SignalsEverywhere: Spectrum Analyzer and Tracking Generator with Pluto SDR

In the latest video on the Signals Everywhere YouTube channel, Sarah investigates how a PlutoSDR can be used as a Spectrum Analyzer with the SATSAGEN software. The SATSAGEN software is able to work as a spectrum analyzer by rapidly sweeping over multiple frequencies and stitching the spectrum slices together. It support SDRs like the HackRF, PlutoSDR and RTL-SDR (in receive mode only). The PlutoSDR can transmit, so it is able to work as a full spectrum analyzer with tracking generator, allowing users to measure RF devices such as filters, tune antennas, and work as a frequency generator.

In the video Sarah demonstrates how to use the PlutoSDR and SATSAGEN to measure our RTL-SDR Blog Broadcast FM filter, and to tune our multipurpose dipole antenna.

Spectrum Analyzer and Tracking Generator with Pluto SDR

13 comments

  1. KB6

    He had me up until the “flashlight scenario” That’s when i knew the jig was up !

    I love a good laugh thanks man I needed that 😉

    Surely you can’t be serious
    I am serious… and don’t call me Shirley!

    ha ha ha

  2. snn47

    Unfortunately it is a bit more complicated, since you forgot to mention the impact of losses/attenuation between signal source (antenna or transmitter) and the sink (receiver or antenna).
    The above assumes that transmission lines like coax cable, chicken wire or waveguide are lossless, in reality losses/attenuation will increase with frequency and vary from cable design to design.
    As consequence the higher the attenuation the lower the meassured SWR or return power will be. If e.g. a cable is long enough to provide attenuation to convert all RF-energy to heat, it doen’t matter if the cable is at the end open, shortened or connected to a any impedance between 0 and infinity, you will always see a SWR of 1.
    Then ther are other effects when the cable length and properties can provide a trasnformation of the sinks impedance, again a shortened or open end coax cable can show a SWR of 1.
    vy 73
    _.. .._. ….. ._.. _.__

    • KB6

      Thank you
      Yes its extremely more complicated then that I was trying to keep it as simple as possible in layman’s terms
      Maybe they should change it to restricted waves instead of reflected ? LOL or just call it Standing Wave Ratio (SWR)
      funny how people actually think that in some weird way RF gets reflected back into the radio down the coax?
      I’m not sure how a transmitter would even see any RF coming back to it as its transmitting (Transmitters do not receive RF) Now if we are talking about (Spurious Radiation) That’s another topic and would go way beyond the scope of this thread I Keep ( RF choke / Balun) on all my coax cables and that has nothing what so ever to do with SWRs !
      Some people even believe that a antenna tuner somehow changes the antenna length Now that’s crazy ! All it dose is change the impedance to something the transmitter can deal with The antenna is still the same exact length with the same SWRs as before and NO the RF power is not returning to the tuner That’s NONSENSE ! However the transmitter is putting out full power and with modern rigs this is a good thing as they automatically back down power with rising SWRs to protect the finals !

      Anyway…. I just wanted to jump in here to make a quick comment about what Sara said “Reflecting RF back into the Pluto SDR”

      Sorry if I offended anyone That was not my intentions

      73s

      Mind blowing thought
      When you shine a flashlight at a mirror dose it reflect back into the flashlight ?
      Think about it dose it really ??? or is it just a illusion ?

  3. Jamie

    No, you are wrong, anytime an rf signal encounters a non-perferct match some power is reflected back. This is the entire point of return loss/vswr measurements. They a measure.ent of the ratio of forward to reverse power and are used as an indication of signal path quality.

    • KB6

      I think you misunderstood me?
      If you watch the video again at 7:37 “the rest of it is being reflected back into the SDR” Now that’s just silly!
      The name ” reflected power” is not correct and is probably one of the most misunderstood term in radio

      When the line and load impedances are identical and the SWR is 1, all of the RF power that reaches a load from a transmission line is utilized by that load. When the load is an antenna, the utilization takes the form of EM-field radiation. If the load is a communications receiver or terminal, the signal power is converted into some other form, such as an audio-visual display. If the impedance of the load is not identical to the impedance of the transmission line, the load does not absorb all the RF power (called forward power) that reaches it. Instead, some of the RF power is sent back toward the signal source when the signal reaches the point where the line is connected to the load. This is known as reflected power or reverse power.
      This sounds confusing ? well don’t feel bad it dose to most people 🙂
      I think the best way to explain this without going into a lot of crazy science

      Take a garden hose and put your thumb on it More pressure and less water coming out the end right ? now think about it Is water somehow getting returned to the supply line ?? Nope !
      Now if you look at my (copyed text) above ” Instead, some of the RF power is sent back toward the signal source when the signal reaches the point where the line is connected to the load”
      This is not exactly true however a SWR meter will read it this way and it sure looks like its doing that right??
      Wrong ! there is no power some how magically bouncing back to the transmitter LOL whats actually happening
      is that the transmitter is seeing something other then lets say a 52 ohm load (mismatch) it would need to get rid of the unwelcome power It dose this by dissipating it as heat A light bulb is a good example of this Is the power being returned to the power company for a brighter bulb ??
      signal source = (coax)
      Load = (Antenna)
      There is nothing going back to the SDR however things are heating up fast !
      Hope this helps 🙂

      • snn47

        Unfortunately you will confuse readers not familiar with the basic ubderstanding, which is why I reply to youe examples.
        The comparissions do not reflect the conditions of a RF-generator connected to a complex transmission line which is terminated by some complex impedance ranging from short to open end:
        – the flashlight scenario is not a transmission line scenario since it lacks the tranmission line, since the light is “radiated” in free space
        – Both the garden water house or power line examples are only influenced by resistance, but do not account for the complex impedance. AC power generator behaves different than a RF-generator since it does not rely on impedance matching of the AC lines, but it has also problems when loaded by complex loads.

        • KB6

          WOW
          What the ??????????????????
          OOOOOK
          You say I’m confusing? are you trying to make a point ? Flashlight??? What?? Garden hose? What??
          A holiday hoodie whaty?????

          Lighten up dude maybe take chill pill?????? 🙂

          I’m sure your joking right????
          If so that’s ok i can take a joke ha ha 🙂
          I honestly thought you was serious from your first reply but now i see I’ve been had

          You got me 🙂 thanks ha ha ha 🙂 very funny !

          Take care

          • snn47

            @all A simple introduction/explanation to SWR and refelected power can be found in ARRL publications e.g. Understanding SWR by Example

            https://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf

            @KB6 Jamie already made my point, youre wrong power is reflected at points along a cable where a missmatch occurs. If a very short pulse is inserted into a cable the reflections can be seen/meassured in a fast oscillospe, for long cables the time delay is used to identify the distance to those points on a cable.You don’t need detailed (mathematical) explanations, but you need to stick to facts and not confuse people.

            If you use an example to provide a simplified explanation it should match the conditions.

            • Anonymous

              NONSENSE !
              I don’t need to prove a point here as its just basic science !

              Please do some some research before you try to prove something that you cant with some crazy far out theories
              Your not getting it apparently and it becoming even more obvious your just a troll and normally I would just ignore people like you !

              But I just cant stop laughing about that “flashlight scenario”

              Anyway…. go ahead and try to prove the earth is flat and we didn’t go to the moon and Covid-19 is a hoax and so on and so on………..

              This is ridiculous !

              “What we’ve got here is… failure to communicate. Some men you just can’t reach”

  4. KB6

    “Reflecting RF back into the Pluto SDR” ???
    Sara
    I love your videos sorry I just had to jump in here as your comment is just plan wrong and I’m sure you would agree with me
    RF Energy dose not magically reflect from the antenna The energy is dissipated as heat at the transmitter finals therefore less energy is transmitted at the antenna “there is no such thing as reflected power” all the power from the antenna is transmitted just less then desired with a poorly matched antenna

    Looking forward to your next video
    Nice job 🙂

    • Nate

      I think you need to find the book Reflections III authored by Walt Maxwell, W2DU (SK) and read it carefully and then read it again. If you still have questions, read it a third time!

      Here is a simple test. Put a Watt meter inline after your transmatch on the line leading to the antenna. Set the transmatch for best match to the transmitter. What does the Watt meter after the transmatch show?

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