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bakerocb
Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA
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 Posted: Fri Jan 17, 2014 5:57 am Post subject: ELT Antenna |
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1/17/2014
Hello Bob Nuckolls, You wrote: “I've heard the rumor that some ELT's were triggered by local radiation from a comm transmitter . . . but
I've not seen any documentation or FMEA that supported the assertion.”
Please let me give you one factual data point on that issue. I was unable to launch on the first attempted test flight of my experimental amateur built KIS TR-1 airplane because every time I keyed the VHF comm transmitter to talk to either ground or tower the ACK ELT-01 began to transmit.
The solution was to move the ELT antenna further away from the VHF comm antenna and to reorient the ELT antenna from its original location and orientation. Both antennas were originally, and remained after relocation of the ELT antenna, inside the fiberglass fuselage.
Kelly McMullen wrote: “Good separation between ELT and com antennas is probably best defense.”
That is what worked for me.
‘OC’
'O C' Baker says "The best investment you can make is the effort to gather and understand information."
===================================
Time: 05:02:21 AM PST US
From: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>
Subject: Re: Re: ELT Antenna
At 09:16 PM 1/15/2014, you wrote:
| Quote: |
Almost all ELTs, especially the older ones do in fact radiate
anytime they are near high powered VHF transmitters in the FM, TV, etc bands.
|
This phenomenon IS demonstrable . . . and rare. The
ELT isn't actually producing any output power unique
to its function. Very strong sources of local radiation
(typically FM stations and the old analog TV stations)
would find its way backwards into the output stage of
the ELT's transmitter (constantly hooked to the antenna -
receiver circuits are not so prone to exhibit this
behavior). If there are two or more strong sources,
then they can MIX or heterodyne against each other
and produce a variety of spurious signals on lots
of frequencies.
Analog TV is gone and about the only source of such
coherent energy would be an FM transmitter but you are unlikely
to spend much time flying around in the FM station's
radiation pattern and in close proximity . . . within
a mile or two.
One might hypothesize that the modern ELT's with
TWO transmitters attached to the antenna are even
more likely to exhibit the behavior . . . but you
still need to be poking around the vicinity of the
strong transmitter at altitudes generally 1000'
AGL or less.
I've heard the rumor that some ELT's were triggered
by local radiation from a comm transmitter . . . but
I've not seen any documentation or FMEA that supported
the assertion. Certainly, modern ELTs are subjected
to the standard DO-160 radiated susceptibility testing
and perhaps even HIRF (high intensity RF) testing.
None of this testing will concern itself with the
intermodulation phenomenon cited above but it would
watch for damage to the ELT and/or false triggering.
Bob . . .
===========================
Time: 07:11:05 AM PST US
From: Kelly McMullen <kellym(at)aviating.com>
Subject: Re: Re: ELT Antenna
Hmm, I don't know why the switch to digital TV will make much
difference. They are still broadcasting in my area on VHF RF channels 7
and 8 through 13 in my local area. I used to have an antenna
configuration on my Mooney that consisted of VHF com in front of
windshield, top of fuselage behind baggage compartment, Loran about 18
in. behind that, followed by ELT antenna right in front of vertical
stab. When flying VFR transition over Phoenix Sky Harbor I would get bad
squelch break anywhere within 10 nm of the antenna farm located on South
Mountain, approx 8 mi south of Sky Harbor. Made hearing controllers very
difficult.
I isolated it to the ELT by doing a flight with external ELT antenna
disconnected, which completely eliminated problem even within a mile of
the transmitters. This was a 1st generation ELT. Moving 1st com antenna
from in front of windshield to the belly virtually eliminated the
interference. Removing Loran antenna (custom version of Comant CI121)
helped as well.
I understand later versions of ELTs have somewhat better isolation of
the transmit oscillator, but doesn't completely eliminate. Good
separation between ELT and com antennas is probably best defense.
Kelly
[quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sat Jan 18, 2014 8:25 am Post subject: ELT Antenna |
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At 07:01 AM 1/16/2014, you wrote:
<nuckolls.bob(at)aeroelectric.com>
I wrote: I've heard the rumor that some ELT's were triggered
by local radiation from a comm transmitter . . . but
I've not seen any documentation or FMEA that supported
the assertion.
----------------------
I was informed by an astute reader that the ACK E-01
series ELTs were demonstrably plagued with a tendency
to trigger in response to on-board COMM transmissions.
This fact (and perhaps others like it) may have been the
foundation for advising installers to add separation
between the ELT and ship's COMM antennas.
While the 'fix' is valid, the premise under which
the fix is exercised assumes that the ELT was
properly qualified to be on the airplane in the
first place. Any ELT that goes through the lab
where I work would have demonstrated immunity to
RF interference many times stronger than the ship's
transceivers.
The consumer has every reason to expect certain
levels of performance in their ELT for it has
been decreed and . . . it's a matter of practical
utility.
With better information, a retraction is in order
for my assertion that the RF triggering event
was a 'rumor'. At the same time I would advise
that compromising the installation of your
own 'antenna farm' in recognition of the
historical fact is unnecessary. If the ELT you
have demonstrates such sensitivity, then
there is good reason to suspect a defect in
design and/or manufacturing that needs attended
to . . . defects that may go beyond the sensitivity
to RF.
If the ELT is the last bastion of defense for
your survival, then offer no quarter in
your expectations for its performance.
Bob . . .
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sat Jan 18, 2014 1:51 pm Post subject: ELT Antenna |
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At 07:56 AM 1/17/2014, you wrote:
| Quote: | 1/17/2014
Hello Bob Nuckolls, You wrote: “I've heard the rumor that some ELT's were triggered by local radiation from a comm transmitter . . . but
I've not seen any documentation or FMEA that supported the assertion.”
Please let me give you one factual data point on that issue. I was unable to launch on the first attempted test flight of my experimental amateur built KIS TR-1 airplane because every time I keyed the VHF comm transmitter to talk to either ground or tower the ACK ELT-01 began to transmit. |
Thanks for the heads-up! I've found a number
of service bulletins against the ELT-01 over
it's market life . . . one involved replacement
of the g-switch. I'd like to know what the
technology is for that switch. Dug around in
the patents a bit but didn't find anything.
Bob . . . [quote][b]
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wschertz(at)comcast.net
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| Posted: Sun Jan 19, 2014 4:11 am Post subject: ELT Antenna |
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My experience indicates that it is not a rumor. When I started testing I
found the ELT being triggered by one of my COM's, and solved that by moving
the position of the ELT itself, not the ELT antenna.
Since repositioning, it has not been a problem
Bill Schertz
--
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nuckolls.bob(at)aeroelect
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| Posted: Sun Jan 19, 2014 6:44 am Post subject: ELT Antenna |
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At 06:10 AM 1/19/2014, you wrote:
| Quote: |
<wschertz(at)comcast.net>
My experience indicates that it is not a rumor. When I started
testing I found the ELT being triggered by one of my COM's, and
solved that by moving the position of the ELT itself, not the ELT antenna.
Since repositioning, it has not been a problem
|
What make and model of ELT was this?
Bob . . .
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berkut13(at)berkut13.com
Guest
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| Posted: Sun Jan 19, 2014 1:04 pm Post subject: ELT Antenna |
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I had the same problem with a client's composite aircraft, it was the
Ameri-King AK-450 ELT unit that was susceptible to com RF triggering. It
could have also been the un-shielded RJ-11 style phone cable used for the
remote display.
-James
--
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Mon Jan 20, 2014 8:29 am Post subject: ELT Antenna |
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At 03:03 PM 1/19/2014, you wrote:
| Quote: |
I had the same problem with a client's composite aircraft, it was
the Ameri-King AK-450 ELT unit that was susceptible to com RF
triggering. It could have also been the un-shielded RJ-11 style
phone cable used for the remote display.
-James
|
Interesting! . . . and no AD's against these
products? Defending a device from the ravages
of external radio frequency energy sources
is virtual child's play in engineering circles.
If I were directing the activities of a engineering
design and development group, EVERBODY . . . but
particularly the rookies . . . would spend some
time in the qual-test facilities followed by
a stint in the packaging design group. Shielding
the i/o wires is not acceptable practice for
the purpose of passing qual-tests.
No matter what you're going to design or program,
it first has to perform and survive in the
intended environment and be packaged in a way
that fits into the end-use while not driving
up cost and MTBF risk.
Too many of our contemporaries have evolved
their careers with little appreciation for where
and how their work-product has to perform only
to find out that 'little problems' surface after
a few thousand units are in the field.
The folks-who-know-more-about-airplanes-than-we-do
have assured us that this isn't supposed to happen . . .
Bob . . .
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