Fixing the Transponder with Duct Tape and Aluminum Foil

"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

I *beg* your pardon? We may be from California, but so far as I know, we are
all straight here.

{;-)

Jim


Ron Wanttaja
shared these priceless pearls of wisdom:


-I was visited by the RST fairy several years back



Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

There are some special versions of copper tape that are specifically labeled
"conductive adhesive". So far as I know, they are only made by 3M and are
gawdawful expensive. And, I got bit in the butt once by assuming (terrible
word) that the adhesive parts would remain adhesive over time. The "conductive
adhesive" only gets its conductivity through copper dust mixed in with the
stickum.

If that stickum comes loose over time or temperature, you've got one hell of a
good slot radiator on your hands, especially at a gigahertz or two.

Jim


"Gerry Caron"
shared these priceless pearls of wisdom:


- 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.

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

On Sun, 07 Mar 2004 07:53:16 -0600, Charlie England
wrote:

Try your local Home Depot 'A/C supply'.

Go to the insulation section & look for 'aluminized mylar' radiant
barrier insulation. It comes in various forms. Best for you would be the
mylar sheet with an aluminum foil bonded to the mylar. Other types may
look like either bubble wrap or foam wrap sheets for protecting delicate
stuff for shipment, but will still have the aluminum foil coating. More
durable than just foil; you can make a 'boot' by stitching a sheet into
a cone shape.

Neat idea, thanks, Charlie. Don't have a Home Depot A/C Supply real close,
but Lowe's Aerospace is right across the road from the airport. :-)


Ron Wanttaja

Gerry Caron wrote:


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



Also only one end of the cables shield should be grounded or else your open
to some ground loops which can cause some of your problems.

On Sun, 07 Mar 2004 21:30:30 GMT, UltraJohn
wrote:

Gerry Caron wrote:


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

Also only one end of the cables shield should be grounded or else your open
to some ground loops which can cause some of your problems.

Thanks, got the same recommendation from others...and contrary advice from
a few.

Fortunately, with a wooden airplane, *not* grounding the other end is a lot
easier than grounding it... :-)

Ron Wanttaja

Charlie England wrote:

Try your local Home Depot 'A/C supply'.

Go to the insulation section & look for 'aluminized mylar' radiant barrier
insulation. It comes in various forms. Best for you would be the mylar sheet
with an aluminum foil bonded to the mylar. Other types may look like either
bubble wrap or foam wrap sheets for protecting delicate stuff for shipment,
but will still have the aluminum foil coating. More durable than just foil;
you can make a 'boot' by stitching a sheet into a cone shape.

Ron Wanttaja replied:

Neat idea, thanks, Charlie. Don't have a Home Depot A/C Supply real close,
but Lowe's Aerospace is right across the road from the airport. :-)

It probably isn't an issue in Ron's case, but having recently seen the Nova
program on the investigation into the causes of the crash of SwissAir #111 I
feel compelled to point out that some aluminized Mylar insulation is
flammable.

Russell Kent

On Mon, 08 Mar 2004 14:06:44 -0600, Russell Kent wrote:

It probably isn't an issue in Ron's case, but having recently seen the Nova
program on the investigation into the causes of the crash of SwissAir #111 I
feel compelled to point out that some aluminized Mylar insulation is
flammable.

Considering that there's a manly pilot leg running on the left side of the
avionics box, and another manly pilot leg on the RIGHT side of the box,
it's something I should keep in mind.

But I actually came up with the perfect solution this afternoon...assuming
the technology cooperates. I was mulling over how I'd cut the sheet, wrap
it around the radio and the wiring harness, and realized that what I needed
was a aluminized mylar funnel.

Something very much like the bottom of a shiny aluminum toy balloon....

So I'm going to pick up a couple of (uninflated) balloons and turn my
multimeter loose on them. Maybe I'll find a left-over Valentine's Day
balloon on sale..

I'm really hoping they'll work out. The idea of buying an aircraft part at
a store called "Party Universe" is almost irresistible. :-)

Ron Wanttaja

Ron Wanttaja wrote:


I'm really hoping they'll work out. The idea of buying an aircraft part at
a store called "Party Universe" is almost irresistible. :-)

Ron Wanttaja


This is just too cool for school, Ron!



Richard

There are those fancy condoms that they sell in the mens' room at the truck
stops... {;-)


Jim


Ron Wanttaja
shared these priceless pearls of wisdom:

-So I'm going to pick up a couple of (uninflated) balloons and turn my
-multimeter loose on them. Maybe I'll find a left-over Valentine's Day
-balloon on sale..

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com