A new idea for ELT antenna placement

[bifft]

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:



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.

[Mel]

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.

[bifft]

Originally Posted by Mel

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.

[Mel]

Doesn't have to be on the deck. It can be mounted on the side panel below the Longeron.

[az_gila]

Originally Posted by Mel

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...

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

[bugsy63]

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

[cfiidon]

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

[az_gila]

Originally Posted by cfiidon

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...

[Canadian_JOY]

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.

[gereed75]

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.