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cjhukill(at)cox.net
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 Posted: Thu Aug 12, 2010 6:45 am Post subject: ELT antenna |
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I took a 7.25 inch piece of 1/16th welding rod, welded a nutplate to the end and used a router to cut a groove in the under side of the cabin top, running forward, just ahead of the F1074 skin and bulkhead. I then attached a second nutplate to the backup strip where the pop rivets go thru the cabin top. Then fill the groove with the wire in it with micro. This was all done with the top upside down on the bench, but could be a (somewhat messy) retrofit, with the top installed. What you now have is a tuned (1/4 wave for 406 mhz) vertically polarized antenna, with ground plane (the aft bulkhead, F1074 skin, aluminum backing strip assembly). The vertical polarization is best for satellite reception, assuming the aircraft isn't standing on it's nose after the crash. The crash protected and completely hidden antenna is wired with the center conductor of the coax to the nutplate on the wire, and the shield to the ground plane nutplate. The 406 tuned aspect of the antenna and vertical polarization is not optimum for the 121.5 mhz transmission function of the ELT, but I prefer to have the antenna optimized for the satellite freqs. I can attest to the function of this setup with an inadvertent activation of this same type of installation I have in my RV8. The ELT antenna in the 8 is mounted in the wingtip the same way, with the rod going out horizontally into the wingtip, and the outboard rib being the ground plane and mounting structure. I accidentally set the ELT off trying to test it , and in less than ten minutes , I received a call from the USAF asking about the ELT hits. I apologized profusely, and explained that I was still trying to get all the screws out of the wingtip to shut the damn thing off. The remote switch turned it on OK, but wouldn't shut it off! I had tried to call the tower, but they didn't even know what a 406 ELT was, let alone the phone number to call to report a false alarm (800 851-3051). Anyway, the nice Zoomie said no problem sir. I then asked him where his satellite was showing me to be, and he said in the northwest corner of KVGT. This was from an aircraft in a closed STEEL hangar (in the NW corner of the airport). This ELT does not have GPS interface! The satellite triangulation was giving him a very accurate position of my ELT, even without the GPS! Maybe the Sats aren't that good in other places of the world, but wow, am I a believer. I can't imagine anybody not availing themselves of this technology. I too have APRS installed (KE7HKK) , which I agree with Tim is fantastic . I also carry a GPS enabled PLB on board, but no way would I travel in an aircraft without a 406 ELT.
Chris Hukill
[quote][b]
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Kellym
Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ
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| Posted: Thu Aug 12, 2010 6:54 am Post subject: ELT antenna |
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Only one small fly in the ointment, if your DAR or FAA decide to be
picky. All ELT's must be TSO'd by regulation. The antenna that they
demonstrate TSO compliance is part of the TSO approval, and it is not
legal to substitute another antenna. Does not matter whether it is on
homebuilt, warbird or ordinary type certificated aircraft.
I'm rather certain what you have does a fine job. Just not what the
paper pushers would bless.
Kelly
On 8/12/2010 7:44 AM, Chris Hukill wrote:
| Quote: | I took a 7.25 inch piece of 1/16th welding rod, welded a nutplate to
the end and used a router to cut a groove in the under side of the
cabin top, running forward, just ahead of the F1074 skin and bulkhead.
I then attached a second nutplate to the backup strip where the pop
rivets go thru the cabin top. Then fill the groove with the wire in it
with micro.
|
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_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
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cjay
Joined: 15 Dec 2007
Posts: 53
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| Posted: Tue Aug 17, 2010 1:24 am Post subject: Re: ELT antenna |
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[quote="cjhukill(at)cox.net"] The satellite triangulation was giving him a very accurate position of my ELT, even without the GPS!
Chris Hukill
Chris,
The Doppler/triangulation processing for the modern 406 Mhz pulsed beacons are accurate to 2 parts per billion, giving a search area of 2 sqkm. A huge improvement over the 121.5 Mhz tone, however, if you want 100m accuracy you need the gps encoded feature.
cjay
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