZ
    [URL]http://www.k7mem.com[/URL]

    In my area, it seems that every pickup truck or SUV comes with one or more dogs. It's so common that I can only assume that the dog(s) must come with the vehicle. So logic tells me that, if you want to keep the truck for a long time, go for the multi-dog option. Otherwise, if the dog dies, you have to buy a new truck. I have five dogs (4 dogs as of 4/4/2013, RIP Katie), so I'm set for a few years.

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