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ELT Antenna Location

bubb2

Registered User
Eagle River, AK
Any recommendations as to the best location for an ELT antenna in a PA-18. I am considering locating the ELT itself in the rear of the top extended baggage.

Thanks
 
Any recommendations as to the best location for an ELT antenna in a PA-18. I am considering locating the ELT itself in the rear of the top extended baggage.

Thanks

Be sure to read up on the requirements for mounting. Frankly, a LOT of ELTs don't meet that criteria. Needs to be REALLY stiff, almost no flex.

As to Antennas, on a fabric plane the issue can be providing a ground plane for the antenna. Ideally, I'd prefer to see the antenna mounted close to the vertical fin, in hopes that if the plane winds up upside down, that vertical fin MIGHT prevent the antenna from being damaged.

MTV
 
What Mike said. Read the install instructions.

Specifically for antennas, any rod or whip antenna is 'vertically polarized'. This means it is designed to transmit (ELT antennas ONLY transmit) from a vertical position, either pointed straight up or straight down. If you could look down at the tip of the antenna the signal would look like the circular ripple created when you drop a pebble into water. it's a concentric circle that starts at the antenna and grows out from there. If you move the antenna to another position, such as sideways, the antenna still sends the signal out the same way. So, in that position the signal pattern is also sideways, so now it looks more like a figure eight, when viewed from above.

Also, metal dampens/deflects the signal so don't place an antenna inside a fuselage with a metal frame and/or metallic coatings (silver dope). Just don't do it.

All antennas need a ground plane. This is a metal surface that the antenna fits in the center of. On a fabric aircraft, I like to use a piece of stainless window screen glued to the inside surface of the fabric, at the mounting location. Fit the antenna through the fabric, through the center of the ground plane material, and secure with a fabricated bracket, to the fuselage tubing. Traditionally ELT antennas have been mounted on the battery box access cover on Cubs. As they are going away, I like to mount them just forward of the vertical stab. Like it was stated above, if the aircraft flips over, there should be some protection for the antenna.

If you ever hear 'good enough' used to describe the installation of safety equipment, it's wrong.

Web
 
Also, metal dampens/deflects the signal so don't place an antenna inside a fuselage with a metal frame and/or metallic coatings (silver dope). Just don't do it.

All antennas need a ground plane. This is a metal surface that the antenna fits in the center of. On a fabric aircraft, I like to use a piece of stainless window screen glued to the inside surface of the fabric, at the mounting location. Fit the antenna through the fabric, through the center of the ground plane material, and secure with a fabricated bracket, to the fuselage tubing.



Web[/QUOTE]Recommended size of the screen?
 
Bigger is better, but on a Cub it's usually a compromise due to the layout of the structure. Try for at least 10" in all directions from the antenna base.

Web
 
Bigger is better, but on a Cub it's usually a compromise due to the layout of the structure. Try for at least 10" in all directions from the antenna base.

Web

Cub crafters mounts them under the fabric in the aft fuselage . Passed all tests for FAA and nothing to break off.

Sandy
 
Cub crafters mounts them under the fabric in the aft fuselage .

Sandy

I won't and I never will. Placing an antenna inside a metal structure WILL degrade the signal. Look up the definition of a 'Faraday cage'. So placing the antenna inside a ferrous metal structure is not a wise idea.

As far as Cub Crafters doing it, I'm not the oldest guy here but I'm old enough to know that just because the faa says it's ok does not mean that it is, in fact, ok.

Web
 
I installed mine right in front of jack screw, if you enlarge the photo you can see the ground plane I attached to the spine and top longerone, its on with just a pressure fit held in place by maybe 4 pc4 screws but mostly the fabric. On a previous bird I used 1" copper tape in a radial pattern to extend the ground plane but it rattled a bit. When I got inspected for a replacement airworthiness certificate I remember one of the FAA guys commenting that was a good idea to attach the 'door' with screws not a hinge and quick tabs that way its an inspection panel and not a door and doesn't require more paperwork. The front door is a door for the rear of the extended baggage
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I mounted the antenna on the top wing root fairing (on the right wing, outside of plane)).

Then I mounted the ELT aft of the right-side fuel sight gauge (inside the plane). The remote switch and buzzer mounted in front of the sight gauge.
 
Right in front of the fin. I know mine works since I set it off one day in parking. The phone call came quicker than I expected.
 

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Mounting the antenna straight up is one thing but when was the last time you saw a wreck that crashed in a way that left the antenna straight up? Not trying to start an argument here, just curious as to the logic train that brought this concept into the station.
 
Mounting the antenna straight up is one thing but when was the last time you saw a wreck that crashed in a way that left the antenna straight up? Not trying to start an argument here, just curious as to the logic train that brought this concept into the station.

The antennas are 'vertically polarized', meaning they are designed to transmit with the whip or rod straight up or straight down. No logic train, just a design fact.

As for the position of the antenna after a crash, ya' play the best odds. End up on the belly and the antenna is straight up. End up on your back and the vertical stab should keep the antenna off the ground. Some operators place the antenna straight down on the tailboom on Bell 206's so I've actually tested them and they work just fine.

Web
 
Or like a handful of years ago when a 182 went on its back in a bog in Galena, antenna on top like every Cessna, and the 406 signal was received. None of us knows for sure what the transmit limits are. We do our best to maximize our chances.

I believe my installation manual says the antenna can be located internally but must be vertical. Don’t remember exactly. Don’t care. I like it external. Given that lots of guys still uses 121.5? Clearly not everyone is concerned about beacon effectiveness.
 
I like the idea of the dipole antenna mounted inside the fuselage. Dipoles are more efficient than groundplane antennas. The cub fuselage is not a faraday cage, but a number of tubes a near the resonant frequency or multiples of the resonant frequency so that they suck up the radiant energy coming from the antenna. But an intact antenna inside a open but bent cage is better than an antenna stuck in the mud or torn off the aircraft structure.
 
A pattern of conductive materiel surrounding an antenna is the definition of a Faraday cage. Even if it was not designed/built for that purpose. Will an antenna inside a steel tube fuselage covered in metallic (silver) dope transmit a strong enough signal? Possibly. Do ELT manufacturers allow for internal antennas? No, with one exception. If you have a non conductive structure, such as wood, fiberglass, etc, you can. But that's because non conductive materials will not dampen a signal. Even then the antenna needs to be oriented vertically and needs a sufficient ground plain to work correctly.

As for the antenna in the mud or broken off, I can't help you if that happens. But this is aviation. We increase the odds in our favor but nothing is %100 guaranteed. If that antenna was placed in a good spot such as in front of the vertical stab and it ends up broken off, then chances are the ELT will not help anyways.

You're a big boy, you do what you feel is correct. But as an 'electrical guy' my job is to help customers get the best equipment/installation possible. I will not accept 'good enough' with safety related equipment.

Web
 
This might be worth a look for an internal antenna option> http://www.advancedaircraft.com Some comparative signal field strength testing after installation would be interesting.
...Gary
I installed one of these in my vertical stabilizer as a comm antenna. Works fine. Signals are relatively weak directly forward and down (blocked by the firewall). Still, it seems work as well as most Cub installations, as nearly every installation results in some compromise and weak coverage areas.

406 MHz ELTs transmit on two frequencies (406 and 121.5), and have monopole antennas designed specifically for them. The Advanced dipoles are unlikely to work well for 406 ELTs.
 
Lol. I didn't even catch that.

You are correct. A com rated dipole would work for the 121.5 mhz signal as that frequency is within the design range of the antenna. But the 406 mhz signal is in an entirely different band.

Web
 
Just to provide a contrary view, I mounted my 406 ELT internally, using the baggage compartment as a ground plane and very close to the ELT, with the antenna tilted quite a bit.

Of course it isn't ideal for transmission, but IMHO, it is more important to keep the antenna attached to the ELT in an accident, than maximizing transmission efficiency in flight. As demonstrated by the success of internal comm antennas, the powdered aluminum layer on fabric is not all that conductive and doesn't make a debilitating Faraday cage. I'm pretty sure that the Husky also has an internal ELT antenna.
 
Those dipoles can be cut down to resonate on 406 but might require a matched signal splitter or ? to also feed a 121.5 antenna from the same coax. The ELT factory antenna setup takes care of that. But....if concerned about impact damage why not mount a second ELT antenna on the belly fed by a splitter?

Gary
 
Mine is on the metal battery hatch behind the passenger (turtledeck).
 
Its handy to put it there but with the 'newer' Cubs and after rebuilds a lot of those covers are going away.

Web
 
Just to provide a contrary view, I mounted my 406 ELT internally, using the baggage compartment as a ground plane and very close to the ELT, with the antenna tilted quite a bit.

Of course it isn't ideal for transmission, but IMHO, it is more important to keep the antenna attached to the ELT in an accident, than maximizing transmission efficiency in flight. As demonstrated by the success of internal comm antennas, the powdered aluminum layer on fabric is not all that conductive and doesn't make a debilitating Faraday cage. I'm pretty sure that the Husky also has an internal ELT antenna.
Ditto
 
Heres the deal, though: You can install your ELT wherever it makes you feel good, and it will work perfectly right up until it’s activated. After that, it’s a bit late to move it.

I only see one “avionics type” engaged in this conversation. I guess I’d follow his recommendations. Or, don’t crash.

MTV
 
Here's a neat experiment in support of WireWeinie's "faraday cage" argument: Find a nearby "big box" store that was built with "tilt-up concrete walls." Those walls are poured with rebar laced together throughout, and when the tilt them upright and tie them together in the corners with steel beams, and install a metal roof, they form a pretty darn good faraday cage. Take a look at your cellphone, and note the number of bars of coverage you have outside, versus inside the center of the store... You could have 5 bars of coverage outside the store, but when you walk in, the signal drops dramatically (in most cases, it goes to almost nothing), and the further you go inside the store, the weaker it gets. That's a faraday cage in action...
 
I recall a number of stories about ELTs being activated for various reasons other than crash situations. I believe MTV related one type at OshKosh, and there is an example above.

Real world examples of antennas working, signals being received. Copy installations like theirs and we should be ok. I bet theirs were by manufacture instructions.

If an antenna comes off in a crash, and you are able- do your best to reattach, even if not perfect some signal is better than none.

Lastly, if everything was perfect we would not need ELTs.
 
Remember those old yellow EBC-102A ELT's? Mounted them somewhere in the cockpit with attached flexible whip antenna. They seemed to work ok especially when installed near a window or skylight. Different frequency than the 406's tho, and made when satellites were scarce.

Gary
 
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