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A new idea for ELT antenna placement

bifft

Well Known Member
So, like all 8(A) builders, I am having to decide between many bad options for the ELT antenna. Looking over past threads, I would summarize the most popular options as under the empennage fairing, or in the passenger armrest. Both of these seem like they have the antenna hidden behind a lot of metal, and wouldn't transmit very well. I have also seen the mid cabin brace mount, but that seems to be lacking a proper ground plane, and I am planning on putting a camera there.

Another option would on the bottom which has a great ground plane, and would be perfect if the plane ends up on its back, as 3/10 of the 8A accidents I find in the NTSB archives did. However, if you stay upright, it would easily be scraped off by brush or flattened under the fuse, turning into nothing at all.

My I think original idea, is to take a quarter wave 406Mhz antenna, which by my calculations is about 7 inches long, and put it behind the passenger's head like so:

img_4911m1.jpg


This could be a homemade rod antenna, or a commercial ducky type like this:

http://www.wpsantennas.com/exc406bnx-antenexcenturion406-420mhzbncmalecoveredtufduckantenna.aspx

Good points: I have measured, this will not hit the canopy either open or closed. It is likely to survive anything the occupants survive. Has a decent (tho not perfect) ground plane. This meets the ELT manufacturor requirements for 406Mhz, but probably not for 121.5. They do keep saying they want to stop listening on 121.5.

According to the manual, I must use an approved antenna, and this is not on the (very short) list. However, I think it would actually work better than the approved antenna in most accident scenarios. So, meeting the spirit if not the letter of the law.

Wanted to get some other opinions before I start drilling holes.
 
Great Idea!

I like it. However, the ELT must be installed in accordance with the manufacturers instructions. I don't think they will allow a "homemade" antenna.

Next best thing would be to mount the factory antenna within the cockpit under the canopy.
 
I like it. However, the ELT must be installed in accordance with the manufacturers instructions. I don't think they will allow a "homemade" antenna.

Next best thing would be to mount the factory antenna within the cockpit under the canopy.

That would be the letter of the law problem. The factory antenna is 15 inches long and doesn't fit under the canopy, unless you let it bend, and let the tip slide along the canopy, which would probably lead to scratches.
 
Doesn't have to be on the deck. It can be mounted on the side panel below the Longeron.
 
Doesn't have to be on the deck. It can be mounted on the side panel below the Longeron.

Which would not meet the letter of the law for the ACK 406 unit...

From the approved installation manual -

The antenna must be mounted externally, on airframes of metallic construction.

The lower part of the 15 inch antenna is the 406 portion, and it would be substantially shielded on the side panel location...:rolleyes:

It might have been an OK idea with 121.5 ELTs but is a poor idea for the newer 406 units.
 
ELT's and 121.5 transmission

Hi Guys,
I speak for the situation in Australia and am confident it applies globally, but wanted to clarify the issue of ELT frequencies. The initial, or long range interagation of activated ELT's by rescue co-ordination centres will in time transition completely to 406, but the final homing and, utimate location of the activated device will remain reliant upon a strong 121.5 transmission. This situation holds for ELT's both with and without GPS assistance.

I would encourage those contemplating the installation of different ELT antennas to not dismiss the importance of the 121.5 signal

As a rescue helicopter crew member, there is nothing more satisying on a SAR task than hearing and homing to a strong 121.5 tone and rendering assistance
 
Ditto

I mounted mine in exactly the same spot, using a "rubber ducky" freq appropriate antenna. I use a piece of angle cut and shaped so as to provide a horizontal mounting plane.

Don
 
I mounted mine in exactly the same spot, using a "rubber ducky" freq appropriate antenna. I use a piece of angle cut and shaped so as to provide a horizontal mounting plane.

Don

Again, not approved under the FAA TSO 126

e. The Federal Aviation Regulations and other TSO Requirements for ELTs. This paragraph adds another option, the 121.5/406 MHz ELT configuration, to the configurations described in RTCA/DO-204, paragraph 1.2, System Overview.
An approved ELT is required for compliance with the FAR. A method of compliance is to obtain approval for installation of a 406 MHz ELT which meets the requirements of this TSO.
The intended configuration of this ELT can be accomplished by either of two approaches:

(1) Installation of a stand-alone 406 MHz ELT to augment an existing 121.5/243.0 MHz ELT installation;

or (2) Installation of an integrated 121.5/406 MHz or a 121.5/243.0/406 MHz ELT, of which the 121.5 or 121.5/243.0 MHz portion meets the requirements of TSO-C91a.


The FAA requirement calls for a 406 ELT in conjunction with a 121.5 ELT. A 406 only device does not meet FAA requirements.

Your choice though if you can persuade a DAR...
 
Just to build on some of the points made above...

Your 406MHz ELT transmits on 406MHz AND 121.5MHz. The duty cycle for 406MHz is very short, less than 1 second every 50 seconds. or less than 1%. The REST of the time the ELT is transmitting its homing signal on 121.5MHz.

If you've stuck a 406MHz-only antenna on your airplane then you are, as other posters have mentioned, NOT meeting the FARs because you are not in compliance with the requirements of the TSO. If you don't meet the FARs then your insurer will no doubt invalidate your insurance should an incident occur. You MUST use ONLY the antennas approved by the manufacturer, no substitutions allowed, period.

Now for the other little nitty-gritty detail... Your 406MHz rubber ducky antenna will present significant VSWR to the transmitter and may cause the transmitter to fail. Not all transmitters can withstand the heat caused by power reflected by poorly matched antennas and transmission lines.

What I can't understand is why on earth anybody would stack the odds against themselves by intentionally designing a sub-optimal ELT installation? That's like buying an insurance policy and then making sure you invalidate it by performing acts not covered under the policy. Not very smart.

One CAN install the antenna on the fuselage so that it is just clear of the aft edge of the canopy when the canopy is slid fully open. My hangar-mate's 8A is done that way and it certainly provides for a much better compromise than some of the solutions mentioned above.
 
I do not have an 8 but I offer up this info about my 6 installation as a data point for thought.

My standard ACK 406 antenna is mounted in one of the typical spots for the 6 - on the seat back cross bar behind the pilot's left shoulder. It is mounted to the smallish triangular plate (about 3" x 5" x 6" right triangle) that is part of the "stock" slider structure. The antenna is bent back at a curve and attached to the aft turtle deck skin. It is held "loosely" at the tip by inserting the tip into a piece of plastic tubing. The tubing is fixed to the edge of the turtle deck skin. The antenna orientation is curved, fits under the canop when closed and I would say it is roughly 45 degrees to the horizontal.

The whole idea was that this antenna location is under the canopy, is unobstructed, and has the potential in a crash to release to the vertical in the event the canopy is shattered and the tip slips out of the plastic tube holder during a crash.

My main concern was that the ground plane provided by the small triangular mounting was insufficient to provide adequate VSWR for effective transmission.

Well I was wrong. The small triangle is adequate. My antenna transmits perfectly, according to the LT Col I talked to at the Tyndall AFB SAR center who called me to discuss my inadvertent activation that they picked up from less than 20 seconds of activation, from inside my hangar (wood roof) with my GPS not powered up.

Embarrassing but a reassuring test. I pass it to you and others to say that there are many options that might work with only a very small ground plane required and the antenna at less than vertical.

The system appears to work very well. And I advise that you list your cell phone as the primary contact when you register with NOAA. Calling that number is the first thing they will do.
 
If you don't meet the FARs then your insurer will no doubt invalidate your insurance should an incident occur. You MUST use ONLY the antennas approved by the manufacturer, no substitutions allowed, period.
Can anyone provide an example of an insurer "invalidating" a policy because of some part not meeting the FAR's?

I have an ACK E-01 121.5 Mhz ELT. The new E-04 comes with an antenna that won't fit under the dorsal fin of our plane. Yet in the instructions, it says that you must use their antenna or one meeting the TSO. Turns our another brand does make a 406 whip antenna.
 
Invalidation of insurance is certainly more common in the "certificated" world, but it has happened in the amateur-built world as well, particularly when dealing with unapproved modifications to engines and props. When there's a large liability suit at stake the insurance companies will do just about anything to avoid a payout.

With respect to antennas, please keep in mind that ELT manufacturers have to test their ELT as a complete "system" and as a result their TSO approval extends only to those components of the system which were tested together. You can't take an ACK antenna and use it on an Ameri-King ELT. In some instances the ELT manufacturer will list in their installation manual the part number of the antenna OEM so, if necessary, you can purchase the OEM part rather than buying the antenna from the ELT manufacturer, but this is not common practice. Listed below is information from the Kannad 406-AF Compact installation manual

KANNAD Designation Manufacturer KANNAD Part Number
ANT200 DAYTON GRANGER ELT 10-773-x 0145621
ANT300 CHELTON 1327-82 0124220
WHIP ANT AV200 RAMI AV-200 0146150
ROD ANT AV300 RAMI AV-300 0146151
ANT100(See note) PROCOM 0124206
 
Still, I never hear of insurance companies refusing to honor a claim, even when the owner messed up or the airplane is technically not airworthy. Not that they don't, but I cannot see a non-conforming antenna as a defense against payment.

According to ACK, you can use a different antenna with the E-04.
From their FAQ:
"Do I have to use the ACK antenna with my E-04 installation?
Yes, you have to use our antenna or a COSPAS/SARSAT / FAA TSO C-126 approved antenna meeting our VSWR requirements."
 
Invalidation of insurance is certainly more common in the "certificated" world, but it has happened in the amateur-built world as well, particularly when dealing with unapproved modifications to engines and props. When there's a large liability suit at stake the insurance companies will do just about anything to avoid a payout.

With respect to antennas, please keep in mind that ELT manufacturers have to test their ELT as a complete "system" and as a result their TSO approval extends only to those components of the system which were tested together. You can't take an ACK antenna and use it on an Ameri-King ELT. In some instances the ELT manufacturer will list in their installation manual the part number of the antenna OEM so, if necessary, you can purchase the OEM part rather than buying the antenna from the ELT manufacturer, but this is not common practice. Listed below is information from the Kannad 406-AF Compact installation manual

KANNAD Designation Manufacturer KANNAD Part Number
ANT200 DAYTON GRANGER ELT 10-773-x 0145621
ANT300 CHELTON 1327-82 0124220
WHIP ANT AV200 RAMI AV-200 0146150
ROD ANT AV300 RAMI AV-300 0146151
ANT100(See note) PROCOM 0124206

From a FAA TSO point of view, substitution is allowed for an antenna IF it meets the manufacturer's electrical requirements, and the FAA requirements (ie, must be an aircraft antenna, not a bent piece of wire) -

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/1ab39b4ed563b08985256a35006d56af/ceba7446f88ecfa1862569c3004ba827/$FILE/ATTZVBXH/ac20-41a.pdf

I would say an antenna change to another antenna that was designed for aircraft and met the published transmitters RF/electrical requirements would be a minor change per the AC above.

The SAR certification may be different though.
 
I mounted mine in exactly the same spot, using a "rubber ducky" freq appropriate antenna. I use a piece of angle cut and shaped so as to provide a horizontal mounting plane.

Don

When you say "freq appropriate", did you measure the SWR on 121.5? I don't have the tools to do that, and don't have a beginning of a way to guess.

From the ACK E-04 manual it has:

Other antennas may be used provided they meet the
minimum VSWR requirements as noted in the specifications section of this manual, (Section 15) and meet COSPAS/SARSAT approval for use with the model E-04 ELT.​

Section 15 has:

VSWR @ 121.5 MHz  2.0:1
VSWR @ 406 MHz  1.4:1​

But then the COSPAS/SARSAT report only includes two antennas, the included 15 inch whip and a large blade that would be even harder to mount.

The extreme rear fuselage option isn't too bad, and aside from being right next to the vertical stab would be great. Probably better than having it inside the fuselage. I suppose that's the best of a bad lot.

I do like gereed's idea of having it inside a plastic tube to protect the canopy. That would be easy, not obstruct radio signals and you could replace the tube at any time. Need to think some more.

I'll look to see if I can find any COSPAS-SARSAT approved antennas that list the SWR measurements.
 
Try this. it fits the E-01 and E-04's requirements, without that stupid 8" rigid plastic section. To me, it sounds legal for an ACK E-04 install.
http://www.aircraftspruce.com/catalog/avpages/ramiav100.php?clickkey=5356

at 13.75 it wouldn't fit under the canopy without bending, but being straight wouldn't lie directly along the VS if mounted at the rear. Thanks, will keep looking, but I do notice that none of the dual frequency antennas I find are 7 inches, probably a basic physics reason for that. (Although, I could probably fit a blade antenna back there, sideways. Expensive and would look really silly)
 
I have my ELT antenna under empenage fairing (AK-451), for year I was wondering if it works in that position. Some time ago I was cleaning the panel from my 8, inside my metal hangar with doors closed and I pushed ELT button by mistake:D and also didn't hear the alarm for 1 or 2 minutes;)

Ten minutes later my wife call me almost crying, someone from SAR call my house and said that my ELT was activated.

Now I know my ELT works and I promised my wife not to clean my panel again:p
 
I have my ELT antenna under empenage fairing (AK-451), for year I was wondering if it works in that position. Some time ago I was cleaning the panel from my 8, inside my metal hangar with doors closed and I pushed ELT button by mistake:D and also didn't hear the alarm for 1 or 2 minutes;)

Ten minutes later my wife call me almost crying, someone from SAR call my house and said that my ELT was activated.

Now I know my ELT works and I promised my wife not to clean my panel again:p

That installation would not meet the required US TSO.

From the ACK-451 manual

The antenna must be installed VERTICALLY (within ? 15o of the vertical plane is acceptable).

Again, a rule bending DAR would be needed....
 
It is not the responsibility of the DAR to see that TSOs are met.
That is the responsibility of the owner/operator.
Same goes for the transponder/altitude reporting system, lighting systems, etc.

It is the DARs responsibility to confirm that the aircraft meets 21.191(g).
 
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It is not the responsibility of the DAR to see that TSOs are met.
That is the responsibility of the owner/operator.
Same goes for the transponder/altitude reporting system, lighting systems, etc.

It is the DARs responsibility to confirm that the aircraft meets 21.191(g).

21.191(g) almost says diddly -

(g) Operating amateur-built aircraft. Operating an aircraft the major portion of which has been fabricated and assembled by persons who undertook the construction project solely for their own education or recreation.


Essentially, if that is your only criteria, all you need to do is check the 51% rule and issue the certificate...:rolleyes:

There must be a few more FARs a DAR must follow...:)

You don't confirm the aircraft meets the requirements of 91.207?
 
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How many FSDO inspectors have you seen checking the TSOs on equipment installed on Amateur-Built Aircraft?

Most everything a DAR inspects is to help the applicant make sure that the aircraft is in a condition for safe operation. This is NOT a requirement, it is simply why most builders opt for a DAR.

Reality check:
How many RV-8s are certificated?
How many of them do you think comply with the ELT TSO?
The RV-8 was simply not designed with this in mind.

Again, final responsibility rests with the owner/operator.
 
Yep, I think it is safe to say there are absolutely zero ways to install an ELT antenna on an RV-8 per the TSO. The sliding canopy design makes a "legal" installation impossible.
 
Yep, I think it is safe to say there are absolutely zero ways to install an ELT antenna on an RV-8 per the TSO. The sliding canopy design makes a "legal" installation impossible.

Interesting dilemma, How do you meet a required FAR when you can't?
 
Mount it on the bottom of the fuselage

I spent considerable time contemplating this issue and concluded that my best option is to mount the ELT antenna on the bottom of the fuselage. In the event of a crash in which I am disabled and unable to activate the ELT myself, there is a reasonable probability that the fuse will be inverted and the antenna oriented just like the engineers and bureaucrats desire. Given the lack of reasonable alternatives in an 8, I just made the executive decision and left the rest up to the good Lord.
 
That installation would not meet the required US TSO.

From the ACK-451 manual

The antenna must be installed VERTICALLY (within ? 15o of the vertical plane is acceptable).

Again, a rule bending DAR would be needed....

Hi Mel,

Yes you are right, I didn't meet TSO, but not because I want to, it's almost impossible to meet TSO in a RV-8 at least you modify the canopy. I choose to install the antenna under fairing because is where "mother ship" installed and I also I know it works even inside my hangar. I am building a second RV-8 and probably will do the same, I haven't see a better place to install ELT antenna here on VAF.

Cheers,

Fernando
 
Hi Mel,

Yes you are right, I didn't meet TSO, but not because I want to, it's almost impossible to meet TSO in a RV-8 at least you modify the canopy. I choose to install the antenna under fairing because is where "mother ship" installed and I also I know it works even inside my hangar. I am building a second RV-8 and probably will do the same, I haven't see a better place to install ELT antenna here on VAF.

Cheers,

Fernando

Good for you Fernando!
I'm in the camp of "living life" and seeking "forgiveness" rather than watching others live life while "seeking permission"!
 
I've heard of folks mounting them inside the wingtip. Same antenna orientation upside down or right side up, and the antenna cable doesn't pass through structure.

aw38z.jpg
 
I've heard of folks mounting them inside the wingtip. Same antenna orientation upside down or right side up, and the antenna cable doesn't pass through structure.

aw38z.jpg

Not sure I like that, the ELT G-switch really should be mounted in primary fuselage structure. I hard-mounted mine in the fuselage just behind the baggage compartment and put my ELT antenna under the fairing for lack of a better location.
 
Why must the antenna be mounted almost vertical, if we crash, are we guaranteed to land up right on the wheel
 
Why must the antenna be mounted almost vertical, if we crash, are we guaranteed to land up right on the wheel

If the gov't requires you to crash upright, then you'd better crash upright or you'll be in BEEEEGGG trouble! :D
 
Whatever you do - do NOT put it in the wing please!

I've heard of folks mounting them inside the wingtip. Same antenna orientation upside down or right side up, and the antenna cable doesn't pass through structure.

aw38z.jpg

After 26 years of Civil Air Patrol Search and Rescue flying, I unfortunately have more experience than I wish I had with respect to the causes and end-results of many airplane crashes. See the following link for more info on this ELT antenna topic:
http://www.vansairforce.com/community/showthread.php?t=81437

In short - the wing is one of the first things to become completely destroyed in a crash situation, so this is about one of the worst locations you can choose if you expect it to provide any benefit.

I too have struggled as to where to put mine, and for the RV8 there is no simple answer. So in the end you are left with little more than to pick the best location you can with the space available that will allow the unit the best possible chance of surviving a significant impact.

I found it interesting that another poster mentioned that the NTSB accidents for RV-8s indicate that many of them end up on their backs. Without knowing the details, it sounds like these may have been ground loop or other landing-related types of problems. The truth about any airplane crash from my own personal SAR experience is that the tail section is the only part of the aircraft that is even remotely recognizeable in most cases, and is thus the reason that ELTs are usually mounted in the aft section of the airplane.

As for the antenna ground plane and adequate SWR comments, all I can say to that is that SAR pilots undergo substantial training that is conducted by using practice beacons with antennas that are configured all sorts of ways. This includes scenarios with no antenna at all, to one that is half broken off, to one that is buried 3 feet in snow, or ones with less than a full battery charge, etc. Perhaps the closest thing we ever did in training that would emmulate an antenna housed inside the fuselage is when we would put one inside a 55 gallon drum and close the lid, and then tip it over on its side.

The end is result is that sometimes you are amazed at how some of them can still be located under less than desireable circumstances. I guess my current perspective is that I am less concerned about where I put the ELT and antenna (wing excepted) as long as it is somewhere aft. I am more concerned about making sure that the ELT batteries are fully charged and replaced when they expire, and that the system is periodically tested (the right way) so I know it is working. Bad or neglected batteries, more than anything else, are the biggest reason why ELTs are rendered useless when they are needed the most.

Fly safe....
 
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As for the antenna ground plane and adequate SWR comments, all I can say to that is that SAR pilots undergo substantial training that is conducted by using practice beacons with antennas that are configured all sorts of ways. This includes scenarios with no antenna at all, to one that is half broken off, to one that is buried 3 feet in snow, or ones with less than a full battery charge, etc. Perhaps the closest thing we ever did in training that would emmulate an antenna housed inside the fuselage is when we would put one inside a 55 gallon drum and close the lid, and then tip it over on its side.

Thanks for the comments Bryan.

Was that 55 gallon drum test with 121.5 or 406 ELTs?

I know several folks have done the 121.5 "no antenna" test by accident and found that they usually work...:)

The shorter burst of the higher (and weaker?) 406 frequencies might be diofferent though...
 
Mostly VHF....

Gil, unfortunately most of my experience was working with 121.5 beacons/antennas. I ceased active participation in CAP and started building my RV 8 a couple years back, just about the same time that the new 406 ELT system change-over was just starting to ramp up, so I have not been able evaluate the differences that one might expect when working with the UHF band units. That said, I would certainly expect line of sight reception to be even more of an issue while on the higher frequency band.

It is certainly one of the more challenging and interesting kinds of flying that I have ever done. There is nothing quite like flying precise search patterns while listening for the faint aural sweeps over a squelch-broken, static-filled radio, while remaining entirely focused on the smallest changes on a directional needle and a signal strength meter, while trying to differentiate between a reflective ghost signal and the real McCoy. Those pilots that are really good at it can nail your position down to within a few feet from an airplane flying overhead at 1000 feet AGL.

As for the "non-antenna" test.......Let me just say that I have also had the pleasure of showing up on someone's doorstep at 0200 to inform them that we have traced an ELT signal to that location. I even went airborne one time to track a signal emminating from a train traveling from Illinois through Denver. So yes, if the batteries are good, and even if you have not attached your antenna on your brand new 406 ELT that UPS just delivered to your door, if you knock it over on the bench while you are unpacking it from the shipping container, that might just be enough to set the SAR wheels in motion! :D
 
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The truth about any airplane crash from my own personal SAR experience is that the tail section is the only part of the aircraft that is even remotely recognizeable in most cases, and is thus the reason that ELTs are usually mounted in the aft section of the airplane.

In your experience, such accidents were uniformly fatal?
 
There I was, Flat on my back...

After 26 years of Civil Air Patrol Search and Rescue flying, I unfortunately have more experience than I wish I had with respect to the causes and end-results of many airplane crashes.

I too have struggled as to where to put mine, and for the RV8 there is no simple answer. So in the end you are left with little more than to pick the best location you can with the space available that will allow the unit the best possible chance of surviving a significant impact.

I found it interesting that another poster mentioned that the NTSB accidents for RV-8s indicate that many of them end up on their backs. Without knowing the details, it sounds like these may have been ground loop or other landing-related types of problems. The truth about any airplane crash from my own personal SAR experience is that the tail section is the only part of the aircraft that is even remotely recognizeable in most cases, and is thus the reason that ELTs are usually mounted in the aft section of the airplane.

Bryan,
Fully concur.
I spent three years in Special Ops flying the mighty OV-10 Bronco. Our tertiary mission was SAR or CSAR depending on where the search and rescue was taking place. I only performed a handful of peacetime US SAR's, all but one were GA aircraft and only one had a happy ending. Amazingly enough though, 100% had working ELT's that through satellite reception or simple triangulation were able to eventually locate the crash site, even with several buried under snow or in a "smokin' hole".
As far as RV's go. I have helped clean up five crashes and all of them (100%) were on their back. That said, I have always mounted my VHF comm antenna on the belly. I used a Co-Ax cable 2 into 1 adapter from radio shack and my comm radio and ELT both feed into my VHF radio external antenna. I have tested it and it works great. You can still transmit VHF radio with the ELT engaged and screaming away. So, the antenna is at the highest point for worst case...:)

My dos centavos...
V/R
Smokey
 
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In your experience, such accidents were uniformly fatal?

Dan, unfortunately yes - mostly fatal. However, I have also seen a fair number of situations where survivors are pulled from wreckage that, after having seen it, leaves you wondering how that was even possible.

I also unfortunately had many experiences where ELTs were not working for one reason or another, and post accident reports reveal that the individuals did survive for a time after the crash, only to succomb to the elements because they could not be located in time. These are the most difficult ones of all for any SAR crew member to deal with, and is the main reason I am so passionate about this subject.

As an example, even here in Colorado with the infamous Rocky Mountains, a good solid ELT signal can mean location and rescue within 1-3 hours after ELT activation, whereas no signal can turn into 1-3 days, assuming a flight plan was filed and activated and followed. With no ELT and no flight plan, it can turn into a week or more, and sometimes not even until the snow melts in the coming spring.

Bryan,
Fully concur.
I spent three years in Special Ops flying the mighty OV-10 Bronco. As far as RV's go. I have helped clean up five crashes and all of them (100%) were on their back. That said, I have always mounted my VHF comm antenna on the belly. I used a Co-Ax cable 2 into 1 adapter from radio shack and my comm radio and ELT both feed into my VHF radio external antenna. I have tested it and it works great. You can still transmit VHF radio with the ELT engaged and screaming away. So, the antenna is at the highest point for worst case...:)

Smokey, thank you for your service. I built a model of the OV-10 Bronco once - cool looking airplane. Wanted to ask if your belly antenna is the standard bent comm antenna that everyone is using? You have enlightened me as to another solution that I had not yet considered, so thanks for that. it may still be a bit too far forward for my liking, but I will certainly mull that one over.
 
I used a Co-Ax cable 2 into 1 adapter from radio shack and my comm radio and ELT both feed into my VHF radio external antenna. I have tested it and it works great. You can still transmit VHF radio with the ELT engaged and screaming away.

I know of one other person that did that successfully in a Sonex. Having said that, it is an absolutely terrible thing to do to both radios. Those bnc tees aren't one way check valves. You are putting 10W of RF into your ELT everytime you transmit. Who knows if, or for how long, it will tolerate that? Will it work when you need it? If your ELT goes off, it is transmitting directly into your receiver, which can blow out sensitive components, and result in a deaf receiver. And then there are the SWR issues that can damage your transmitter. It might work for some, it might work for a while, but there's a good chance that you will damage one or both devices.
 
We went through this.

We went through this and wanted to be in full happy argreement with the "F.A.R.'s" and the "O.E.M." call-out, as you have all knoked around here.
It can be done. Retracktable, mounted in a tube.
 
Thanks for all the input, especially from those with SAR experience. If the SAR guys are still going to be listening on 121.5, I don't want to use a 403 only antenna. So, for me that leaves only on the bottom or on the top, back by the VS.

In my initial research for this question, I searched the NTSB archives for 8A crashes, I only found 11. Reading the text for these, ELT was activated in 3. In the two fatal accidents, ELT activation was not recorded, and the accident was such that help arriving sooner wouldn't have made a difference. Three ended up inverted (no fatalities), one fatal was vertical in a pond, the rest were upright, except for one which sank.

Reading these accidents, I felt I would want ELT activation in 6 of the 11 (I didn't count the two fatal, as it wouldn't have made a difference to survival, others were only minor damage). Of the 6, three ended up inverted, one of which they were not able to exit the plane until help arrived. So, about 1/2 the time I would want a ELT, I would want on the top.

Doesn't help me decide top or bottom. Bottom is easier to install, what with flat surfaces. Top is less likely to suffer damage in an accident, but could easily end up crushed along with the VS. I would have to make a notch in the rear of the canopy skirt, as I only have 0.25 inch or so clearance there. I would need to sweep the antenna forward so as to have it not hit the VS, but that would probably help with the broadcast.

Thinking about it some more, based on the "more likely to survive in the rear" idea, if it is on the bottom rear, it would scrape during takeoff and landing when you rotate, so that narrows it down to top rear, just in front of the VS. The notch will make it harder to seal the rear canopy, but my wife has never flown in a small plane that isn't open cockpit, so any cover at all will be a big difference.

Really, thanks a lot for the discussion, helped me make up my mind on something I've been debating for about a year.
 
Thinking about it some more, based on the "more likely to survive in the rear" idea, if it is on the bottom rear, it would scrape during takeoff and landing when you rotate, so that narrows it down to top rear, just in front of the VS. The notch will make it harder to seal the rear canopy, but my wife has never flown in a small plane that isn't open cockpit, so any cover at all will be a big difference.

Really, thanks a lot for the discussion, helped me make up my mind on something I've been debating for about a year.

Your analysis is why so many ELT antennae have been installed inside the emp fairing. I would be hesitant to cut a notch in the canopy skirt, that is a problem area as it is.
 
Using a common antenna for both the com radio and the ELT is a very bad thing. You will damage the transmitter output stage of the ELT. You probably already have. Just because you can still hear the ELT does not mean that it is radiating the proper output signal. Placing the antenna in the cabin may not be the best place to put it and will not meet the published specs but it has a much better chance of doing some good than trying to share antennae with other equipment.
 
All good points Bryan. No question, a wingtip mount increases the chance of ELT unit damage.

I too have struggled as to where to put mine, and for the RV8 there is no simple answer.

So where have you put yours? Every location has its compromises.

The most studied GA accident in recent history is probably the CFIT crash of the GCI DHC-3T in Alaska, August 2010. It is noteworthy here because a crash-related disconnect of the antenna cable rendered its 121.5/406 ELT useless. Although it was operating inside the intact metal fuselage, the signal was not heard by satellite, nor by any of the search aircraft working the well-defined 52 mile route of flight.

Anecdotes about metal barrels aside, this and other reports raises serious doubts about the performance of an ELT inside a metal fuselage with no external antenna. The literature is full of reported failures to transmit an effective signal due to antenna disconnect or antenna damage. The boxes themselves seem to be doing just fine, even when they ricochet around inside crashed structure.

Point is, location of the ELT unit may be moot compared to the issue of antenna and cable damage. A disconnected ELT inside a fuselage appears to be a brick.
 
In closing

I think that most of us understand that with regards to an "ELT" that you are dealing with an actuary. Were you put the thing, how it is lagit, and if it will work. We are all trying to figure our best odds. You will all put it were you want it and it will are may not help you.
But a closing thought you may wish to take in. If you have a unit with a remote panel you may wish to put "arm-ELT"," ELT-on" on your emergency check list. As the extra "what-ever" time you set it off as you go down just gives you better odds. I think doing some paper work after the fact is better than not being found in time. If you need it.

Thanks, Yours as always R.E.A. III #80888
 
Location, Location, Location

An antenna mounted that close to the rear seat passenger's head could be a bad thing.

Make sure it will not whip around and hit anyone!
 
A new idea

Here is my thinking out of the box idea for an ELT antenna placement on the RV-8 where it is on top of the fuselage, vertical and with a proper ground plane. You need a little imagination so bear with me. If you were to run a hollow plastic tube, small diameter (1/8?), up the front of the vertical stabilizer, on the outside of the leading edge. Then glass in the tube to fair it in with the stabilizer. You would mount the antenna on an aluminum plate that is located in the tail cone between the two top longerons just below the leading edge of the vertical stabilizer. The whip portion of the antenna would protrude out the top of the fuselage into the plastic tube on the leading edge of the vertical stabilizer. There would be some shadowing to the rear, just the width of the vertical stabilizer, but the rest would be unrestricted. I am not an antenna guru so I do not know the effect of having it in close proximity to the vertical stab, but not in direct contact. Also I would not know what effect the slight change in the aerodynamics to the vertical stabilizer would have on the aircraft. This is just intended to start the discussion for new ideas.
 
The issue of having the ELT antenna so close to the vertical stab is the antenna radiation pattern would most likely be fairly radically altered. The "some shadowing" would likely be more like "blanked to the rear", or close to it. My best guess is that a cardiod pattern would result (same thing happens when putting a reflector behind the driven element in a yagi).

All that having been said, this particular airplane doesn't provide many options for ELT antenna installation, and all of them are sub-optimal compromises. Without actually performing a 3D far-field analysis of the radiation pattern it would be impossible to know which compromise ultimately yields the best RF performance. One can only guess, and make the best compromise between RF performance and crash survivability.
 
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