"Ron Wanttaja" wrote in message
...
(answering several responses)

Well, the problem isn't in the Microair's DB-25, since I had the same
problem with my Terra, which had a hard-wired rack. It must be some sort
of bleedover from the antenna itself, since the Microair is machined out
of
a solid hunk a' aluminum.

So I figured the problem was in the Narco, not the transponder.

On Sun, 07 Mar 2004 03:50:28 GMT, "Gerry Caron" wrote:

]OTOH, 6 - 16" long pieces of tape overlapped 1/4" results in a
]sticky sheet about 16" square.

One bit of curiosity I have, about the copper tape: How does the "sticky
side" affect electrical connection? Is the adhesive conductive?

IIRC it's not conductive, but it's not much of a dielectric either so I
imagine there's some coupling across the layers. The tape acts more as a
reflector than a conductive shield.

]If the unit is metal, you shouldn't need to shield the unit itself
]with only a couple exceptions. Since we're dealing with a
]transponder (1090 MHz), the only other issue would be an aperture
]which could act as a slot antenna.

This may be *exactly* the problem I have. The metal case of the radio is
cut back to allow the connector to be external without grounding out...
and
the cutout is just shy of the full width of the case. Also, there's a
wider slot orthogonal to the connector slot, where the plastic lock for
the
connector slides in to.

That's where I would start. There are two basic EMI paths: conducted and
radiated.

The conducted enters thru the wiring harness. To stop this, you need to
keep the EMI out of the harness. It gets into the harness either thru an
e-field or m-field. The overbraid shield is the first line of defense. The
second line of defense is cable routing. Cables running parallel create an
opportunity for inductive coupling. Keep the Xpdr antenna cable away from
comm and audio cables. If they have to be close, try to have the routes
cross at 90 deg. The overbraid needs to be grounded because you can get
some pretty big currents induced along a cable.

The radiated path is generally blocked by the case which is designed as a
shield. Any gaps in that shield let rf in. The size of the gaps will tune
the susceptibility to specific frequencies. That cut out along the back for
the connector sounds like a pretty good 1/4 wave slot for the transponder
freq. Covering the gaps with the tape will block the rf. Grounding
generally isn't an issue because the tape doesn't accumulate much of a
charge.

Gerry