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406 ELT antenna location?

Ron B.

Well Known Member
We have a wip antenna for our 406 ELT. We mounted it on top of the tailcone and I don't like the look of it. I have seen at least one picture of an antenna mounted horizontally on the F-1009 fuselage frame. It's hidden under the Emp. trim fairing. I doubt it's the ideal location for transmitting, but it's less likely to get damaged from the "event" .
I would like to move it but would like the view of those in the know, here first.
Thanks Ron
 
Ron, I have an RV-8a I've about finished. I mounted the new 406 mh type ELT behind the rear baggage compartment lip beside the bellcrank with the antenna mounted horizontally below and to the side of the vertical stabilizer under the fiberglass fairing. Was not sure it would work there until last week. I was previously told the antenna had to point toward the sky and no obstructions to be picked up by the satellite and that there was small to no chance the satellite could pick it up from there. Not true. The plane was inside my brick two story shop with steel doors closed and a bunch of metal directly above it, I.E. both wings, ailerons & flaps, two garden tillers and an edger plus yard tools, table saws, bikes, etc. and when I boneheadly flipped the ELT on instead of off the CAP, Emergncy Management Folks and Sheriff's office, U.S. Air Force and Boy Scouts of America let me know the signal was received and my ELT located very fast. Damned I felt dumb! Soooo, long story-short answer. I don't think it matters much where the antenna is located to bang a signal into a satellite. What matters most is will it survive the impact ? Best regards, Bill
 
Ron;

I have an overhead plenum for air. I put mine inside that overhead and can access it when I remove the baggage compartment corrugated wall. The antennae ends up being horizontal. Made a mount and placed the ELT unit on the right side fuse stringers.

Fortunately, never had to use it. Easy to get to the batteries to change when doing the annual.

Dr Fred.
515FW
 
I just relocated my ELT antenna from the tailcone just ahead of the vertical stabilizer to beneath the empennage fairing in a horizontal orientation. What a positive difference in the appearance it makes!

My ship has two blade antennas on the belly for XPDR and ADS-B, and a bent whip under one wing for COM2. Everything else (both GPS pucks and COM1 whip) is "stealth" under the canopy. I like the clean look. Maybe 1/2 kt faster, too. :D
 
Ron - ask yourself one question and answer it honestly.

When the chips are down and your airplane is a mangled wreck and you can feel your lifeblood literally ebbing from you... Does it matter a darn what your ELT antenna installation looks like, or is it more important that it works and summons rescuers to your location?

If your answer is in favour of summoning rescuers I would strongly suggest avoiding the "under the empennage fairing" installation location.

Yes, we read anecdotal stories of ELT's getting the signal out to the satellites when we "oops" and hit the wrong switch. What we don't think about when we read these stories is that many of them originate from an aircraft which is in fairly ideal conditions... In a hangar, on a flat airport, with no vegetation within a couple of miles. Sure, it worked from inside a hangar so we think that's just great. The reality is it's a test which is totally unrepresentative of a real world crash. Those often happen in highly inhospitable terrain, in crappy weather, with the wreckage having settled at the bottom of dense foliage. The real world is a much tougher place than the hangar floor.

Too often we have discussions like this one and somebody chimes in with the rhetorical statement that goes along the lines of "well, ELT's don't work anyway, so why should I care how it's installed?" The answer to that rhetorical question is clear; why on earth wouldn't we install our "last resort" lifesaving equipment in a manner which provides it with the maximum probability of saving our precious backside?

An under-the-empennage-fairing installation is an installation which hobbles the horse before it's even left the gate. For your own good and for that of the folks risking their lives to save yours, install the ELT such that its antenna is not severely compromised right out of the gate. Follow the manufacturer's guidance. Mount it on top of the tail cone where it is protected by the vertical stabilizer from rollover damage. While you're at it, spend a little coin and purchase enough fire sleeve (yup, that orange Aeroquip fire sleeve) to put a fire-resistant sheath over the coax run between the ELT and the antenna. The ELT may survive the post-crash fire but what good is it if the coax is melted?

If you've read some of my previous posts here you'll know this is something of a personal soapbox topic for me. It's not because I have any commercial interest. It's because I have seen lives saved and lives lost. I've seen ELT's make a real difference. They are an imperfect tool but they are the tool we are mandated to use. Let's do our best to get the best performance from that tool. When the chips are really down you'll be at least somewhat relieved to know you did the best you could in installing your ELT and maybe, just maybe, the cavalry is just over the next hill, summoned by that ELT's little beeping heart.
 
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Mark - I disagree with your logic.

Consider:
- What is the most survivable location on the plane? Looking at crumpled up RVs (upside down or not) the intersection of the VS and HS seem to always remain mostly intact.
- The “antenna must be vertical” argument. How will you know the orientation of the antenna after a crash? Also consider the antenna at 406mhz ~3/4 of a wavelength doing burst communication to a satellite - direction unknown other that above the horizon. In other words the vertical orientation may have marginal benefit at 121.5mhz, but none at 406.

The RV-10, RV-8A and RV-8 all have the ACK ELT antenna under the empennage fairing, base mounted to the last top skin bulkhead. On the RV-10 the owner inadvertently set it off - it got immediate attention.

Carl
 
ELT installation compliance

It must be that time of year when we discuss ELT antenna installations...:rolleyes:

Sarcasm aside, this is a great question - and a critical decision.

There really isn't much of an argument about whether the ELT antenna can be installed vertically or not - it's more a matter of whether you want to comply or not. ELT installations are one of the few items installed in experimental aircraft that actually must comply with a TSO. In this case TSO-C91 as required by FAR 91.207 (if you're in an RV-3, you're exempted...one-seater). Anyway, I've installed ACK and Artex ELT's - both installation manuals have text similar to below:

From Artex's installation manual:
"Place the antenna in a location where the antenna can be installed vertically and with at least 32 inches (0.8 m) of clearance from other antennas, particularly VHF, in accordance with RTCA/DO-204, § 3.1.10.2."

From ACK E04 manual:
"The antenna may be mounted internally in composite construction, and tubular fabric covered aircraft, as long as the fabric or composite material is of a non conductive nature. The antenna must be mounted externally, on airframes of metallic construction. The antenna should be mounted as close to the ELT transmitter as practical. The coaxial cable connecting the antenna to the ELT transmitter, should not run in close proximity to comm radio coaxial cables, and should avoid crossing aircraft production breaks. (i.e. Riveted fuselage sections) The antenna must be mounted within 30 degrees of vertical, when the aircraft is in normal flight attitude. The installed antenna must be able to withstand a static load of 100 (one hundred) times it’s weight (13 lbs.) applied to the base of the antenna, along the longitudinal axis of the aircraft. The antenna should be mounted a minimum dis- tance of three feet (1 meter) from any vertically polarized communication antennas. (i.e. Antennas radiating in the 118-137 MHz band.)"
 
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Mark - I disagree with your logic.

Consider:
- What is the most survivable location on the plane? Looking at crumpled up RVs (upside down or not) the intersection of the VS and HS seem to always remain mostly intact.
- The “antenna must be vertical” argument. How will you know the orientation of the antenna after a crash? Also consider the antenna at 406mhz ~3/4 of a wavelength doing burst communication to a satellite - direction unknown other that above the horizon. In other words the vertical orientation may have marginal benefit at 121.5mhz, but none at 406.

The RV-10, RV-8A and RV-8 all have the ACK ELT antenna under the empennage fairing, base mounted to the last top skin bulkhead. On the RV-10 the owner inadvertently set it off - it got immediate attention.

Carl

Respectfully, Carl, your statement fails to address some of the more serious implications of an "under the emp fairing" installation.

One of the greatest challenges in ELT antenna placement is, as you have acknowledged, finding a location which is both functional and crash-worthy. The area around the intersection of horizontal stabilizer, vertical stabilizer and tail cone does indeed seem to provide one of the more robust mounting locations. The amount of structure required to support the loads imposed by those tail surfaces is considerable, thus likely to survive a crash.

BUT... when we bury an antenna within metal its radiation pattern can change quite significantly. As I pointed out previously, setting the ELT off accidentally in the rather pristine environs of the airport is anything but a real-world test. Try it at the bottom of a mountain ravine with iron-laden granite and huge coniferous trees surrounding the aircraft. Now we're talking real world. All of that "stuff" around the airplane sucks up, bends, reflects, re-radiates etc etc etc. That's why the 406MHz signal is a thumping 5 Watts as compared to the typical 100 milliwatts of the 121.5MHz signal. In designing a better mousetrap we recognized that too many times that puny 121.5MHz signal just didn't make it out of the crash site.

Still, we need that puny 121.5MHz signal for the "last mile" people to find us via brute-force ground search. How well is that signal going to get out of the mangled metal at the empennage fairing? How true will be the radiation pattern? Both of these, as just two variables, make the jobs of those "last mile" searchers anything from more difficult to darned-near impossible with an under-the-emp-fairing installation.

While it is your prerogative to disagree with my logic I might suggest that disagreeing with the logic of a large team of technical experts may be inadvisable. That large team of technical experts would be the folks who designed the Minimum Operational Performance Specification for the ELT - the MOPS is one of the fundamental documents which eventually drives the performance definition which ultimately makes its way to us in the form of TSO C126 and its follow-on variants. Those technical experts aren't just the avionics weenies that build ELTs - they're also the folks who build the satellites that listen for the ELTs, the folks who run the Search and Rescue organisations which go searching for those ELT signals in the most horrific weather conditions, not to mention the NTSB folks who bring with them their sheaves of accident reports which are intended to better inform future equipment designs.

With this in mind I will suggest you are well within your right to disagree with my logic. After all, what can I possibly know? By the same token, it would behoove us to pay heed to the logic of experts as it is manifested in TSO C126.

If we wish to really examine logic I would ask you to perform some introspection. Your logic about the installation of your own particular ELT antennas doesn't stand up in light of the ultimate test. Have you crashed any of those aircraft to determine how well those ELT antenna mounting locations work in the real world? Once you've got this empirical test data in hand let's rejoin this conversation. Until then, let's rely on the guidance of the expert logic expressed in TSO C126.
 
So

So if the aircraft is on its back after the crash, where is that top mounted antenna? It is UNDER the aircraft, if not broken off, and being shadowed by the fuselage above it.

That said, the mounting location is also a function of what your DAR wants. My particular DAR wanted it installed per the manufacturer’s recommendation, therefore, mine was mounted on the top of the fuse as a requirement to get the a/w certificate…
 
Mine is mounted roughly halfway between the baggage bulkhead and the VS on top of the tailcone. Click on the pic below and you can clearly see it.
 

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Mine is mounted horizontal under the tail fairing

My ARTEX 345 had a Mfg software issue and the ELT self activated in my metal hangar. The satellite received the signal and I was notified 🤔
 
I mounted mine right behind the right door in the 'B' pillar. There is a hollow in the fiberglass of the canopy that hides it well. It is very nearly vertical bending a little bit to follow the inside canopy curvature. The stock coax is just long enough to mount the ELT in the right side wall behind the baggage bulkhead.

Here are a few pics.
http://www.mykitlog.com/users/display_log.php?user=leok&project=2446&category=0&log=220527&row=223

Very innovative…I like it!
 
Antenae Locaton for the rv-4

Artex 345 406 ELT and a rv-4…… has anyone come up with a location that is inside the airplane…. Perhaps vertical mounted next to rear seat. Under the canopy for protection? I have seen were several have mounted under VS fairing but was hoping for something different if there is such a thing.

Thanks fo your consideration.
Pokey[/CODE]
 
I don't have a picture, but have several like this on -8 models

ELT on 8.jpg

Basically, a little gusset to mount the base of the antenna to, riveted to the bulkhead the rear seat rests against. The coax runs down a little and then back to the rear mounted ELT.

The antenna is flexed just a little to follow the contour of the bulkhead/canopy, and secured with a hole drilled and a zip tie poked though, with the ratchet end hidden behind the bulk head itself.

Not sure how that maps onto the -4, but hopefully that helps
 
Try it at the bottom of a mountain ravine with iron-laden granite and huge coniferous trees surrounding the aircraft. Now we're talking real world. All of that "stuff" around the airplane sucks up, bends, reflects, re-radiates etc etc etc. That's why the 406MHz signal is a thumping 5 Watts as compared to the typical 100 milliwatts of the 121.5MHz signal. In designing a better mousetrap we recognized that too many times that puny 121.5MHz signal just didn't make it out of the crash site.

All the more reason that the solution we need here is an emergency button for our ADS-B installation, that you hit before you go down, not a "hope it goes off, hope the signal gets out" device that only *starts* broadcasting when you're at the bottom of that ravine.
 
All the more reason that the solution we need here is an emergency button for our ADS-B installation, that you hit before you go down, not a "hope it goes off, hope the signal gets out" device that only *starts* broadcasting when you're at the bottom of that ravine.

You have that ability already if you installed the remote switch and connected a GPS signal to your ELT.

If you know you're going to land off-field (crash), in your emergency landing checklist, along with all of the other fun things like ensuring your fuel is turned off and canopy/door cracked open, include "Switch ELT to "ON".

With a GPS input, your ELT will immediately start transmitting signal with your current location.
 
ELT Antenna installation

The problem with mounting the antenna under tail faring (VS n HS) is the antenna is near metal which affects the VSWR. A VSWR of 2:1 has a reflected power back to the unit of 10% meaning 90% of the power is being sent out. A VSWR of 3:1 reflects 25% of the power. You can buy a pocket network analyzer for $50-$75.

With Artex units you can test an installation by purchasing a subscription for $40-45 which will text and email you if the satellite receives the signal. In my case I didn’t want to mount the antenna on the fuselage. I do long xcountry flying and if I go down in a forest and forget to activate the ELT in the air, branches can easily shear off the antenna and now no one has my exact location. Even if I was with ATC before going down and can give rescuers a good location but it could still take hours for rescuers to find you. A gps coordinate gives your location with 50-100 yds. When a 406 ELT goes off the first 406 signal burst is 50-60 seconds upon activation and about every minute after that where as the 121.5 signal is on continuously but fairly useless in my opinion. I have an RV3 and I mounted the antenna under the canopy to protect it. I was able to test the installation to verify the satellite was receiving the signal. Technically the installation instructions call for the antenna to be mounted with 30 deg of vertical but like someone said earlier, the plane could end up in any orientation. Initially I tested the antenna mounted in the baggage compartment because I’ve heard people having their plane inside a metal hangar and the ELT was accidentally activated and the satellite still received a signal. Someone’s earlier post stated their signal was received being inside a hangar. Well when I tested the installation of the antenna inside the baggage compartment with the plane out on the tarmac, the satellite didn’t receive the signal. So it’s interesting that a satellite can receive a 406 ELT in an enclosed metal hangar but failed when I tested the antenna in the baggage compartment. Hopefully someone on here can explain how or why.

Although the current antenna location is not ideal and has a VSWR of closer to 3:1, I rather have the antenna protected. And yes I did test the antenna with the canopy closed
 

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