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nuckolls.bob(at)aeroelect
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 Posted: Fri Jan 28, 2011 7:03 am Post subject: ELT antenna performance |
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At 11:54 PM 1/27/2011, you wrote:
| Quote: | Bob and all,
During an emergency landing too many aircraft go over on their backs. |
or suffer severe deformation/disassembly
| Quote: | | I suspect that the back contact and sliding in too many instances scrapes off or otherwise damages antenna like the ELT typically use. |
which is why the preferred location for ELT antenna
is just forward of the vertical fin. Studies of
wreckage remains determined that this was the lowest
risk location for the antenna . . . unfortunately,
"lowest" is not a nice number near zero.
| Quote: |
From that situation, I have wondered about a design concept that would allow the ELT to feed either two or one antennas with enough radiation to get someone's attention. The thought goes something like this. Have an antenna both top and bottom of the fuselage region. Hoping that one survives the impact and will radiate the ELT signal. |
Having the system upside down is not as deleterious
as one might imagine. UHF behaves quite a bit differently
than VHF.
When I first got into electronics as a profession,
I was working for a two-way radio company in Wichita.
We serviced a few systems in the 72 mHz band but
most were in the vicinity of 150 mHz and a few
were up about 450.
This was all vacuum tube stuff. The 450 mHz
equipment was at the upper fringe of what
was practical. It was harder to develop power,
losses in feedlines was higher, keeping
receiver sensitivity to less than 1 microvolt
was more demanding. All-in-all, higher
cost of ownership.
You'd think that customers would shy away from
this equipment. On the contrary. Antennas were
more efficient. Terrestrial noise was a small
influence. Signals behaved much better in the
confines of tall buildings. The cab companies
wouldn't have anything different. The police
wanted it but the FCC decreed that public
services will conduct business in the 150 mHz
band. 150 mHz was subject to much more multi-
path nulls and noise.
121.5 mHz was picked for the first ELTs
because it was a frequency already serviced
by aviation communication services. Same
with 243 albeit military. The idea was
that folks having nothing better to do
on long trips might monitor the emergency
frequencies and report any contacts. Heaven
knew that the satellite location technology
was EXCEEDINGLY crude. Location accuracy was
poor (100 square miles or more), it took
three or more satellite passes to get that
good (meaning many hours after first contact)
and that assumed that they could sort out
the distressed aircraft from several dozen
false triggers that every satellite could
hear within it's line-of-sight cone.
Combine this with the relatively poor
performance of VHF propagation in general
and you begin to see why the system was
doomed to poor performance from the
beginning. 6% find-rates was considered
great performance by some.
Then came solid state UHF electronics
where communications at 2,000 Mhz was
no big deal. Add ship's i.d. GPS location
features to the ELT's transmitted signals and
that 6% number took a big jump.
They have a better chance of sorting
out your distress from other people's
false alarms.Haven't seen the latest
figures but I'll bet there are no
more than 25% of found wrecks where
attributed to ELT performance where
rescue outcomes were good . . .
I'd be delighted to be wrong.
Getting back to upside-down wrecks.
The UHF propagation performance is so
much better than the old 121.5/243
junk that antenna orientation and
shadowing is much less critical. I've
not been privy to detailed discussion
on ELT systems. But given what I know
of UHF propagation behavior, if the
system is not physically disabled,
the system is going to perform about
as well as expected irrespective
of wreckage configuration.
But the real bottom line is, what
is the return-on-investment for having
installed an ELT? How many wrecks
in the history of all aviation crashes
would have had a better outcome for
the occupants had the guys in white
coats with stretchers arrived say
within two hours of the crash?
In other words, what percentage of
all events resulted in occupants
surviving for hours to days but
unable to fend for themselves due
to non-lethal injury or simple lack
of transport?
The ELT makes a lot of sense for
over-water flight where there is
higher probability of non-lethal
landing and perhaps days of post
crash survival on a float. But
pile an airplane into a mountain
or even average terrain at night
and the ELT becomes more of a
whistle in the dark. If you really
want the folks in white coats to
show up fast, make sure you've
added the GPS position data option.
At least they KNOW where you are.
Now the problem is to get to
you with what ever resources are
available for the task. Those
resources are dwindling . . . but
that's another issue.
| Quote: |
Any thoughts about a design that could "reasonably" match and feed either two (if both survive) or the remaining antenna.... |
Sure. You could install rugged, flush
mounted slot antennas on top and bottom
of the tail. Use a power divider to feed
both antennas.
This would demand some analysis and perhaps
demonstrated experiments to determine
it was really that much better. I suspect
that the numbers guys would find that
going to dual, robust antennas would
double installation costs while making that
25% go up to 26.2% or some such.
I'm pretty sure it's been explored. The
ELT guys would be DELIGHTED to have dual
robust antennas added to the mandate for
installing such systems. If it made any
sense at all, they'd be doing it.
Bob . . . [quote][b]
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retasker(at)optonline.net
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| Posted: Fri Jan 28, 2011 8:52 am Post subject: ELT antenna performance |
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Hey Bob, not to pick nits , but you are the example here after all... 
mHz - millihertz
MHz - megahertz
Dick Tasker
Robert L. Nuckolls, III wrote:
<snip>
| Quote: | When I first got into electronics as a profession,
I was working for a two-way radio company in Wichita.
We serviced a few systems in the 72 mHz band but
most were in the vicinity of 150 mHz and a few
were up about 450.
This was all vacuum tube stuff. The 450 mHz
equipment was at the upper fringe of what
was practical. It was harder to develop power,
losses in feedlines was higher, keeping
receiver sensitivity to less than 1 microvolt
was more demanding. All-in-all, higher
cost of ownership.
You'd think that customers would shy away from
this equipment. On the contrary. Antennas were
more efficient. Terrestrial noise was a small
influence. Signals behaved much better in the
confines of tall buildings. The cab companies
wouldn't have anything different. The police
wanted it but the FCC decreed that public
services will conduct business in the 150 mHz
band. 150 mHz was subject to much more multi-
path nulls and noise.
121.5 mHz was picked for the first ELTs
because it was a frequency already serviced
by aviation communication services. Same
with 243 albeit military. |
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nuckolls.bob(at)aeroelect
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| Posted: Fri Jan 28, 2011 10:21 am Post subject: ELT antenna performance |
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At 11:15 AM 1/28/2011, you wrote:
| Quote: | Hey Bob, not to pick nits , but you are the example here after all...
mHz - millihertz
MHz - megahertz
Dick Tasker
|
You're quite correct. Unfortunately, when trying
to bang out a timely but extensive reply, I don't
have the luxury of coming back to edit it tomorrow
to attempt a 99% clean posting.
When I go back and read my past postings, it's
not unusual to get whacked by spelling/semantics
issues. I'll just have to beg the Lists indulgence.
That doesn't mean I don't want to hear from
folks when I've stubbed my toe. Just be forewarned
that in spite of your most honorable efforts, my
head will probably continue to run out in front of
fingers on the keyboard.
Thanks for the heads-up!
Bob . . .
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kuffel(at)cyberport.net
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| Posted: Fri Jan 28, 2011 12:30 pm Post subject: ELT antenna performance |
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After my first couple hundred aircraft searches up in
Alaska, and only two crashes myself, I've come to the strong
opinion a terrible ELT antenna inside the structure works
infinitely better than any exterior one removed in the
crash. Suggest builders consider a tailcone location for
fiberglass aircraft or somewhere, anywhere, in the cockpit
for metal ones.
It appears the 406 MHz ELTs will require one to use the
manufacturer's antenna to be legal so we probably can no
longer roll our own ELT dipoles or whips. But using a
simple ground plane in a glass tail or any handy metal
surface in the cockpit will do the job. Don't worry too
much about "seeing the sky". Orientation doesn't matter.
If you really need ELT operation you are unlikely to be
upright anyway.
Tom Kuffel
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Fri Jan 28, 2011 4:28 pm Post subject: ELT antenna performance |
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Don't feel in the slightest bad Bob... I'm guilty too.
Noel
--
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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Fri Jan 28, 2011 4:28 pm Post subject: ELT antenna performance |
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Thanks Dick... I was unaware there even was a millihertz. In English class many snows ago we were told when it came to abbreviations, the rule was only capitalize measurements which were peoples’ names. Under that rule mHz would be correct for either millihertz or megahertz or even gHz...gigahertz.
Noel
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Richard Tasker
Sent: January 28, 2011 12:45 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: ELT antenna performance
Hey Bob, not to pick nits , but you are the example here after all...
mHz - millihertz
MHz - megahertz
Dick Tasker
Robert L. Nuckolls, III wrote:
<snip>
When I first got into electronics as a profession,
I was working for a two-way radio company in Wichita.
We serviced a few systems in the 72 mHz band but
most were in the vicinity of 150 mHz and a few
were up about 450.
This was all vacuum tube stuff. The 450 mHz
equipment was at the upper fringe of what
was practical. It was harder to develop power,
losses in feedlines was higher, keeping
receiver sensitivity to less than 1 microvolt
was more demanding. All-in-all, higher
cost of ownership.
You'd think that customers would shy away from
this equipment. On the contrary. Antennas were
more efficient. Terrestrial noise was a small
influence. Signals behaved much better in the
confines of tall buildings. The cab companies
wouldn't have anything different. The police
wanted it but the FCC decreed that public
services will conduct business in the 150 mHz
band. 150 mHz was subject to much more multi-
path nulls and noise.
121.5 mHz was picked for the first ELTs
because it was a frequency already serviced
by aviation communication services. Same
with 243 albeit military.
[quote][b]
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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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mdnanwelch7(at)hotmail.co
Guest
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| Posted: Fri Jan 28, 2011 4:56 pm Post subject: ELT antenna performance |
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> Don't feel in the slightest bad Bob... I'm guilty too.
| Quote: | >mHz - millihertz
>
>MHz - megahertz
>
>Dick Tasker
|
Fortunately, I can honstly sya, I nver make any tpos. : )
Mik e
[quote][b]
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sportav8r(at)gmail.com
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| Posted: Sat Jan 29, 2011 7:55 am Post subject: ELT antenna performance |
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Millihertz antennas are impractically long. At least, the resonant ones are. Your mileage won't vary on this one.
-Bill B
On Fri, Jan 28, 2011 at 7:15 PM, Noel Loveys <noelloveys(at)yahoo.ca (noelloveys(at)yahoo.ca)> wrote:
[quote]
Thanks Dick... I was unaware there even was a millihertz. In English class many snows ago we were told when it came to abbreviations, the rule was only capitalize measurements which were peoples’ names. Under that rule mHz would be correct for either millihertz or megahertz or even gHz...gigahertz.
Noel
From: owner-aeroelectric-list-server(at)matronics.com (owner-aeroelectric-list-server(at)matronics.com) [mailto:owner-aeroelectric-list-server(at)matronics.com (owner-aeroelectric-list-server(at)matronics.com)] On Behalf Of Richard Tasker
Sent: January 28, 2011 12:45 PM
To: aeroelectric-list(at)matronics.com (aeroelectric-list(at)matronics.com)
Subject: Re: AeroElectric-List: ELT antenna performance
Hey Bob, not to pick nits , but you are the example here after all...
mHz - millihertz
MHz - megahertz
Dick Tasker
Robert L. Nuckolls, III wrote:
<snip>
When I first got into electronics as a profession,
I was working for a two-way radio company in Wichita.
We serviced a few systems in the 72 mHz band but
most were in the vicinity of 150 mHz and a few
were up about 450.
This was all vacuum tube stuff. The 450 mHz
equipment was at the upper fringe of what
was practical. It was harder to develop power,
losses in feedlines was higher, keeping
receiver sensitivity to less than 1 microvolt
was more demanding. All-in-all, higher
cost of ownership.
You'd think that customers would shy away from
this equipment. On the contrary. Antennas were
more efficient. Terrestrial noise was a small
influence. Signals behaved much better in the
confines of tall buildings. The cab companies
wouldn't have anything different. The police
wanted it but the FCC decreed that public
services will conduct business in the 150 mHz
band. 150 mHz was subject to much more multi-
path nulls and noise.
121.5 mHz was picked for the first ELTs
because it was a frequency already serviced
by aviation communication services. Same
with 243 albeit military.
| Quote: |
ist" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List
tp://forums.matronics.com
_blank">http://www.matronics.com/contribution
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[b]
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