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ELT antenna location

PostPosted: Mon Jan 15, 2018 12:38 am
by pt17
I'm planning a move to NZ which requires mandatory fittment of fixed ELT. I'm after some ideas as to where to put the ELT antenna which requires some form of ground plane, and how this could be done to a completed Highlander. I used the turtle deck for the radio antenna.
Any ideas or experience with this would be helpful. Pete

Re: ELT antenna location

PostPosted: Mon Jan 15, 2018 10:07 am
by alan
Hey Pete.

If you have access to the tailcone it's easy. I guess you need to remove the rear cargo bulkhead. Make a groundplane by using a thin piece of aluminium. You'll have to measure the available space and make it as large as you can, at least a foot square. You really need to mount it on the floor behind the baggage compartment because of the high intensity RF it emits. .020" is very light and available in most hardware stores. Melt a small hole in the fabric right in the middle of the groundplane and screw it all together, antennae outside, groundplane inside. Attach a groundwire to the groundplane and run the wire to your ground bus.

I did this during construction almost 12 years ago and have had no problems with it since, even after installing ADS-B last year.

Good luck.

Alan

Re: ELT antenna location

PostPosted: Mon Jan 15, 2018 11:35 am
by FlyerChief
Hi Pete,
If I had to redo mine, I would likely just drill a hole near the top-center of the rear cargo bulkhead and mount the antenna pointing into the fuselage behind it, keeping the tip end as clear of metal tubing and bracing as possible. KISS principle.
Since the ELT antenna is only used in a crash situation there are a few considerations:
- It must be mounted securely and survive a very high G impact (both ELT and antenna)
- It must be serviceable, so access is important
- Many crashes involve the aircraft ending up nose down or inverted, so the antenna should be placed accordingly
By mounting it on the rear bulkhead pointing it down into the tail of the plane you satisfy most if not all of these issues. It does place the wire (or at least part of it) for the ELT antenna inside the cargo area so it would have to be protected from damage or disconnection when loading cargo.

In my Highlander, I have a Lithium Iron Phosphate battery on the firewall, so I used the plate designed for the battery to mount my ELT transmitter on. I have a small hinged access door on the rear cargo bulkhead, which allows me easy access to the ELT, rather than having to access it through the tiny triangular shaped hole in the tail of the aircraft. Keep in mind when you mount the ELT it needs to be able to survive a High G crash and easily removable to service it with battery changes every 4-5 years and possibly, annual re-certification. (depending upon local regulations)

I hope you figure out a solution that works well for you and your Highlander!
Dan

Re: ELT antenna location

PostPosted: Mon Jan 15, 2018 11:37 am
by Tralika
The instructions that came with my Artex ELT said to install the antenna in accordance with AC 43.13-2B.310. Using the formula to determine ground plane size, a 406 MHz ELT needs a 6.75 inch ground plane made from thin sheet metal or wire mesh. The AC says the bigger the better so what the heck, call it 7 inches.

Re: ELT antenna location

PostPosted: Tue Jan 16, 2018 9:56 am
by alan
I must apologise to all. I must have had a major chip slip when reading the first post. ELT transposed into Transponder somehow in my brain and stayed there until reading other, more correct, posts. All I can say is "Oops". And maybe "Dang".

Alan

Re: ELT antenna location

PostPosted: Wed Jan 17, 2018 12:31 am
by pt17
No problem. Thanks for the ideas.
I like the idea of keeping the antenna "inside", not sure whether I can get way with having it mounted horizontal though.
Turns out if I register as microlight in NZ, I can get away with PLB instead of ELT. More thought required.

Re: ELT antenna location

PostPosted: Thu Jan 18, 2018 9:18 am
by danerazz
alan wrote: Attach a groundwire to the groundplane and run the wire to your ground bus.


Alan



The ground plane does not need to be grounded to the airframe, just the base of the antenna. The antenna mount must make good electrical contact with the ground plane. You could use a piece of wood with foil tape as a ground plane, as long as the antenna itself is electrically connected to the foil.

Also, there is some speculation as to how harmful XPDR radiation is, my understanding is just an adequate ground plane directs the signal away enough to mitigate. But I guess better safe..,

Re: ELT antenna location

PostPosted: Thu Jan 18, 2018 11:39 pm
by RexEasley
Here is how I did mine. The plate just in front of the rear access panel just in front of the tailwheel. Had to bend the antenna near the tip to clear stuff. Not too fun to change the battery later, kind of by braille, but I'd do it again. It's only fell out a few times. Kidding.