406 ELT retrofit: antenna

maus92 · #1 05-15-2015, 08:11 PM

I am replacing my 121 AK ELT with an ACK E04 406 ELT. The current installation has a horizontally mounted antenna buried in the horizontal stab fairing. I have two problems. One, the current antenna is designed for 121. Two, it's almost impossible to replace the current antenna with the one supplied with the new ELT without kidnapping a small child to climb down the fuselage to disconnect the BNC and put a wrench to the old antenna mount. Considering that the old 121 antenna is sub-optimally mounted horizontally, should I just reuse the current antenna? The new 406 ELTs broadcast a homing signal on 121 in addition to the 406 signal encoded with GPS data, so the current antenna theoretically should work for the homing signal. But is the antenna hopelessly out-of-band for the 406 signal? Maybe I should cut the old antenna to perform on the 406 frequency at the expense of the homing signal?

kevinh · #2 05-15-2015, 08:43 PM

Alas. You need to use the new antenna if you want to get the full (or even much) of the 406MHz benefit you just paid for ;-).

lrgabriel · #3 05-17-2015, 02:45 PM

I have no advice; I merely wanted to commiserate.

I have a composite airplane, and the only clue as to the location of my 121 MHz antenna is a piece of coax disappearing suspiciously into the fuselage sidewall.

Perhaps I'll just hold the new antenna in one hand while flying...

tim2542 · #4 05-18-2015, 03:55 PM

You need a dual band antenna for that ELT, and there is a TSO issue. Not that anyone would call you on it, but the unit is certified with the supplied antenna.
I know the ACK units do an antenna check on boot up, so it would not see the proper impedance from the transmission line and antenna and fault out.
Tim

maus92 · #5 05-18-2015, 04:13 PM

Quote:

Originally Posted by tim2542

You need a dual band antenna for that ELT, and there is a TSO issue. Not that anyone would call you on it, but the unit is certified with the supplied antenna.
I know the ACK units do an antenna check on boot up, so it would not see the proper impedance from the transmission line and antenna and fault out.
Tim

Thanks. I just got off the phone with them and explained my dilemma. The rep commiserated - he was aware of the RV-8 issue, but was non-committal about what to do since the TSO installation is on the top of the plane <30 degrees of vertical. However, he did say that any external mounting would provide good signal strength, and that many aircraft end up on their backs after crashes. I took that to mean that mounting on the bottom of the plane is a viable option, although not meeting the TSO.