RFI is best squelched at the souce, i.e. on the PC board. The biggest
source of RFI is the clock that drives the processor, etc. If it isn't
routed on the board very well or properly terminated, it will radiate like a
pig. Also, effective grounding of the PC board to the case is important.

Given that you have a noisy PC board, the only way to effectively shield it
is the fully enclose it in a conductive case, preferrable made out of steel
since that will also block the magnetic portion of the radiation. Any gaps
or slots in the case that are as large as the wavelengths of the radiated
signals will allow the signals to escape. In an airplane, aluminum cases
are preferred due to their light weight and lack of magnetic effects on the
compass.

Also, wires that run out of the enclosure will often carry the RFI signals
and re-radiate them by acting as antennas unless they are also well shielded
and properly terminated to the case ground. Ferrite cores can also be
effective in blocking such signals from traveling out through the wires.

Dean

These are all band-aids for the original problem of having an overly noisy
PC board.
"jcpearce" wrote in message
ups.com...
As a pet/learning project I made a data aquisition unit using an 8051
microprocessor and an EPIA M motherboard running a variant of Linux to
process and display the information. It all works but the EMI from the
EPIA M causes way too much noise to the aircraft radios. I have tried
shielding the whole device in an aluminum case with very little
improvement.

Any ideas on how to smother the EMI or some other small motherboard
which may not have as much an issue (as a test I took my portable
aviation radio and within 6' of any my home computers the same occurs
which gives me little hope)

Thanks