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Old March 23rd 07, 02:36 AM posted to rec.aviation.homebuilt
COLIN LAMB
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Posts: 94
Default ELT turns on with comm transmitting ???

I do not know the layout of the ELT, but have experience in servicing solid
state devices and reducing interference, so I am responding in general.

I expect that the problem is created because the rf from the transmitter is
being picked up by the control cable of the ELT. That cable is acting as an
antenna. Rf entering into the ELT is rectified by a diode junction (solid
state devices can act as crystal sets). That rectification creates a
voltage which activates the ELT.

The factory specified a ferrite bead to solve the problem. That reduces the
rf getting inside the ELT. If that does not solve the problem, then use
more drastic measures to prevent rf from getting inside the ELT.

Some additional steps that can be taken a

1. Loop the control cable through the ferrite bead 2 or 3 times. That will
multiply the effectiveness of the ferrite.
2. If you cannot do that, you can simply use 2 or 3 ferrite beads.
3. If the control cable is not shielded, replace it with shielded cable.
4. Add a capacitor across the terminals at the input of the ELT. A .001
ufd (1000 pf) capacitor should help.
5. Moving the comm antenna further away from the wiring.

I have not addressed the certification issue, which should be considered.
My aircraft is experimental and I feel like I have a bit more license to
work on it.

Colin Lamb


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