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Thread: ELT Antenna: vertical or horizontal?

  1. #51
    Senior Member Andy's Avatar
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    Default Re: ELT Antenna: vertical or horizontal?

    I measured my completely factory standard FX-3 ELT antenna today. I used an MFJ antenna analyzer and a Vector Network Analyzer. The MFJ has limited frequency range but the VNA was set to sweep 100 to 500 MHz.

    The results for 121.5 can only be described as horrible. At 121.5 MHz the antenna shows an SWR of 4.4:1. The closest resonance is actually at 108.8 MHz where the antenna has an SWR of 2.5:1. The results for MFJ and VNA showed good agreement. The fact that the resonant frequency is lower than the design frequency is consistent with the supposition that the bracket makes the antenna electrically longer.

    It looks better at 406 MHz where the SWR according to the VNA was 1.5:1. This may explain why CubCrafters live COSPAS/SARSAT test was satisfactory.

    I would like to hear what measurements CubCrafters made on the ELT antenna. It's hard to believe that anyone with any knowledge of antenna theory or practice could have designed such a poor installation.

  2. #52
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    Default Re: ELT Antenna: vertical or horizontal?

    What great information.

    For someone that is flying and followed CC build manual (anetnna inside fuse), what are the possible corrective actions or solutions?

    The antenna is impossible to get to, there fore a new antenna solution is probably the best.

    There is an extensive thread on ELT antenna location on SuperCub.org. It explains a lot of what the issues are similair to what has been discussed here. But the solutions are all pre-covering.
    Last edited by aeroaddict; 10-14-2021 at 02:45 PM.
    Dan Arnold
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  3. #53
    Senior Member Andy's Avatar
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    Default Re: ELT Antenna: vertical or horizontal?

    Quote Originally Posted by Andy View Post
    I measured my completely factory standard FX-3 ELT antenna today. I used an MFJ antenna analyzer and a Vector Network Analyzer. The MFJ has limited frequency range but the VNA was set to sweep 100 to 500 MHz.
    Test results as documented by rather poor cell phone images:

    VNA sweep, marker at 108 MHz
    ELT sweep marker 108 MHz.PNG

    VNA sweep, marker at 408 MHz
    ELT sweep marker 408 Mhz.PNG

    MFJ at 121.5 MHz
    MFJ SWR at 121.5 Mhz.jpg

    MFJ at 108.8 MHz
    MFJ resonance at 108.8 MHz.jpg

  4. #54
    Senior Member ceslaw's Avatar
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    Default Re: ELT Antenna: vertical or horizontal?

    Andy:

    Looks like a good news / bad news story.

    The good news is that the 406MHz antenna has a good match.

    The bad news is that 121.5 has a poor match.

    The good news is that your testing and mine are consistent revealing an SWR in excess of 4.5 at 121.5

    The bad news is that it confirms a fundamental design flaw due to that two inch standoff used by CC to support the base of the antenna

    The good news is that the problem is easily corrected by adding a 23 1/4" radial as detailed on my separate post. Further, it should be possible to add that radial on a finished aircraft provided the antenna was mounted in the lower position. If mounted in the higher position used by CC it will be a bit more of a challenge requiring the antenna be relocated in the lower position to provide the needed clearance for the radial.

    It is indeed surprising that the issue was not recognized long ago. Perhaps the thought process was that only 406 would be of concern to the engineers responsible for this design?

    Thanks for posting your findings.

  5. #55
    Senior Member Andy's Avatar
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    Default Re: ELT Antenna: vertical or horizontal?

    Amateur radio whip antennas for VHF and UHF bands are usually adjustable for length and resonant frequency. Does the CubCrafters supplied ELT antenna have a grub screw that allows the whip length to be adjusted?

  6. #56
    Senior Member ceslaw's Avatar
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    Default Re: ELT Antenna: vertical or horizontal?

    There are no adjustments. The length of the whip is fixed.

  7. #57
    Senior Member Andy's Avatar
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    Default Re: ELT Antenna: vertical or horizontal?

    Quote Originally Posted by ceslaw View Post
    There are no adjustments. The length of the whip is fixed.
    Not unexpected. It would be easy enough to trim the length to bring it into resonance at 121.5 but that may hurt the match at 406.

    I'd like to measure Dave's separate ground plane installation. Was that used in Blue Bird which I think is based near me now?

  8. #58
    Senior Member ceslaw's Avatar
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    Default Re: ELT Antenna: vertical or horizontal?

    The antenna is a RAMI AV-200 which sells for around $100 plus.

  9. #59
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    Default Re: ELT Antenna: vertical or horizontal?

    No Bluebird I got going by changing the length of the cable (which, like CC said they had to do with some of theirs as well). I have a kit shipping to me next week so Iíll do more experimentation with it.

    Quote Originally Posted by Andy View Post
    Not unexpected. It would be easy enough to trim the length to bring it into resonance at 121.5 but that may hurt the match at 406.

    I'd like to measure Dave's separate ground plane installation. Was that used in Blue Bird which I think is based near me now?
    Dave Embry
    "You only live once.......but if you do it right.........once is enough."..

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