gmcjetpilot
Joined: 04 Nov 2006
Posts: 170
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 Posted: Fri Apr 25, 2008 1:51 pm Post subject: ELT Antenna for Fiberglass aircraft (any aircraft) |
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Yes loss of satellite monitoring of 121.5 Mhz will make
little difference, but you are also losing 246Mhz
completely. That is a big difference. that means the only
ELT you will have is something that transmit less than a
kids walkie-talkie (100mW) and has a range of a mile or
two at BEST. Basically they have to know where you are
first before they can find you.
The only way you really will be FOUND is with a
satellite signal, at least in a large and/or remote area.
What does that mean, no one will come to look for you.
Remember Steve Fossett was well-known for his world
record-setting adventures in balloons, sailboats, gliders,
and powered aircraft. He took off in Sep 2007. Never
been found despite a search that was very extensive, a
1000 times more extensive than if you or I went
missing.
406Mhz is better in every way. Resist all you want but I
find that foolish if flying over any remotely sparsely
populated area, especially if you are responsible for
other peoples lives, aka passengers, not to have one.
At least get a PLB.
BTW 406Mhz is 5 watt pulses which is one of the many
reasons its better. 121.5Mhz 100mW, do the math.
RUBBER DUCKIE ANTENNA - STUPID ON A GOOD DAY
Its best to use the antenna that comes with the ELT. If
you know anything about RUBBER DUCKIE antennas
you know they suck. There is NO magic in antennas
and you need length (to resonate at the proper freq).
"Duckie" base loaded coil non-ground plane mono-pole
antennas are a HUGE sacrifice. They really are terrible
even for the little handheld receiver or transceiver they
where designed for, meaning a super big compromise.
BTW key words, "they where designed for", meand they
where never made for your ELT ever; they WILL cause
HUGE loss in efficiency. It's you life, so beware.
LET ME SHOW YOU WHAT WE ARE TALKING ABOUT
http://tiny.cc/9tGKn
-Verses-
http://www.edmo.com/modules/products/xarimages/Large/455-5044.jpg
http://img292.imageshack.us/img292/5631/eltantennasls8.jpg
(RV's w/ ELT antenna's zoom in and see if you can see them)
"THERE IS NO SUBSTITUTE FOR AN EXTERNALLY MOUNTED 1/2 WAVE DIPOLE"
(Our whips are 1/4 wave but act as 1/2 wave on a ground plane)
You are dealing with 121.5Mhz (at) 100 mW (milli watts)
power. You might as well have a string-N-can if you
screw with these dumb little excuses for an antenna.
Forget 246Mhz its gone. No birds in the sky are looking.
If talking 406Mhz they are already super short. 406Mhz
is UHF and the antenna is SMALL, PUT IT OUTSIDE
WHERE IT BELONGS.
If you get a new 406Mhz and I think you should, you
will most likely have two antennas one for 121.5Mhz
and one for 406Mhz. As you know as the freq goes up
the wave length goes down and so does antenna
length. Artex has the one antenna version that handles
both freqs (121.5 and 406). It is OK to have a longer
antenna (for 121.5mhz) that also handles the higher
UHF freq (ie 406Mhz) if designed properly.
It is not OK to go the other way, have a short antenna
(rubber duckie) for a radio that needs a long antenna. You
can always go longer bit not shorter. Again there is NO
magic or violations of physics. "Trick" the radio single all
you want but it knows it and there will be loss of efficiency.
A 406 Mhz antenna is so short no rubber duckie need apply.
This C-130 doing a grid at 6,000 agl (mile up)
at 220kts is looking for you. If your weak signal is
made weaker with a rubber duck or directional loop they
might not "hear you", even if they fly directly overhead.
121.5Mhz is for LOCAL DF steer only after they found
you.
Antenna's are a special deal, engineered to work with
that specific transmitter, power & freq. Unless a radio expert,
who knows antenna theory and how to test it, have
the equipment and knowledge to do so, screwing with it
just to make your plane look good is kind silly in my
opinion.
LOOP ANTENNAS BAD FOR ELT's (my opinion)
Loop Antenna? Well when I hear loop, normally I get
excited, because loops are directional, give good GAIN
if pointed and tuned properly and have good noise
rejection. However loop antennas claim to fame is
for lower freqs (HF or MW). Since HF and MW wave
lengths are long, their antennas are long (10' to ++100'),
so a loop is a good solution for getting the required length
to resonate at those long wave freqs a still be somewhat
compact in size. However the directional part is a draw back
for a "FIND ME" Beacon which should be omnidirectional.
THE WHEEL, REINVENTED, WHY?
Why do we need to reinvent the wheel when its already
been done for us. Yes I know you glue and string
airplane guys want to hide your antennas. OK than put
a 1/2 wave in the vertical stab or put a 1/4 wave
reflected dipole (acts like 1/2 wave) with a good grd plane.
A good 1/2 wave or 1/4 wave reflected dipole is great for
the HF and UHF band.
The Wings and Wheels antennas for gliders, like their
non ground plane 1/2 wave will work. Again who knows
how it will really work if the real world; again 121.5Mhz is
pretty lame "to be found" with anyway. At least these
antennas are professionally designed and made. There
is nothing wrong with making your own antenna if you
know what you are doing, but if your are making a smaller
antenna or one stuffed in a wing top or in between the
vert and horz stabs with horz polarization or screwing on
a rubber duckie antenna thinking you are being clever you
are not.
Do some test with your home-brew deal. Go to a big airport
and with-in the first 5 min of the hour with no more than
5 sec "on time" check range with a receiver and sensitive
signal meter. May be the CAP can help w/ some gear? The
reciver antenna can be a whip, loop or beam, great for DF
(Direction finding), but the latter two req you know where point.
Even have get some one to fly over, with receiver/meter. See
how far out you can transmit in different directions. I think you
will be disappointed with hidden antennas laying down inside
fuselages or rubber duckies. External and vertical with good
ground plane will maximize performance. Now we can get into
what if the plane flips but save that for a different time.
406Mhz antennas are so small just put them
outside the plane with a ground plane. It will not cause
drag or distract from the beauty of your plane.
#1 advice, USE THE ANTENNA THAT CAME WITH IT
and mount it as instructed. It really is the Law, since
the ELT is a TYPE certificate item. The ELT and its
installation is one of the few things an experimental
must have, must be TYPE CERT (including installation)
if operations require you having an ELT at all. I know
we get away with some crazy stuff but its your life.
Unless you are a Ham radio Gen / Extra and have the
wherewithal (& equipment) to test and evaluate your
custom ELT antenna system, than I would use the KISS
method and mount the antenna outside per the ELT's
instructions. A 121.5 Mhz elt whip cost you about 1/8 to
1/4 mph loss at 220 mph. I don't think you'll notice it. A
406Mhz Ant is the size of transponder & almost invisible
with no drag. Make it an airplane. Airplanes have antennas.
Fiberglass guys have more antenna options but a 406Mhz elt
should be the first option.
Just MY opinion, George
Be a better friend, newshound, and [quote][b]
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