ELT Antenna Placement

[GraemeSmith]

I've search the forums and it's been discussed in general - but I'm trying to nail this down a bit better.

Anyone on here with radio / antenna experience care to pitch in?

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So a combination 406/121.5 ELT whip antenna is about 16 inches in length. It's a matter of physics. And for certification purposes you have to use the manufacturer's antenna so substituting some glass fibre blade types (about 10" tall) is not permitted.

The antenna has to see the sky. So on top of turtle deck somewhere..... A lot of Sonex end up on their backs - antenna gets smooshed. So that doesn't work. And as I just discovered - it's vulnerable to an errant cover blowing in the wind......... :-(

Can't put it inside - the metal box would shield the 5w signal.

My builder put the antenna in the corner of the glareshield and bent the whip to follow the windshield bow. But does that modify the signal or mask the signal too much? Newer whips have a thickened base and can't be bent to follow that curve anyway. Which is why I could not reuse that spot.

How about flat on the edge of the glareshield? From my limited understanding - that doesn't work because the needed ground plane provided by metal structure is not "below" the whip. Neven mind half the signal will be lost into the glareshield.

Anyone?

[racaldwell]

Mine is in the cockpit right rear corner with the end just shy to touching the canopy. If I had a passenger, it would be just behind their right shoulder. I saw this location used on a friend's Rocket and it seems like a good solution to keep it out of the breeze. He unknowingly turned his on and the CAP came looking for him so it seems to work even with the bulkhead behind it proximity.

Rick Caldwell
Xenos 0057

[Direct C51]

Mine is on the turtle deck about half way to the tail. A lot of Sonex builders put it there. If it flips, your vertical stabilizer should keep it from completely smashing the antenna.

That being said, I hate how it looks and can’t help but think it causes a good bit of drag. I will probably bring it inside once I finally start the aerodynamic clean up of my plane.

[n502pd]

Good afternoon! Antennas, radios, placements are right in my cone of experience! I have been a ham operator since I was 13, now 76! I have built lots of antennas and placed them to work as well as possible.j
So, for maximum "radiation efficency" ( the ability to be heard?) an antenna should be as close to 'free space' as possible, that is, nothing conductive nearby. laying the antenna down on the glare shield, as you said is improper, because of ground plane as you said, but also that metal next to the antenna detunes it away from its resonant frequency, making the antenna way way less effective for being 'heard'. The best placement for any antenna, at any frequency, is to be as far as possible away from conducting surfaces that are not being utilized as a ground plane. Inside the cockpit, IMHO, is no-no, because of nearby conductors, including the passengers shoulder, detunning the antenna, and partially absorbing energy fron the signal (dont stand infront or near an active radar antenna...it will heat you up from the inside microwave oven style). So, all this said, IMHO, and experience, is to mount the antenna near the aft end of the fuse. I mounted my 121.5 antenna roughly 1/4 to 1/3 the distance from the vert stab-fuse joint, which can protect the radiation ability in case of a flip over. Some have com antennas on the belly, and have trouble with comms while on the ground. the signal isnt going far enouge because of proximity to the partial conductiveness of the runway, earth, concrete,etc.

'The 406 mhz antennas have a matching coil in their base to allow reasonable radiation efficency at both 121.5 and 406 mhz, and that is why they cant be bent over and be expected to function per specs. Further, 406 mhz radiation is more easily interfeered with by nearby objects because of higher absorbsion.

INHO, all that being said, the best place in the aft 1/4...1/3 of the fuse, as long as it is as far away as possible to the com antenna, which is another good absorber of energy.

I know I talk too much! But, beside aviation, radio/electronics has been in my blood for a very long time! Hope I have not confused anyone too much!

And, thanks for your recent conversations on your flying! Keep them up!

Joe

[13brv3]

I've been struggling with this myself, for a Onex. Sadly, my Artex antenna is 23" long, though I think they have a shorter version now. Even with the shorter one (which I won't pay $305 for), there's still no easy option.

Mounting it on the centerline of the turtledeck doesn't seem like a great option, since you'd be cutting into the center channel and where the skins overlap. It could be done, but the next issue I have is that it would be located in a part of the rear fuselage that you can never get to after the bottom skin is riveted in place. Functionally, it would be the best location, but it's not a clear winner with the other drawbacks. If anyone has pictures of how they installed theirs, I'd be interested in seeing the mounting details.

I had pretty much settled on the rear corner of the fuselage, keeping it as high as possible without interfering with the canopy. This wouldn't be ideal from an RF standpoint, but I have to imagine an ELT is expected to still function in far less than ideal situations.

If there's any doubt about my ability to put the plane down safely (pretty much all of East TN) I'm going to activate the ELT manually. I'm told the first fix will be sent in 50 seconds or less, so it's good to get at least one set of coordinates to the satellite before having to rely on the system to survive the landing and work automatically.

Rusty (mounting the ELT antenna somewhere)

[Direct C51]

You don’t have to mount it on the centerline. Both my comm and ELT antenna are offset slightly to clear the channel and overlapping skin on the turtleneck. It is hardly noticeable unless you are looking for it.

[builderflyer]

Joe, I don't disagree with anything you said and, in fact, I initially located the 121.5 ELT stainless steel whip antenna just about at your recommended location (on the turtle deck forward of the vertical stabilizer). But when I relocated the ELT itself forward into the baggage compartment, I took that opportunity to move the ELT antenna as well to the same location as used on the Sonex factory prototype, out of the slipstream. That is, inside of the windshield with the glareshield serving as the ground plane (ouch!). You may or may not be aware that the Sonex prototype utilized a single antenna for both the com antenna and the ELT antenna (I never completely understood how they were able to do the cabling to make that work).

You and I share a common interest in ham radio. I've had a ham ticket for 2 years longer than my pilots license that I've had for 56 years. Also, when I exchanged my original ham call sign for a one by two call sign when they came available in the 70s, I ended up with N6UZ which subsequently became the n number on my Sonex as well as my car license plate number. A bit of trivia that you may be interested in.

Art,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,,.Jabiru 3300 #261

[daleandee]

I must be doing this wrong because like Art I placed my ELT antenna inside on the glare shield on both Sonex aircraft that I have owned.

And I did place my com radio antenna on the belly along with my transponder and ADSB out antennas (spacing is among these was very important).
The only antenna on the turtle deck is my ADSB GPS antenna but it's white and matches very well.

I also confess to having a ham radio license although it's been a few years since I've been active. I was gonna go with a shorter number but the one I received was quite unique, KG4FCC. LOL!

As to the point of the thread and the use of ELTs ... consider that if you have ADSB out and are on flight following, your location and need for assistance should be known to those that can assist you, perhaps even before the aircraft has landed once you declare an emergency.

[n502pd]

To all the repliers... I have no real problem with anywhere on the aft fuse, except for maybe near the canopy soley because of most likely total distruction of the antenna durring a flip over. the rudder and aft canopy are susposed to act like a roll cage...sort of... in the flip over, and protect most of the antenna from being destroyed, as well as the occupunts.

The 23 inch thing for length indicates that is a 1/4 wave antenna, used because at that physical length, the impeadence of the antenna is very close to 72 and 50 ohms, which is what the little transmitters are designed to feed, and to the coax value of 50 ohms for RG58xx, or 72 ohms for RG-59xx, both the 1/4 inch or so diameter coax. Other coaxs are double shielded, as the orange-ish stuff usually used for transponders (I forget the actual number on the coax). the shorter antennas that have a cylindrical thumb sized part at their base, or part way up the actual antenna, that part is a matching network need for either dual frequency matching, or for matching a shorer than 1/4 wavelength antenna. the antenna efficency next to the windscreen plastic wont really affect it much if at all, but the curvalinear positioning to the metal glare shield by the bent over antenna may very well mess with the efficency by detunning, and by distorting the radiation pattern from ideal omnidirectional.

IMHO keep all antennas seperated as far away from each other as possible. Com on the top, elt on the top, gps on top, transponder on bottom. unless you have a VERY strong com transmitter ( more than 20 watt), gps wont be interfered with because of the great frequency differences. since I have skybeacon on the lh wing tip, it is quite far from the com transmitter, and is not in direcdt sight line with transponder stub antenna on the bottom, per directions.

Lastly, everyone fly safe and i hope and pray none of us ever have to actually need an elt!

Have a safe holliday!

[GraemeSmith]

Thanks all!