ELT Antenna Mount

I've seen pictures somewhere of ELT antennas mounted horizontally on a bulkhead in the tail under the upper fairing between the horizontal and vertical stabilizers. The instructions with the ELT say the antenna must be mounted vertically.

To mount it vertically means on the belly on the RV-8. I would like to keep from hanging things out in the breeze.

Where are people mounting their ELT antennas and why? Any concerns about vertical vs horizontal?

Thanks,

John

I'm very pragmatic about it - I decided that it is very hard to predict what attitude the aircraft is going to end up in after a crash, and I know that fiberglass is transparent to VHF frequencies, so I hid it horizontally under the empennage fairing. I know that someone ill instantly respond that this does not meet the TSO. OK. It satisfied my DAR. There are MANY RV's with the ELT antenna in this location.

Besides, I carry a GPS-equipped PLB on my shoulder harness, and the first step of my "I'm going down" checklist is to activate it. I give that a lot more chance of getting rescuers to me than a 121.5 ELT.

As I said - very pragmatic.

Paul

Bubblehead said:

To mount it vertically means on the belly on the RV-8. I would like to keep from hanging things out in the breeze.

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An ELT antenna in the breeze does not have a measurable effect on aircraft speed. The weight of the system probably has more effect on performance. Since the FAA mandates carring an ELT so I decided to give it every chance on working. External antenna mount for me.

Paul and Phil,

Thanks for your perspectives. Like the politician here iin Illinois said once, "I believe strongly both ways."

I realize the speed penalty is low, but I just broke the comm antenna off that is on the belly (of the plane) so durability is also a factor. I am planning on getting a PLB soon, because I think that is so much more effective than the current model of ELT, and I can use it when I backpack and canoe.

Paul - do you have pictures of your installation?

Thanks.,

John

Potaytoe Potaatoe

Bubblehead said:

........Where are people mounting their ELT antennas and why? Any concerns about vertical vs horizontal?.....

Click to expand...

John,

I don't have any concerns about horizontal vs vertical orientation at all. In fact, I mounted the ELT antenna underneath the empennage fairing on the -6A. This time around however, I decided to reserve that location for a *possible* installation of a remote EFIS component or ballast or whatever.

I cannot claim to be the first to install the ELT antenna here, but its vertical location works for me. I mounted the unit itself as far back as practical. I figure that ELT could at least serve to enhance the CG given its prodigious battery capacity. Might as well put it to work doing something useful.

Bubblehead said:

Paul - do you have pictures of your installation?

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I'm not Paul but here is the ELT antenna in my RV-6:



There is also a Dynon magnetometer peacefully sharing the area with the antenna.

The new elt has a frequency of 406MHz. I haven't seen an antenna for these yet, but a 1/4 wave antenna at that freq is around 7" long, so you might just be able to sneak it under the fairing and have most of it vertical.

I'm waiting to buy one based on the advice from an avionics company. He said there may be something happening price wise (for the 406MHz units) around the end of the year.

This is all conjecture on my part by the way.

Mine looks pretty much like Sam's, except I substituted a rubber ducky for the long whip. Just put a bulkhead BNC connector on the bulkhead, and snapped on the antenna.

Also 121.5

tomcostanza said:

The new elt has a frequency of 406MHz. I haven't seen an antenna for these yet, but a 1/4 wave antenna at that freq is around 7" long, so you might just be able to sneak it under the fairing and have most of it vertical.

I'm waiting to buy one based on the advice from an avionics company. He said there may be something happening price wise (for the 406MHz units) around the end of the year.

This is all conjecture on my part by the way.

Click to expand...

Tom... these new ELTs also transmit on 121.5 for short range search.

I would expect a similar size antenna to the present ones. If the new 406 ELT has a built in GPS, expct some very specific mouting requirements.

gil A

Rick - I agree with your comments. In fact, the current location for my ELT antenna is along the passenger's seat like your picture shows, but the antenna is a longer whip. The original builder did not provide insulating stand offs like you have so the antenna is grounded out on the airframe. That may be why the original ELT will no longer work.

Paul and Sam - thanks for the info/picture. I think I'll put the whip under the fairing mounted on a small tab like Sam's picture and keep the rubber ducky by the ELT unit in case I need to take it out of the airplane and use it "remote."

Tom... these new ELTs also transmit on 121.5 for short range search

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.

Touche' Gil. Thanks.

I copied Roberta's

I, too, have a tipup canopy. I mounted the ELT antenna on the aft side of the rollbar, starting vertically on the canopy rail and then bending inboard somewhat, shamelessly copying Roberta's. I like the under the T fairing idea except that if the airplane is upside down, it could effectively shield the broadcast even more than where I have it. Just another version. I have no idea what the ideal solution is.

Empennage attenuation?

The horizontal antenna mounting in the empennage is tempting but has anyone actually tested the radiation pattern from this arrangement, especially at 406 MHz? I have to believe that there will be substantial attenuation by the adjacent metal even though there are fiberglass ?windows? that will allow some radiation. Artex specifies vertical orientation (within 15 degrees) and at least 30? separation from other antennas and the VS. The combination of horizontal orientation and attenuation may severely compromise effectiveness in an actual emergency. Indeed the close proximity of metal may, with crash related deformity, simply short the antenna to ground and disable the unit completely.

This arrangement may meet the letter of the regulation, but not provide any real protection. One might argue that using this antenna placement, the installation of a low cost 121.5 MHz fixed unit actually makes sense if the pilot is carrying a PLB and can count on it to provide actual locator function

Skyflyer said:

One might argue that using this antenna placement, the installation of a low cost 121.5 MHz fixed unit actually makes sense if the pilot is carrying a PLB and can count on it to provide actual locator function

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That's how I'm flying - the ELT meets the requirements, and the PLB on my shoulder harness is what's going to save my life....

Paul

My location is not RV-8-friendly

I put mine on the turtledeck, just past the point where the canopy slides back to. I bet there's not enough room on an RV-8 aft fuselage, since the canopy is longer.

As for my opinion on the "clever, drag-reducing" locations some builders choose for their ELT antennas, I am saving my firmly-held opinions for my own web site... to avoid cluttering Doug's!

http://www.bashingaluminum.com/ELT/ELT.html

Cheers,
Martin

Your family....

Skyflyer

One might argue that using this antenna placement, the installation of a low cost 121.5 MHz fixed unit actually makes sense if the pilot is carrying a PLB and can count on it to provide actual locator function

Click to expand...

Ironflight said:

That's how I'm flying - the ELT meets the requirements, and the PLB on my shoulder harness is what's going to save my life....

Click to expand...

Paul... I just heard a disturbing story about the cost to a widowed pilot's wife to have her husband declared officially dead.

The legal fees were in the order of $100,000, and were needed before any life insurance policies, etc. would pay out....

I was blind to the fact that closure for the family (i.e., finding the wreckage) might come with such a high cost....

So yes, the PLB will help get you rescued (I just bought one......), but a properly activated ELT might be a big help to your family....

gil A

Antenna length

tomcostanza said:

The new elt has a frequency of 406MHz. I haven't seen an antenna for these yet, but a 1/4 wave antenna at that freq is around 7" long, so you might just be able to sneak it under the fairing and have most of it vertical.

Click to expand...

I measured the antenna that came with my Artex 406 MHz ELT. It's 24" long. This unit transmits on both 121.5 MHz and 406 MHz, so that may explain why it's longer than it "should" be.

Regards,
Martin

mgomez said:

I measured the antenna that came with my Artex 406 MHz ELT. It's 24" long. This unit transmits on both 121.5 MHz and 406 MHz, so that may explain why it's longer than it "should" be.

Regards,
Martin

Click to expand...

Martin,

Where in the world do you mount a 24" antenna on an RV?

N941WR said:

Martin,

Where in the world do you mount a 24" antenna on an RV?

Click to expand...

On the outside

In Virginia

To be more specific (and less of a wise***), here:



Like a good rocket scientist, my web site shows every step except the final one!

mgomez said:

On the outside

In Virginia

To be more specific (and less of a wise***), here:

Like a good rocket scientist, my web site shows every step except the final one!

Click to expand...

Looks like a plan. The antenna that came with my ELT begs to be fitted on the spine of the aeroplane. It even looks like the one on the back of my old Passat wagon!

Also the ELT comes with a quick release mounting bracket and an extra telescopic antenna - so I assume that before the ship sinks, or is engulfed in a ball of flame you are supposed to grab the bright yellow transmitter so that you can summon help on your treck though the wilderness or Shackelton style open life raft voyage to safety. Mounting the ELT in the tail cone might make that difficult. Just a thought.

Jim Sharkey

I don't think there is a really good location to mount an ELT antenna on an RV-8. Every location I have considered has some significant issue.

The ELT antenna on my RV-8 will be between the seats in the cockpit.



I thought about putting it horizontally under the empennage fairing, but I was concerned about the likely poor performance. I also considered running it along the 807 bulkhead at the rear seat back. But, an antenna that runs closely parallel to a bulkhead will have very poor performance. My chosen location doesn't have a much of a ground plane, so it might not work that well either. But, it is vertical, not too close to other metal, and out of the wind.

I also plan to buy a PLB.

Unless you can exactly predict the attitude of your aircraft at the "future crash" site, then you can't know which location is best. They all have problems; on the top (what if you flipped over), on the belly (what if the gear collapse or you are in water/ditch), ect, ect?

Paul and Kevin have the best option, PLB. If you hit hard enough to damage the PLB on your harness, the odds are extremely high you will be beyond the point of help. Also a good idea to make allownces to retrieve the ELT for hand held use if forced down and able.

It seems like everyone else has one of the shorter, flexible antennas with the rubber base. The antenna that came with my AK-450 is solid wire with no rubber base, and it is not real flexible.

I am curious about the "creative" ways in which others have mounted this more solid antenna? Or, has everyone else pitched it and purchased the flexible style?

Thanks,

Scott
7A Finishing

My ELT antenna

Mine is a little different. Works well here, as I can pick up the warble 11 miles away on my handheld with the plane in my garage, and the door shut. Unscientific testing for sure, but seems adequate.

Regards....Chris

DARs may not understand RF

mgomez said:

I measured the antenna that came with my Artex 406 MHz ELT. It's 24" long. This unit transmits on both 121.5 MHz and 406 MHz, so that may explain why it's longer than it "should" be.

Regards,
Martin

Click to expand...

Ideally these antenna should be 1/4 wave dipole perpendicular over ground plane. Multiples of that will work, but the radiation pattern changes significantly.

Putting anything within 1/4-to-1/2 wavelength of the antenna is asking for trouble. So, the problem is more than 3x worse at 121.5 than at 406.

My 406Mhz ELT antenna is mounted in the rear window just behind the roll bar. Its too close to the roll bar, so one of my "things to do" is to move it.

I'd like to move my W&B forward, so I'm thinking of moving the ELT to near the firewall somewhere, and putting the antenna on top the glare shield.

I've got my GPS & weather antenna on the glare shield and both work great.

As far aft as practical.

Steve,
The problem with mounting the ELT forward is that in a crash, this is where most of the damage usually occurs. That's why the ELT mounting instructions say the ELT should be mounted as far aft as practical.
In the event that a crash occurs, the ELT is the last thing you want damaged.

That makes sense

Mel said:

Steve,
The problem with mounting the ELT forward is that in a crash, this is where most of the damage usually occurs. That's why the ELT mounting instructions say the ELT should be mounted as far aft as practical.
In the event that a crash occurs, the ELT is the last thing you want damaged.

Click to expand...

OK. I guess I should have figured that out.

Mounting the antenna so that it won't radiate well enough or mounting the box so that it is destroyed in the crash are two sides of the same coin.

Both leave you without help when you need it.

And - on word choice

My entitling a post: "DARs may not understand RF", was a very bad idea. When I read it this morning it felt very much different than when I typed it yesterday morning.

Obviously, some may, some may not. No one is an expert in every field, and airplane construction covers several.

What I meant to say was: "just because the DAR bought it, doesn't mean its a good idea"

Apologies to DAR's in general.

What is trouble?

Steve Brown said:

Putting anything within 1/4-to-1/2 wavelength of the antenna is asking for trouble. So, the problem is more than 3x worse at 121.5 than at 406.

Click to expand...

There where several comments about 406 antennas being short. The New 406 ELTS still have 121.5 Mhz (for local DF). Yes the 406 antenna is short, but Artex for example has combined both 406/121.5Mhz in one antenna, which is still the same length as current 121.5Mhz antennas. Other 406 Mhz units have two separate antennas for the respective 406/121.5 freqs.

Having two antennas real close is a problem or possible trouble, but define trouble. You will polarize the signal somewhat, but it will still transmit. It's no worse than burying ELT antennas flat Horz or inside and next to metal structure. The "radiation pattern" will be very distorted. Like the empennage intersection fairing area. Yes fiberglass is RF transparent but that vert stab at 12 O'clock, horz stab at 3 & 9 O'clock and the fuselage at 5 to 7 O'clock is like kryptonite to Superman. So you hope the vert polarized antenna laying horizontal will shoot out a signal between little slots you left. I'm a libertarian and think the Gov should have min influence on our lives, but just be aware that is the score. Now on the other side of the coin, an antenna mounted outside and on the back of the plane is not any good if the plane flips over and crushes it into the ground under the plane.

I personally think a cockpit mounted ELT antenna, in the vertical orintation, with a predominate portion exposed to Plexiglass and not metal structure, as far away from all metal as possible, is an OK compromise. I know the EBC ELT's have integral antennas, and the units are designed for cockpit mounting. EBC's don't have any AD's on them. There is some goodness about NOT having coaxial cable and a remote antenna to fail. If you buy a EBC unit and cockpit mount it, there is no argument, its installed per the TSO instructions. I hope EBC will make a 406 unit with like fit and function to their existing units.

I would love to see some builders join forces with a ham radio guy and get an antenna spectrum analyzer out, to evaluate some of these innovative mounting schemes. I think some set-ups will be really really poor. There are no facts just guesses. There are other threads that have beat this topic the death, so I won't repeat them again, but if you are interested they are worth searching and a read.

Using a signal generator, reference antenna, spectrum analyzer and a directional receiving antenna, I've played a bit with antenna locations on a few different aircraft and can say with great certainty that most of the installation locations suggested in the many posts above will result in less-than-optimal antenna radiation patterns. Some of them, such as the mount at the rear corner of the baggage area and the one under the fiberglass fairing in the tail, will result in gross distortions of the radiation pattern and are, frankly, poor choices if you're hoping this thing might some day save your life or locate your bleached bones.

The best location from a radation pattern perspective is on the spine of the aircraft. The vertical stab does cause some pattern shaping, but very little when compared to any other location. In the instance of a crash, the vertical stab tends to protect the antenna in a rollover. An additional plus is that when mounted back in the tail the coax and antenna are further from the heat source in a post-crash fire.

One important point to note is that ELT antennas survive crashes best if they're flexible. I guess what I'm trying to say is that it's not wise to hook your ELT to a fixed rod-style antenna that's likely to snap off under impact forces.