• Building a QFH antenna for 137.9MHz

    Leonardo • 04/13/2026 at 17:52 • 0 comments

    Hi everyone!

    This will be the first project log for the B.R.A.S.S. ground station. 

    Today I finally tested the first antenna of the project, and it's working great. For this reason, I decided to make a post about it, to start well.

    WHAT IS A QFH ANTENNA & WHY BUILD ONE?

    The Quadrifilar Helix (QFH) antenna is a popular type of antenna in the Amateur Satellite reception world.
    It consists of four strands of copper wires twisted into a precise helical shape, forming two nested loops of slightly different sizes, and by having one loop slightly larger than the other, the antenna creates a 90° phase shift between them.
    This internal phasing enables the Quadrifilar Helix antenna to capture circularly polarized radio frequency transmissions, such as those from NOAA or Meteor-M series satellites.
    The antenna is also omnidirectional, so it can receive the satellites without the need for rotors or other pointing systems.

    HOW I BUILT ONE

    First things first, you need to perform some calculations to design the antenna correctly for your chosen frequency.
    Luckily, there are some great tools online that simplify this, just like the one I used.
    You can find it at this link: jcoppens qfh antenna calculator online

    I inserted the values that I wanted in the calculator to get the values/measurements I needed to build the antenna

    Design frequency137.9MHzThe frequency of the antenna. It will be tuned for Meteor-M LRPT
    Number of turns0.5Default
    Length of one turn1 [wavelength]Default
    Bending radius2 mmI used 2mm wires, so the radius will be small
    Conductor diameter2 mmI used 2mm copper wire, but the bigger the diameter,
    the larger the bandwidth of the antenna, and easier will the
    tuning/construction be
    Width/height ratio0.44Default

    After calculating all the dimensions, you can easily follow the images/diagrams provided in the online calculator to build the antenna.
    I used one central 32mm wide PVC pipe as the main pole for the antenna. Then, I cut 12 smaller rigid plastic poles from trash I had lying around (6 red for the outer loop, 6 yellow for the inner loop).
    I 3D printed some adaptors to connect the smaller poles to the main pole.

    Twisting the wire can be a little tricky, but if you pre-cut the wire to the (almost) right length (value provided by the calculator) and fold it and twist it a bit before inserting it in the antenna structure, you'll be fine.

    Instead of a balun or a coax cable around the main tube, to limit common-mode currents, I inserted 5 ferrite cores at the top part of the coaxial cable inside the tube.

    Make sure to solder the top coaxial connector correctly with the correct loop sections.
    If you need it, at this link you'll find a good video that lets you see how to build a QFH antenna and how to properly solder it: qfh antenna build video

    The completed antenna:

    TEST RESULTS

    I first tested the antenna with my NanoVNA to get an idea of how good the antenna I built was. I had to do some tuning (very small), but in the end, the antenna was luckily very well matched with 137.9MHz. I managed to get an S11 of about -20dB at 137.9MHz, which is really nice in terms of antenna efficiency (1.22 VSWR).

    Then, I attached the antenna to a filter and an RF amplifier and tried receiving some data from Meteor-M 2 4 passage. The test was successful, and I managed to achieve about 15dB SNR on SatDump during the passage. I was holding the antenna with my hands and a chair for some part of the passage, so not the best setup, but it was enough.

    Here you have the data I got: