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wschertz(at)comcast.net
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 Posted: Sun Nov 02, 2008 12:35 pm Post subject: COM-ELT INTERFERENCE |
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I have just finished a composite airplane and am preparing for FAA inspection. Testing final systems, etc. I have run into a vexing problem that I need help with, when I transmit on COM 1, it triggers the ELT to start broadcasting. Transmitting on COM 2 does not cause the problem.
Below are the facts that I have uncovered so far:
Plane: KIS Cruiser, composite low wing 4 place airplane
COM 1 SL-30
COM 2 SL-40
Intercom: PS Engineering 6000
COM2 Antenna Copper foil in vertical Stabilizer
COM1 Antenna Bob Archer folded dipole on fuselage wall behind baggage compartment bulkhead
ELT ACK Model E-01
ELT Antenna 18" whip with copper foil strips bonded to fuselage body to form ground plane, located close to centerline of compartment that COM 1 antenna is located.
ELT has remote panel that allows you to reset the ELT if it is transmitting, along with flashing diode. Disconnecting this remote panel does not affect the behavior.
Disconnecting the ELT antenna from the ELT, does not alleviate the problem
If I insert my head and shoulders into the space, the problem goes away. As I withdraw, when I exit the compartment fully, the problem returns. Therefore it appears that having some mass/capacitance? near the antennas has a positive affect.
What kind of better solution can be implemented/tried to eliminate this interference? I cannot carry a body around in that compartment because of weight and balance considerations.
Bill Schertz
KIS Cruiser #4045
N343BS
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Bruce(at)glasair.org
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| Posted: Sun Nov 02, 2008 1:26 pm Post subject: COM-ELT INTERFERENCE |
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You have to move the COM 1 antenna to give more separation or buy a higher
quality ELT. The low end ELT's are susceptible to resonance from nearby
transmitters broadcasting. The resonance causes the ELT to trigger. Most of
the better ELT's have circuitry to block this action.
Bruce
www.Glasair.org
--
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nuckolls.bob(at)cox.net
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| Posted: Sun Nov 02, 2008 6:07 pm Post subject: COM-ELT INTERFERENCE |
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At 08:27 PM 11/2/2008 +0000, you wrote:
| Quote: |
I have just finished a composite airplane and am preparing for FAA
inspection. Testing final systems, etc. I have run into a vexing problem
that I need help with, when I transmit on COM 1, it triggers the ELT to
start broadcasting. Transmitting on COM 2 does not cause the problem.
Below are the facts that I have uncovered so far:
Plane: KIS Cruiser, composite low wing 4 place airplane
COM 1 SL-30
COM 2 SL-40
Intercom: PS Engineering 6000
COM2 Antenna Copper foil in vertical Stabilizer
COM1 Antenna Bob Archer folded dipole on fuselage wall behind baggage
compartment bulkhead
ELT ACK Model E-01
ELT Antenna 18" whip with copper foil strips bonded to fuselage body to
form ground plane, located close to centerline of compartment that COM 1
antenna is located.
ELT has remote panel that allows you to reset the ELT if it is
transmitting, along with flashing diode. Disconnecting this remote panel
does not affect the behavior.
Disconnecting the ELT antenna from the ELT, does not alleviate the problem
|
Your ELT was probably not qualified to conditions that
are now routinely levied on new designs for resistance
to high-intensity radio frequency fields. These antagonist
fields have always been around and include transmitters of
all types aboard the aircraft including comm, xpndr, dme, radar,
hf, telephone services, etc.
External antagonists include radio and television stations,
ground and airborne radar, etc.
We had little need to concern ourselves with these threats
before plastic airplanes came along.
It's almost a sure bet that your ELT is suffering a HIRF
moment due to proximity of the transmitting antenna. A
useful experiment is to swap atennas between the two
comm radios and see if the antagonist shifts to the other
radio.
| Quote: | If I insert my head and shoulders into the space, the problem goes
away. As I withdraw, when I exit the compartment fully, the problem
returns. Therefore it appears that having some mass/capacitance? near the
antennas has a positive affect.
What kind of better solution can be implemented/tried to eliminate this
interference? I cannot carry a body around in that compartment because of
weight and balance considerations.
|
Assuming the experiment described above confirms
vulnerability to signals emitted from the close
proximity antenna, Just because the Com 2 radio doesn't
trip the ELT doesn't mean that it's sufficiently weak
to NEVER trip it. The fact that your body alters the
coupling between antenna and ELT sufficiently to stop
the problem suggests a marginal condition . . . that
could shift either way with either radio in time.
This is why we test most devices at 10x to 100x the
energy we ever expect them to see. LOTS of head-room!
I'd go for shielding the ELT. Is this one of those plastic
case things? Alternatively, you would need a better
location for the ELT or the offending antenna. But
shielding is likely to offer a 10x reduction in
coupling for both radios and be a much more robust
solution.
Have you talked to the ELT folks? This can't be
the first time this has happened.
But if push comes to shove, you can get a really
whippy GPS enhanced crash-locator beacon that's
about 100x more likely to get you found than a
121.5/243 ELT. See:
http://www.marvgolden.com/elt-encoders/acrterrafix.htm
An aluminum foil shield that is connected to the outer
shell of the antenna coax connector may make your
problem go away.
Bob . . .
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h&jeuropa
Joined: 07 Nov 2006
Posts: 645
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| Posted: Mon Nov 03, 2008 4:16 am Post subject: Re: COM-ELT INTERFERENCE |
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Bill,
We had a similar problem in our composite Europa. We solved it by shielding. Changed the cable from the remote to the ELT to a shielded cable and grounded the shield at the remote end to the common ground point for the aircraft. Also wrapped the ELT in aluminum foil and mechanically connected the ground in the shielded cable to the foil. Cured the problem!
Jim & Heather Butcher
N241BW Europa XS
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