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More advice - ELT aerial

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  • More advice - ELT aerial

    Last night I had the local chapter of the Sport Aircraft Association visit and look over my Bearhawk, which provided a great opportunity to get some advice from members. Many of them have built more than one aircraft and several of them are current or retired aircraft engineers.

    One suggestion was concerning my location of the ELT aerial inside the rear fuselage, and whether it would work given that it is surrounded by the airframe. My understanding (having not read further yet) is that I need to add a ground-plane around the base of the aerial.

    Has anyone done it in this manner ? Comments ?

    Another suggestion was to invert the lines from my static ports to prevent water ingestion which made immediate sense and was a 2 minute job to fix.

    3A8A0425-2FB8-4E12-884F-D01E6C8DA64D.jpeg

    D5AF7BB5-8B25-47B0-A753-3081DED91288.jpeg
    Nev Bailey
    Christchurch, NZ

    BearhawkBlog.com - Safety & Maintenance Notes
    YouTube - Build and flying channel
    Builders Log - We build planes

  • #2
    I noticed a lot of folks mount their ELT's in the rear of the fuselage. I couldn't find any info on any specific requirement on locating it. I was hoping go put it under the pilot seat to keep the wires short and I can reach down and turn it off and on while seated. That portion of the fuselage is the strongest part of the airframe. If the ELT doesn't survive the crash.................

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    • #3
      Lots of builders place their ELT's in the back but with a ground plane underneath. That seems to work fine. Mark

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      • #4
        Not sure about the Artex but ACK specifically says their ELT / antenna is fine inside a tube structure. Provide a good ground plane and you should be good. There's quite a discussion on antenna location and the benefits of having it protected inside a structure. A buddy of mine flipped his Super Cub and buried the tail mounted ELT antenna in the mud, broke it off and it became useless, so there is a case to be made that inside is the safest place for it.

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        • #5
          I was hoping go put it under the pilot seat to keep the wires short and I can reach down and turn it off and on while seated.
          This was my original plan too. Battson pointed out to me that it could easily be triggered inadvertently by a pax foot, hence me locating it aft of the rear bulkhead. I’ve actually bolted it through the aluminium plate into the frame tabs so it now pretty secure. I plan to locate the ON/OFF/TEST panel in the right-side wing root panel.
          Nev Bailey
          Christchurch, NZ

          BearhawkBlog.com - Safety & Maintenance Notes
          YouTube - Build and flying channel
          Builders Log - We build planes

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          • #6
            I placed the elt and antenna very similar to yours in the pic. The aluminum pan they are mounted on might be a bit bigger in ours but not much. I will say it all works fine there. During my initial airworthiness inspection, the inspector wanted me to test it and I inadvertently pushed the wrong button and activated it. I quickly discovered my error and turned it back off. Within about 5 minutes my phone was ringing from the Air Force in Florida questioning the signal they had picked up. I explained what happened and apologized and all was fine. Was glad to know that such a quick response was generated and with the airplane in the hangar.

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            • #8
              That location is fine. American Champion locates all of theirs just behind the baggage bulkhead. I am planning on mounting mine under the passenger seat. You will want a ground plane of at least 1 square foot.

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              • #9
                I'm planning to mounting the ACK 121.5/406 ELT similar to how Nev mounted the Artex in the pictures above but I have a question about the ground plane. Are most people assuming once a metal plate is attached to the mounting tabs welded to the fuselage tubing, the fuselage tubing acts as the ground plane? The installation instructions I received say the antenna needs to be mounted in the center of a ground plane with a minimum diameter of 24", preferably 48" for maximum power. 24" inches is ~1/4 wavelength of 121.5 MHz, so I understand they're sizing the ground plane to that frequency. If the fuselage tubing isn't the ground plane, then are people just sizing the ground plane to the 406 MHz wavelength since that's the modern frequency? Appreciate the help.
                Colby Osborn
                Mullen/Lincoln Nebraska
                Model 5 Quick Build Kit

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                • #10
                  This is one of those frustrating situations when the manufacturer cooks up the most ideal scenario they can imagine for their product only, without any regard for the airplane it is going onto, or any other products that will be on it. Sort of like on the first day of school when each of the teachers says their class is the one class that really matters and is going to be the hardest. As a result we are left to decode what is good enough, because clearly we aren't going to be providing the ideal. I considered the fuselage tubes to be good enough.

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                  • #11
                    I'm installing the same ELT right now. I called ACK to ask this specific question. They were confident the airframe, if correctly bonded to the antenna mount bracket and the antenna, will act as a ground plane. I'm going to go with that for now and test it once it's installed. The ELT came in the mail yesterday so I'll mount and test it in the next couple weeks and report back.
                    4-Place QB kit #111. First flight May 2022.
                    IO-470 - 260hp

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