On Thursday, March 15, 2012 12:32:02 PM UTC-7, Bob Kuykendall wrote:
On Mar 14, 11:51*pm, WaltWX wrote:
Does anyone have success finding an aircraft installed 406 ELT for a
Schempp-HIrth glider (i.e. internal installed antenna somewhere in
fuselage)?

A quarter wave of 406.25 is about 6.9", so a quarter-wave whip antenna
optimized for that frequency would stand about 7" tall. A dipole would
be about 14" tall. It's probably a vertically-polarized signal, so you
want the antenna standing vertically, and not horizontally.

If your glider has a fiberglass fuselage, it is relatively transparent
at the MHz frequencies, so you can use either an internal dipole or an
internal quarter-wave whip with a ground plane. The dipole is probably
the better bet.

If it's an all-carbon fuselage, it is pretty much opaque to the MHz
frequencies, about like an all-aluminum fuselage. No internal antenna
could offer anything like reasonable performance.

If the fuselage is primarily carbon, there may be a few places where
you might make an antenna work. Sometimes the vertical fin or rudder
are fiberglass or aramid; you might be able to put an antenna in one
of those places. I used Jim Weir's design guide to design the internal
123 MHz dipole antenna in the rudder of Brad Hill's Tetra-15
sailplane, and it seems to be working pretty well. The turtledeck area
might also be fiberglass or aramid, so you might be able to install
something there.

Thanks, Bob K.
http://www.hpaircraft.com/hp-24

The tail area is a poor choice for an ELT antenna because the tail boom is quite likely to break in a crash, possible taking the ELT coax cable with it. Typically there is just really no good way to mount an ELT antenna inside a carbon fuselage. The turtle deck and nose deck areas don't have enough space for a vertical antenna and ground plane, even if they are RF opaque glass/kevlar. You might want to consider a 406 MHz PLB attached to the pilot/parachute harness. The PLB won't activate on impact, but then its not clear that ELTs will reliably active on impact either...especially for what may be 'low energy' glider impacts.... all ideally after starting with a SPOT tracker which can deliver many of the benefits of an ELT/PLB and quite a few the ELT/PLB cannot. I still like to also have a 406 MHz PLB since SAR organizations really do understand them and the old fashioned 121.5Mhz local homing beacon built into them can still be useful.

Darryl