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ELT antenna placement

S

springcanyon

Member
Greetings,

I just installed a "AmeriKing" ELT. I am looking for suggestions as to placement of the ELT antenna. The AmeriKing manual recommends a 36" ground plane. That pretty much dictates an outside mount, yet I don't recall seeing very many ELT antennas on RV's. I really hate to stick an ELT antenna on top of my fusalage, but if ever required, I would like the thing to work. Any suggestions?

Thanks,
Don Owens
 
My ELT antenna is on the turtle-deck between the canopy and the vertical stabilizer. Most installation instructions state that the antenna must be within 30 degrees of vertical. Some people mount them inside the cockpit.
 
Don:

Being that RVs are all aluminum, any location exept the engine cowling, wing tips and empanage fairing will provide the necessary ground plane. Internal mounting in the bagage area and under the empenage fairing (attached to the vertical bulkhead) seem popular. I'm mounting mine in the baggage area.

Jekyll
 
RV-9 Slider & ELT antenna placement

I've scanned a few threads looking for ELT antenna placement for my RV-9 slider with super tracks. I laid out the tracks and wonder if the canopy will extend beyond the end of track. Are there other considerations that would affect mounting the antenna a few inches aft?
 
When I upgraded my old 121.5 MHz ELT to the 406 MHz ELT I installed the antenna under the empennage fairing. The instructions require it be within 30 degrees of vertical but I figured the aircraft would be in a nose down position in most likely crash scenarios therefore the antenna would definitely be within the 30 criteria. Others have seemed to agree with me.
 
Is is fine

hoyden said:
I've scanned a few threads looking for ELT antenna placement for my RV-9 slider with super tracks. I laid out the tracks and wonder if the canopy will extend beyond the end of track. Are there other considerations that would affect mounting the antenna a few inches aft?
Click to expand...

This is the configuration I have and I moved my antenna aft to clear tracks. I have to older antenna which is longer; there is a shorter ELT antenna available now. Make sure you follow the instructions and leave lots of slack in the antenna’s cable run
 
ELT Antenna placement considerations

Tankerpilot75 said:
When I upgraded my old 121.5 MHz ELT to the 406 MHz ELT I installed the antenna under the empennage fairing. The instructions require it be within 30 degrees of vertical but I figured the aircraft would be in a nose down position in most likely crash scenarios therefore the antenna would definitely be within the 30 criteria. Others have seemed to agree with me.
Click to expand...

I really don’t think the position of the antenna within 30 deg of vertical matters. This is the reason why I say this. I have an Artex345 and Acrartex has a $40 subscription that lasts for 4-6 months where when you do self tests on the unit. The subscription will text and email you if the signal was received by the satellite. The only other way to easily test the unit without the subscription is to trigger the unit and see if the Air Force calls you which is illegal/frowned upon.

I mounted my antenna horizontally under my canopy to protect it from a crash. Most people don’t realize that when the 406 ELT is triggered, it takes 50-60 seconds before the 406 to send a burst signal. So if you forget to activate the ELT in flight, which is a possibility during an emergency, and you crash through trees and your antenna gets sheared off or if the plane flips over and snaps the antenna, you’re screwed. The subscription verified the signal was received by the satellite with how I mounted my antenna. And by the way I tested it with the canopy closed

When you are operating at such high frequencies, metal near the antenna affects the output on the antenna by reflecting the power back. So mounting the antenna under the tail fairing most likely will possibly reduce the signal output to where the signal won’t reach the satellite.

You can get a handheld network analyzer for about $80 to check the output of the antenna after you mount it.

D9D55755-8024-4218-8831-C77E54F65698.jpg
 
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So

So does anyone have data on how many rvs end up on their back during an accident? How’s that signal strength with the aircraft shadowing the antenna?
 
Antenna Location

I placed my antenna on the small surface area just behind the pilot seat next to the rear window in my RV9. Gusset Plate F-655L in the plans. The antenna takes the curve of the window inside the cabin. The ELT is remote mounted in the rear with antenna cable routed along the inner fuselage. The antenna sits alongside the roll bar, which provides a bit of crash protection, at least as good as my head would get in a crash.

The ELT is an Artex ELT 345 W/Whip Antenna with the remote panel mounted activation switch.

I was concerned that a ground plane might not be the best but got a call from Airforce Search and Rescue one night that the ELT was activated. The plane was in a large metal hangar and was still able to get a signal to the satellite. I went to the airport and turned the ELT off. It activated by itself so I had to send the ELT 345 in for repair, it was 6 years old and on its second battery.

So that answered my ground plane concern question.
 
RV7

I looked up the Gusset Plate number I referenced in the prior post and I think the number in the RV7 is the F-695-L. I don't have access to the RV7 plans but it appears very similar to the RV9 F-655-L gusset.
 
When I came out of A&P school my first job was working on helicopters and I asked why all the ELTs were mounted on a 30 degree (IIRC) angle and was told that's how they usually crash. Given the dismal record of ELT "saves" and the number of different ways an aircraft can crash, this whole what orientation question seems to me to be a lot like the what is the best seat to be in when an airliner crashes discussion. I like the old EBC ELTs where the antenna was integral to the unit. Better yet, I think we should all do like the glider guys do and convert to PLBs. (Of course, nobody cares what I think and I've stopped caring that they care what I think,, so there is that.) :D:p
 
Perhaps the recommend 30 degree orientation has nothing to do with crash or final aircraft orientation, but has everything to do with the relationship between the antenna and it's ground plane.
 
CATPart said:
Perhaps the recommend 30 degree orientation has nothing to do with crash or final aircraft orientation, but has everything to do with the relationship between the antenna and it's ground plane.
Click to expand...

In the case of a quarter wave whip antenna the best performance is obtained with the vertical portion of the antenna being perpendicular to the ground plane. The selection of a 30 degree recline angle is based on esthetics, drag and the ability to make such an antenna more durable than a vertical pole. That durability goes toward crashworthiness as well.
 
FireMedic_2009 said:
<snip>
The only other way to easily test the unit without the subscription is to trigger the unit and see if the Air Force calls you which is illegal/frowned upon.
<more snip>
When you are operating at such high frequencies, metal near the antenna affects the output on the antenna by reflecting the power back. So mounting the antenna under the tail fairing most likely will possibly reduce the signal output to where the signal won’t reach the satellite.
Click to expand...

One data point: My 406 Antenna is in the empennage intersection, under the fairing, horizontal, with a bent tip to make it fit. An inadvertent activation inside a door closed metal T-hangar generated a call from a friendly Airman standing watch, who correctly identified the airport.
 
The biggest threat to a transmitted signal is the antenna cable. Too many get ripped off in an accident or burned.
If the ELT works, and the antenna is connected, it is very likely that the signal will get received, regardless of orientation, even sticking in the dirt.
Chad Stimson at NASA did a lot of research on ELT crash survivability and improving the g-force curve for activation as inputs to ED-62B.

So, put the ELT near the antenna, or leave lots of slack in the cable. I used weak wire ties on a cable loop.

And I carry a PLB... I'd rather sleep in my bed that night than make a fire with sticks and a flint.

https://www.nasa.gov/langley/crash-test-assesses-plane-emergency-locator-transmitters

Report:
https://ntrs.nasa.gov/citations/20160009372
 
ELT signals received through a metal hangar

I’ve heard of several people saying their ELT’s went off in an enclosed metal hangar and were called by Search n Rescue. So when I was installing the 406 in the RV3 I first tested the antenna mounted in the cargo area behind the seat back. No signal received (no text or email on my test subscription) and I did a few tests. The satellite only received the signal when the antenna was mounted on the outside. It could be the antenna is too close to the enclosed fuselage reflecting the power back, but when it’s far away from an enclosed metal hangar the signal gets through somehow, I don’t know.
 
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I have had a conversation in the past with one of the major manufactures, and they told me that there are instances of 406 MHz ELT‘s riding in metal delivery trucks without antennas attached to them, be tracked as they moved across the country from one side to the other.
Not that that is a reason to deviate from installation recommendations, but I think it does show that there is quite a bit of latitude as far as performance goes. Don’t count on this if you only have a 121.5 MHz.
 
Thanks everyone for your perspectives. Sounds like mounting to the top skin between the slider canopy rail and vertical stabilizer is less desirable unless I want to add extra coax. There is almost zero slack otherwise. Now I am thinking attaching to the baggage wall bulkhead with the antenna facing forward just under the canopy. Or maybe perpendicular to the baggage wall with the antenna facing up. Either space is wide open to the sky and very close to the ELT. My ELT is attached to a bracket riveted to the right side lower longerons just aft of the baggage wall. The mounting angle is within 10 degrees of the longitudinal axis. The location is easy to access for inspection. Just have to be antenna aware when the canopy is open.
IMG-8464.jpg
 
fl-mike said:
The biggest threat to a transmitted signal is the antenna cable. Too many get ripped off in an accident or burned.
If the ELT works, and the antenna is connected, it is very likely that the signal will get received, regardless of orientation, even sticking in the dirt.
Chad Stimson at NASA did a lot of research on ELT crash survivability and improving the g-force curve for activation as inputs to ED-62B.
Click to expand...

I very often run across ELT antenna cables running through bulkheads. Often, in a crash, a bulkhead gets crushed thereby cutting the antenna cable. Wherever possible the cable should run around a bulkhead and not through it.
 
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The technically correct answer

Canadian_JOY said:
In the case of a quarter wave whip antenna the best performance is obtained with the vertical portion of the antenna being perpendicular to the ground plane. The selection of a 30 degree recline angle is based on esthetics, drag and the ability to make such an antenna more durable than a vertical pole. That durability goes toward crashworthiness as well.
Click to expand...

My dad was an electrical engineer and ham radio connoisseur, I became interested and we made a number of antennas. In one of the pictures shown with the antennas mounted 90 degrees, the grounding plane is so small that the power transmitted will be significantly reduced. The grounding plane works like a radio frequency mirror so that it behaves as if there is a comparable length of antenna on the opposite side.
 
Howdy Everyone. My ELT is behind the baggage area and the antenna is behind the roll bar, passenger side. (RV-7)
 

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Karetaker said:
Howdy Everyone. My ELT is behind the baggage area and the antenna is behind the roll bar, passenger side. (RV-7)
Click to expand...

Mike,

Recommend adding some stand off insulators on that antenna. It looks like you have it shorted to aircraft ground.

Keep in mind the end of a quarter wave antenna is the high voltage part. This end is most affected by proximity to any metal structure - so I’d go a good 3”-4” of spacing.

Carl
 
You definitely want some Delrin standoffs there, or similar - no part of the antenna should touch any part of the metallic airframe, that's a dead short for the RF trying to get out.
 
Thanks for the tips. I believe the base of the antenna is insulated, but the antenna wire is not isolated from the roll bar. (Apologies to the OP for the thread drift.)
 
ELT antenna in wingtip

I placed my 406 ELT and the antenna inside the wingtip of my RV8 with the outboard rib acting as a ground plane. I cut down a rod antenna to the right length (about 11 inches) and mounted it with vertical polarization. I used my signal strength meter to verify the efficiency. Months later while testing the elt, I was unable to turn it off because a wire in the crimp had come loose. While frantically trying to remove all the wingtip screws to access the switch on the elt to shut it down, I received a call from a USAF officer. I explained the situation to him, and he was very cool about it.
Here is the thing...
I asked him how strong my signal was and were they are showing my airplane's location. He said the signal was 5x5, and was located at VGT airport. I asked where on the airport and was told very precisely were.
My 406 ELT was not equipped with GPS and it was located inside my steel hangar and the door was closed!
So if their satellite could receive my ELT, and determine the position without GPS very accurately (within 200 feet) inside my steel hangar with no windows and closed doors, I wonder if we are getting the full story on this technology.
Inside the wingtip provides good crash survivability for the antenna and the elt (away from a fire). If the wing is sheared off in a crash, the ELT will survive. I think that the vertical polarization is also helpful, as the satellite is usually above you, right? I have no idea why they want you to mount the antennas with horizontal polarization, unless they are thinking that would optimize for ground receivers? Again, we may not be getting the full story.
Just saying...
 
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BenNabors said:
. . . . In one of the pictures shown with the antennas mounted 90 degrees, the grounding plane is so small that the power transmitted will be significantly reduced. The grounding plane works like a radio frequency mirror so that it behaves as if there is a comparable length of antenna on the opposite side.
Click to expand...

Hi Ben
I am the one who has the antenna mounted 90 deg on the RV3. Question: Is the antenna only looking at the at the vertical portion of the bracket as the ground plane even though the bracket is grounded to the fuselage meaning the antenna is not looking at the fuselage as part of the ground plane?

If I increase the width of the bracket should I also increase the length above the antenna so the antenna is not at the edge of bracket/ground plane?

Also am I correct that the ground plane thickness can be fairly thin? My bracket is thick to provide rigidity so I guess I can add a circular or oblong ground plain between the antenna and the bracket, correct?

Thanks,
Daren
 
cjhukill said:
I think that the vertical polarization is also helpful, as the satellite is usually above you, right? I have no idea why they want you to mount the antennas with horizontal polarization, unless they are thinking that would optimize for ground receivers?
Click to expand...

I think you are confusing polarization with radiation patterns. A vertical monopole is vertically polarized, and its radiation pattern strongest in the horizontal direction, so toward the horizon, but weakest straight up and down. A horizontally mounted monopole, as you have described i think, is horizontally polarized, and the radiation pattern is strongest well, pointing straight up, down, anywhere except where the antenna is pointing. Should you be doing a wingover, you will certainly for a moment at least have a vertically polarized antenna :)

The addition of the ground plane to the monopole increases the strength of the radiation pattern, growing it both horizontally and vertically. This relates to my earlier post that the 30 degree limitation is about as far from perpendicular as you can be to the ground plane before having diminished or deleterious effects, a compromise between looks and functionality.

Agreed, some of the evidence shows that none of this "seems" to matter much.
 
The COSPAS SARSAT satellite receiving antennas are circularly polarized, so they can handle a vertically or horizontally polarized signal. The polarization loss is factored into the RF link budget.
 
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