406 ELT Dipole Antenna Inside Plastic Dorsal Fin

[Gary0747]

For those that have the plastic Dorsal fin has anybody found a dipole ELT antenna that would mount inside it?  Paul Lowen used to talk about doing this back with the old 121.5 MHz ELT and  with his dorsal fin speed mod.   I was looking for a 406 MHz dipole that would fit inside the dorsal.   I think internal ELT antennas are done in plastic and fabric airplanes.  Not sure where Cirrus puts theirs?

[N201MKTurbo]

https://www.vr2woa.com/blog/diy-half-wave-dipole-antenna/

 

[Gary0747]

It would be easy to make a dipole antenna for 406 MHz and mount it inside the plastic dorsal fin, but I was wondering about the legalities of making your own?  Half wave dipoles have more gain than vertical quarter wave antennas so it seems to make sense to go this way. I had a PM from a person with a link to a dual band vertical saying that Mooney uses that antenna.  If they mount it inside the plastic dorsal they must not be mounting it vertically?  Vertical antennas have to work against a metal or conductive ground plane so if Mooney or Cirrus has figured out how to do this it would be nice to know. 

[KSMooniac]

All of the 406 ELT's are certified with specific antennas only, so you can't just find a random antenna you like and connect it to a modern ELT.

Sent from my LG-US996 using Tapatalk

[jaylw314]

4 hours ago, Gary0747 said:

For those that have the plastic Dorsal fin has anybody found a dipole ELT antenna that would mount inside it?  Paul Lowen used to talk about doing this back with the old 121.5 MHz ELT and  with his dorsal fin speed mod.   I was looking for a 406 MHz dipole that would fit inside the dorsal.   I think internal ELT antennas are done in plastic and fabric airplanes.  Not sure where Cirrus puts theirs?

There's a Service Instruction from Mooney and an installation kit for the Artex ME406 ELT, with an ELT antenna to replace and install in the dorsal fin.  M20-116.  The antenna is apparently an Artex 110-773?  I've looked at the instructions but didn't really understand it fully

[jetdriven]

11 hours ago, KSMooniac said:

All of the 406 ELT's are certified with specific antennas only, so you can't just find a random antenna you like and connect it to a modern ELT.

Sent from my LG-US996 using Tapatalk
 

I have a TSO’d whip antenna. So technically I installed the 406 ELT with their approved anyenna. Then the next day remove that and install the TSO’d replacement. 

[johncuyle]

I went through this.  The ELT antenna is mounted inside the tail root fairing.  There's a variety of antennas which fit or can be bent and made to fit with a little heat, but the fairing is riveted on and swapping the 121.5 antenna for the 406 requires pulling the tail off.  I ended up with an extra hole in my plane because the added cost wasn't worth it.  Don't know anything about the Artex option or whether it would be better/is somehow easier to install.

[PaulM]

As per the SB it is the Artex 110-773 (Rev B)... I believe it has to be ordered by a MSC via Mooney.  

>NOTE:

>When ordering Antenna Kit, specify if Antenna is going to be mounted on Exterior or inside Dorsal Fin.

 

the 110-773 is certified for the Artex ME406 and 1000 ELT's.   the 345 is not certified for that antenna. 

[jaylw314]

1 hour ago, johncuyle said:

I went through this.  The ELT antenna is mounted inside the tail root fairing.  There's a variety of antennas which fit or can be bent and made to fit with a little heat, but the fairing is riveted on and swapping the 121.5 antenna for the 406 requires pulling the tail off.  I ended up with an extra hole in my plane because the added cost wasn't worth it.  Don't know anything about the Artex option or whether it would be better/is somehow easier to install.

The Service Instruction says at the beginning that in some cases, the antenna can be replaced without removing the dorsal fin, but I just can't make sense of the instructions.

[Gary0747]

The service instruction gives two locations for mounting inside the dorsal and outside the dorsal.  It specifies the (B) version antenna for the inside the dorsal fin. The inside the dorsal fin will only work if you have a plastic dorsal fin. My fin currently is metal but I just ordered the Laser speedmod dorsal which is currently on sale and it is plastic.

[OR75]

On January 15, 2019 at 8:32 PM, KSMooniac said:

All of the 406 ELT's are certified with specific antennas only, so you can't just find a random antenna you like and connect it to a modern ELT.

Sent from my LG-US996 using Tapatalk
 

Random certainly would not work 

but the install manual may make it looks like it is specific

[EricJ]

The 406 antennas are fairly unique in that they have to have the characteristics of an ELT antenna (e.g., less likely to break off), and also have to support both 121.5MHz and 406MHz operation.   That's not a typical VHF antenna, nor is it a typical old-school ELT antenna.    So be careful what you connect to it as it would be reasonably easy to diminish its capability with an inappropriate antenna.

 

[N201MKTurbo]

A dipole has a very different radiation pattern then a 1/4 or 5/8 wave vertical. The vertical is omnidirectional in the horizontal plane, the dipole has two strong lobes perpendicular to its orientation. 

[jetdriven]

Is the ELT a dipole antenna? I thought it was simply a whip.  

[Gary0747]

The current antennas are apparently vertical but dont have to be quarter wave. It is hard to say what the radiation pattern is when they are bent over sideways when put inside the dorsal fin like Mooney does.  If the objective is to put out a signal upward towards a sattelite it seems a vertical antena would be the least effective. 

[EricJ]

8 hours ago, Gary0747 said:

The current antennas are apparently vertical but dont have to be quarter wave. It is hard to say what the radiation pattern is when they are bent over sideways when put inside the dorsal fin like Mooney does.  If the objective is to put out a signal upward towards a sattelite it seems a vertical antena would be the least effective. 

There are multiple layers of 406 MHz SAR satellites at GEOstationary, LEO and (projected on GNSS satellites) MEO orbits.   As long as there is sufficient radiation in an upward hemispherical direction you have a decent chance of the signal being heard.   

Regardless, I'd be very careful about matching an appropriate antenna to a particular VHF/406MHz ELT system, not only for frequency response and antenna pattern but also to match the expected impedances to the transmit amplifiers.  If the antenna isn't recommended by the manufacturer of the unit, I'd proceed with caution.   There are many ways in this particular endeavor to inadvertently defeat the performance of the system.

[Gary0747]

It all dependes where you crash and assumes you are not upside down. Vertical antennas have their transmitting radiating lobes perpendicular to the antenna which makes the straight up direction weakest.  Perhaps if you are not in a ravine you can hit a satellite in a somewhat horizontal direction.   Anytime any kind of transmitter is installed hooked to a new antenna a simple SWR/Power check should be made to prevent the reflected power from damaging the finals and make sure adequate power is being radiated. 

[jetdriven]

I feel a lot better about having the ELT antenna under the dorsal  fin than on the top of the fuselage. A lot of these get ripped off during the crash sequence which defeats the functionality of the ELT. 

[Gary0747]

Another thing worth noting here is not all ELTs sold now are dual band. Both 406 and 121.5 MHz made more sense a few years ago but with 121.5 no longer being satellite monitored and the old single band 121.5 equipment no longer being sold the future viability of dual band units is diminishing.   So apparently dual band transmitters and antennas are not required in the regs?

[Steve W]

8 hours ago, Gary0747 said:

Another thing worth noting here is not all ELTs sold now are dual band. Both 406 and 121.5 MHz made more sense a few years ago but with 121.5 no longer being satellite monitored and the old single band 121.5 equipment no longer being sold the future viability of dual band units is diminishing.   So apparently dual band transmitters and antennas are not required in the regs?

Even if it's technically legal to only have a unit that transmits on 406(and it's hard to find the official rule one way or the other), I'd never consider buying one. If the GPS on your 406 only doesn't work or gives an old position a couple miles away then they still have to find you, the way that's done is with the lower powered 121.5 homing signal. Sure, if you crash in nice weather they may be able to find you visually, but in other instances they're going to have to find you by radio, and hand-held homing on the continuous 121.5 signal is so much easier than the 406 that only gives a burst once per minute.

 

[EricJ]

3 hours ago, Steve W said:

Even if it's technically legal to only have a unit that transmits on 406(and it's hard to find the official rule one way or the other), I'd never consider buying one. If the GPS on your 406 only doesn't work or gives an old position a couple miles away then they still have to find you, the way that's done is with the lower powered 121.5 homing signal. Sure, if you crash in nice weather they may be able to find you visually, but in other instances they're going to have to find you by radio, and hand-held homing on the continuous 121.5 signal is so much easier than the 406 that only gives a burst once per minute.

 

Lots of people still listen to Guard, too, so even if the 406 is completely inop for some reason, having the 121.5 transmitting does help improve your odds.   Plus it makes testing easy.   Mine goes off by itself once in a while which I used to catch just by the LED blinking.    Now I also have the audible unit under the panel, and when it went off by itself a couple months ago on takeoff I had to figure out what that new beeping noise was.   Listening to Guard once in a while is a good thing and can also help with self test.

 

[Gary0747]

The thing that confuses me about finding the antenna recommended to install in the dorsal according to Mooney SIM20-116 is there appears a large number of different antennas styles with the same model number Airtex 110-773.  Even talking to a MSC and several different vendors didnt clarify anything.   No one knew about a revision (B) The MSC said they use the standard antenna that come with the Airtex ME-406 that Spruce carries. I would order one to see but the items are not returnable.  I am looking for anyone who has done the 121.5 to 406 upgrade in their dorsal for advice. 

[Deb]

Eric and Paul at Oasis Aero in Willmar, MN allowed us to participate in this endeavor. We drilled out the rivets holding down the dorsal fin and then used a plastic tool to cut the adhesive. We also had to remove the interior panels in the top of baggage area. We replaced the existing antenna with the 110-773 whip antenna. We let the professionals reseal and expertly rivet the dorsal fin back in place. We touched up the rivets with rustoleum glossy white paint (thanks @jetdriven). ACR has a kit to use the existing wiring to the remote switch which includes the new switch. 

Here’s a drawing of the antenna from the installation manual https://www.acrartex.com/download-product-attachment?id=1361.

Edited January 19, 2019 by Deb

[Gary0747]

That photo of the 110-773 antenna does not appear to be able to bent over to fit under the dorsal. This drawing of a 110-773 is a very different antenna and appears to be able to bend and fit.  That is, look at the difference in height of the loading coil on the bottom. 

[MarcK]

On 1/18/2019 at 2:08 AM, Gary0747 said:

Another thing worth noting here is not all ELTs sold now are dual band. Both 406 and 121.5 MHz made more sense a few years ago but with 121.5 no longer being satellite monitored and the old single band 121.5 equipment no longer being sold the future viability of dual band units is diminishing.   So apparently dual band transmitters and antennas are not required in the regs?

Dual band is required by the latest TSO and by the FCC. 406 for general location, but that's one burst every 50 seconds. 121.5 for final direction finding.