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  1. #1

    Join Date
    Jul 2019
    Location
    North Saanich, BC
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    2

    Default Balun for Underwater Antenna

    I have designed and built a 27 MHz QFH antenna for a fresh-water wireless sensor system (1.2 m wavelength in water). The QFH presents a balanced load. Since the medium is water, the antenna impedance is low, measuring 13+j0 ohms at 27 MHz with an antenna analyzer (MFJ-259C with a half-wavelength of coax to the antenna).
    The radio module is 50 ohm unbalanced, hence the need for a balun.

    I wound a balun on a TDK B62152A0001X001 binocular core using #26 magnet wire: 8 turns and 4 turns.
    After installing the balun at the antenna feed point, I measure Z = 52+j34 ohms. The real part of Z is good but not the reactive part.
    Should I use a capacitor to compensate?

    Alternatively, given that this is a low-power (+15 dBm) narrowband application, would I get better performance by using a LC balun network?

  2. #2

    Join Date
    Oct 2009
    Location
    Oulu, Finland
    Posts
    614

    Default

    Unless your anticipated range is less than one metre, I wouldn't use 27 MHz. In most cases I have heard UW comms takes place below 30 kHz (reference: https://en.wikipedia.org/wiki/Commun..._low_frequency)

    There are cases where folk are trying to use 2,4 GHz (reference: https://www.mdpi.com/1424-8220/16/6/890/pdf)

  3. #3

    Join Date
    Jul 2019
    Location
    North Saanich, BC
    Posts
    2

    Default 27 MHz is OK in Fresh Water

    Quote Originally Posted by OH8GAD View Post
    Unless your anticipated range is less than one metre, I wouldn't use 27 MHz. In most cases I have heard UW comms takes place below 30 kHz (reference: https://en.wikipedia.org/wiki/Commun..._low_frequency)

    There are cases where folk are trying to use 2,4 GHz (reference: https://www.mdpi.com/1424-8220/16/6/890/pdf)
    Thanks for your interest in this topic. When I started this project I was surprised how well HF and VHF propagate in fresh water.

    As you can see in the following paper, the losses are much lower in fresh water than in sea water.
    https://file.scirp.org/pdf/JEMAA2011...1_18390291.pdf
    I have validated and extended these calculations to better address our particular needs.

    Our proof-of-concept used an Archimedean spiral antenna (because the math and antenna construction are straightforward) with +10 dBm input and we had good rx strength in nearly 10 m of lake water and a distance to shore of 200 m.
    https://vtechworks.lib.vt.edu/bitstr...ll_etd_Ch2.pdf

    However, the form factor of the Archimedean spiral won't work for our application (diameter is way too large) so I designed a QFH, which, although tall, has a diameter of only 13 cm and does not require a ground plane. I simulated the antenna in air using openEMS (by scaling due to e_r of about 80) and got good results. I tried in-water simulations but it looks like we can't have water as a boundary matching layer in openEMS.

    Now, I need to better match the impedance so that I get a good SWR. The system is battery powered, so efficiency is important.

    Any help with impedance matching would be appreciated.

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