My antenna measurements with RTL-SDR

In my Antenna measurements with RF noise source posting I described how antenna measurements can be done with RF noise source, directional coupler and SDR radio (DVB-T stick). To do measurement we connect the noise source to the output of the coupler, the antenna to the input of the coupler and the RTL-SDR dongle to the coupling (CPL) port of the coupler.

Because I did not have a nice 50 ohms directional coupler at the moment and getting one would need some investment (time and money to get one), thought if I had something suitable already.

I found some TV antenna network taps and remember that they are actually directional couplers. The downside with them is that they would be made for 75 ohms impedance and work best only at TV antenna network frequencies (around 40 MHz to 900 MHz or slightly more). But I though that they might be somehow used as directional couplers for 50 ohms antenna measurements if I an lucky. Let’s try to test them. I fortunately had access to some RF measurement instruments to see how those antenna taps work as directional couplers. And I also had antenna to measure (antenna tuned for 450 MHz).

First directional coupler I had: FA 1-8

Taps 5-1000 MHz

FA 1-8

Abzweigdämpfung / Tap loss (IN-TAP) 8,5 dB

Durchgangsdämpfung / Through loss (IN-OUT) 2 dB

Entkopplung / Isolation (OUT-TAP) 23 dB

Rückflussdämpfung / Return loss (IN/OUT/TAP) 18 dB

Measurements of this directional coupler on 50 ohms system (50 ohms source, 50 ohms load):



Results with 5o ohms terminator,  FA 1-8 and Spektrum RTL-SDR Spectrum Analyzer Software:



Measurement setup with antenna:

Results with 450 MHz antenna connected:



Second directional coupler: FA 2-12

FA 2-12 D

Abzweiger/Taps 5-1000 MHz

Technische FA 2-12 D Daten:

  • Frequenzbereich: 5-1000 MHz
  • 12dB Abzweigedämmpfung
  • Schirmungsklasse A
  • CE-Prüfzeichen
  • Anschlüsse: F-Connectoren
  • Massives Gehäuse aus Zink-Spritzguss
  • Ein- und Ausgänge brummentstört

 1 Signal Eingang
1 Signal Ausgang
2 Signal Abzweigung 12dB


Measurements of this directional coupler on 50 ohms system (50 ohms source, 50 ohms load):

Results with 450 MHz antenna:


End results

The end result was that those antenna taps worked quite acceptably as directional coupler for antenna measurement. They are not ideal, but they are useful tools. In my measurements it seems that the first 8 dB tap was slightly better for this application.



  1. Tomi Engdahl says:

    This RF BRIDGE 0.1 – 3000 MHz from
    looks really interesting. I have to test out it.
    I ordered one and plan to review it when I receive it.

  2. Tomi Engdahl says:

    #158: Directional Coupler Basics & how to sweep SWR of an antenna | Return Loss | VSWR

    This video describes the basic properties and specifications for directional couplers, and shows their basic operation on an oscilloscope. Typical applications are discussed, along with a practical example showing how to sweep the VSWR, or Return Loss, of an antenna using a directional coupler and a spectrum analyzer with a tracking generator. The resulting return loss measurement is compared to the SWR measured on an MFJ-259B antenna analyzer.

  3. Tomi Engdahl says:

    Return Loss Bridge-Setup Procedure

    In this informational video, Jim Beck at Eagle guides you through the proper procedures for taking antenna measurements using the Eagle Return Loss Bridge.

  4. Tomi Engdahl says:

    RIGOL DSA815 Spectrum Analyser discovers real-time VSWR of 144MHz 2m antenna… in real time

    Using RIGOL Spectrum Analyzer and VSWR Bridge to Measure Antenna


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