406 ELT antenna

[Skyland]

I'd like to replace my 121.5 ELT with a 406 but reluctant to install a long whip antenna outside the fuselage somewhere.  I like that the old antenna is hidden underneath the fiberglass dorsal fin.  Any solutions to this dilemma?

[PilotX]

Depends on how the ELT is certified. One I saw said the only approved antenna is the whip that it came with.

[KSMooniac]

Mooney has a SB or SI to do this with a 406 ELT.  There is one antenna certified for the Artex that will fit under the fairing.  Most of the kits you'll see on the web have the other antenna, though, but my distributor was able to order one with the shorter antenna.

[Fly Boomer]

25 minutes ago, PilotX said:

Depends on how the ELT is certified. One I saw said the only approved antenna is the whip that it came with.

ACK lists the following alternate antennas for their E-04:

ACK E-04.8 (whip)
ACK S65-1231-1 (blade)
COMANT CI 319-1 (blade)
Sensor Systems S65-1231 (blade)
ARTEX 870-3137 (blade) ARTEX 110-340 (blade)
ARTEX 110-340 -1 (blade)
ACK Cl 319-1-1 (blade)
ACK AV-300-1
RAMI AV-300

For the fastest among us, at least some of the blade antennas are certified up to Mach 1.

[Fly Boomer]

19 minutes ago, KSMooniac said:

Mooney has a SB or SI to do this with a 406 ELT.  There is one antenna certified for the Artex that will fit under the fairing.  Most of the kits you'll see on the web have the other antenna, though, but my distributor was able to order one with the shorter antenna.

I have an ELT antenna mounted to the top of the fuselage just in front of the vertical.  As usual, it's not centered, and I hate it.  I would love to use the (now abandoned) antenna inside the fairing.  Do you happen to know if it can be done?

[KSMooniac]

You cannot reuse the old antenna, so you would need to remove the fairing, replace the antenna, and reinstall the fairing. It is an effort.

Sent from my motorola edge plus 2023 using Tapatalk

[Fly Boomer]

5 minutes ago, KSMooniac said:

You cannot reuse the old antenna, so you would need to remove the fairing, replace the antenna, and reinstall the fairing. It is an effort.

Sent from my motorola edge plus 2023 using Tapatalk
 

I'll put it on my "future" list along with about a zillion other things.

Thanks.

[KSMooniac]

I'll put it on my "future" list along with about a zillion other things.
Thanks.

As it was for me, for years!

Sent from my motorola edge plus 2023 using Tapatalk

[Skyland]

Antenna length is inversely proportional to frequency.  The higher the frequency, the shorter the wavelength, the shorter the antenna.  That's why I can't understand why a 406 antenna can't be offered in a short version.  A quarter wavelength 121.5 antenna computes to about 62cm (24").  Could be it's wire wound and can be 1/4 that length (as I understand it) which is probably the length of the antennas under the tail fin.  All the 406 antennas I've seen are straight wire whips and computes to about 36cm (14") 1/2 wave.  That seems to be about the length of those.

Additionally, don't the new ELTs transmit both frequencies?  Maybe the 121.5 antenna is in the base while the 406 antenna is the wire whip.  Any antenna gurus in the group?

[Fly Boomer]

1 hour ago, Skyland said:

Antenna length is inversely proportional to frequency.  The higher the frequency, the shorter the wavelength, the shorter the antenna.  That's why I can't understand why a 406 antenna can't be offered in a short version.  A quarter wavelength 121.5 antenna computes to about 62cm (24").  Could be it's wire wound and can be 1/4 that length (as I understand it) which is probably the length of the antennas under the tail fin.  All the 406 antennas I've seen are straight wire whips and computes to about 36cm (14") 1/2 wave.  That seems to be about the length of those.

Additionally, don't the new ELTs transmit both frequencies?  Maybe the 121.5 antenna is in the base while the 406 antenna is the wire whip.  Any antenna gurus in the group?

If you look at some of the alternate antennas I posted above, the first blade type I looked at is 7.5 inches.  They can be shorter, but I suspect the blade type is more expensive.

[jetdriven]

3 hours ago, KSMooniac said:

Mooney has a SB or SI to do this with a 406 ELT.  There is one antenna certified for the Artex that will fit under the fairing.  Most of the kits you'll see on the web have the other antenna, though, but my distributor was able to order one with the shorter antenna.

the Rami AV100 is a certified 406/121.4 ELT whip antenna its TSO'd. it also fits under the dorsal fin if you bent it a little.

[EricJ]

5 hours ago, Skyland said:

Antenna length is inversely proportional to frequency.  The higher the frequency, the shorter the wavelength, the shorter the antenna.  That's why I can't understand why a 406 antenna can't be offered in a short version.  A quarter wavelength 121.5 antenna computes to about 62cm (24").  Could be it's wire wound and can be 1/4 that length (as I understand it) which is probably the length of the antennas under the tail fin.  All the 406 antennas I've seen are straight wire whips and computes to about 36cm (14") 1/2 wave.  That seems to be about the length of those.

Additionally, don't the new ELTs transmit both frequencies?  Maybe the 121.5 antenna is in the base while the 406 antenna is the wire whip.  Any antenna gurus in the group?

Physical length and electrical length can be different, like with a coil wire-wound antenna (as you mention).    The electrical length can stay the same, but the antenna gets physically smaller by doing tricks like that.   This is done in cell phones, etc., and anything with BlueTooth and most things with WiFi antennas, where they use a "fractal" antenna, often printed on a circuit board, that is physically small but electrically resonant to the desired wavelength.    That said, there's a penalty to making an antenna physically very small, which is that it loses some "affective area", which essentially costs sensitivity in the receiver and cuts range a bit.   For devices where you actually want the range to be limited, like BT, WiFi, and cellular, this is a desirable tradeoff and gets designed around in the system engineering.

ELTs and airborne comm really want as much sensitivity/range/antenna affective area as possible, so our antennas don't get to shrink quite so much.   Practical limitations still apply, so some coiling is often still done in the antennas to get a compromise between size and affective area..   So the 121.5 MHz ELT antenna is generally still the longer whip part of the antenna, and the UHF 406 MHz part is in the shorter, stiffer part (at least in the one I have).   The difference is the stiff, short part is added to accomodate the new UHF 406 MHz antenna.

And yes, when fully activated the ELT transmits on both frequencies.   The 121.5 MHz is continuous and low power, while the 406 MHz makes a short burst every 50 seconds or so (it's a little random on purpose), at a much higher power level.