Fixing the Transponder with Duct Tape and Aluminum Foil

On Sun, 07 Mar 2004 02:15:39 GMT, "Gerry Caron" wrote:

"Ron Wanttaja" wrote in message
news

I figure the foil isn't worthwhile as a permanent solution. A guy at the
airport suggested using aluminum window-screen material. It appeals to
the
cheap side of me. Any drawbacks, or suggestions for alternates?

Use what the pros use -- copper tape. Call friendly neighborhood EMI
engineer or go to McMaster-Carr; www.mcmaster.com. Search for "copper
tape".

The trouble is, to shield everything, I need to completely wrap around the
back end of the radio. It's 3.375" per side, so I'd need copper tape about
16" wide, and about the same dimension long to have some overlap on the
back of the radio and still go along the wiring harness a bit."
McMaster-Carr's got copper tape 50 feet wide, but it's only 3" long. :-)

I see M/C's got some nice copper mesh sheet. It probably would work
nicely, but they want $700 for a 11" square sheet....and I'd need two of
them. I bought a 36" square sheet of aluminum window screening for $5.
I'm hoping it'll work....

Ron Wanttaja

"Ron Wanttaja" wrote in message
...
On Sun, 07 Mar 2004 02:15:39 GMT, "Gerry Caron" wrote:

"Ron Wanttaja" wrote in message
news

I figure the foil isn't worthwhile as a permanent solution. A guy at
the
airport suggested using aluminum window-screen material. It appeals to
the
cheap side of me. Any drawbacks, or suggestions for alternates?

Use what the pros use -- copper tape. Call friendly neighborhood EMI
engineer or go to McMaster-Carr; www.mcmaster.com. Search for "copper
tape".

The trouble is, to shield everything, I need to completely wrap around the
back end of the radio. It's 3.375" per side, so I'd need copper tape
about
16" wide, and about the same dimension long to have some overlap on the
back of the radio and still go along the wiring harness a bit."
McMaster-Carr's got copper tape 50 feet wide, but it's only 3" long. :-)

I see M/C's got some nice copper mesh sheet. It probably would work
nicely, but they want $700 for a 11" square sheet....and I'd need two of
them. I bought a 36" square sheet of aluminum window screening for $5.
I'm hoping it'll work....

Ron Wanttaja

Have you checked www.digikey.com for a "shielded hood" for the
DB-whatever-it-is on the Microair? Their quantity 1 price will seem kind of
pricey, but it's certainly cheaper than 700 bucks -- probably more like 7.

Tim Ward

(answering several responses)

On Sat, 6 Mar 2004 18:57:48 -0800, "Tim Ward"
wrote:

I see M/C's got some nice copper mesh sheet. It probably would work
nicely, but they want $700 for a 11" square sheet....and I'd need two of
them. I bought a 36" square sheet of aluminum window screening for $5.
I'm hoping it'll work....

Have you checked www.digikey.com for a "shielded hood" for the
DB-whatever-it-is on the Microair? Their quantity 1 price will seem kind of
pricey, but it's certainly cheaper than 700 bucks -- probably more like 7.

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.

Every bit of wire associated with the transponder was replaced when I
switched to the Microair...the transponder got its own circuit
breaker/power wire (the Terra shared a single breaker and a single 18-gauge
wire with the Comm radio...hey, *I* didn't install it), new encoder
harness, new coaxial cable to the repositioned antenna. So I figured the
problem was in the Narco, not the transponder.

I *am* glad I bought the unit from my friendly avionics dealer on my own
home field, instead of mail-ordering it. It gave me real-time access to
some technical advice. I emailed Microair with my problems and didn't
receive a response. I did state, in my email, that I knew the problem
wasn't with their device, but was hoping they'd have some suggestions.

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?

(Snip)

]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.

Neat suggestion, though...I might try to wrap that last couple inches of
wiring harness and shield the openings somehow.

On Sat, 06 Mar 2004 18:48:11 -0800, Jim Weir wrote:

} Or send Jim a SASpaddedE and he'll send you what is left of a few
} rolls we've used around here for "magazine projects", if y'know
} what I mean.

I was visited by the RST fairy several years back, and still have a good
bit of a roll left after doing the ground plane for my plastic-pipe-fitting
antenna.

Also, little strips of it on the edges of the suitcase work neat for
picking out your suitcase as soon as it comes off the airport luggage
carosel...subtle, but instantly recognizable. Though, from the frequency
of the "Hi, we're the TSA, and we pawed through your bag" notes I get, I
get the impression the Federales get a bit suspicious... :-)

Ron Wanttaja

"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

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

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

Every body worries about "GROUND LOOPS" but in many cases they do not
really know what they are talking about.

Antenna coax is shielded wire that is also controlled impedance wire.
You will find for every transmitter that the shield is connected at
BOTH ends of the wire and for most antennas the shield is also
grounded at both ends. If you disconnect the shield at one end and
measure the VSWR you will notice that it goes toward infinity due the
shield not being connected.

CURRENT ALWAYS HAS TO RETURN TO THE SOURCE.

The current may not flow where you want of even where you think it is
flowing but it will seek a way(s) back to the source.

LOW FREQUENCY CURRENT follows the path of least RESISTANCE.
HIGH FREQUENCY CURRENT follows the path of least INDUCTANCE.

The current will divide inversely proportional to the impedance of
each individual path among the many paths available and seek the path
of least total impedance.

Now what is the difference in low frequency and high frequency current
you might ask. The difference is the length of the connecting wire
in terms of wavelengths of the signal. If the wire is longer than
1/20 of a wavelength then the signal is considered high frequency and
you must think of the wire as a transmission line not just as a common
wire.

For LOW frequency shielding the wire shield is only connected at one
end. Low frequency in this case is defined as DC up to the top of the
audio band, 20 KHz. This keeps the magnetic coupling of near by
cables from inducing a voltage in the shield that couples to the
signal. Connecting the shield at only one end will NOT keep out high
frequency signals!

For very high frequency signals where the shield thickness is many
skin depths of the signal frequency you can have different independent
currents on both the outside and the inside of the shield. For this
reason you must have a 360 degree shield connection or the inner and
outer currents on the shield couple and mix due to the inductance of
the shield pigtail connection. To keep out, or in, high frequency
signals you must connect the shield at BOTH ends.

Here is something to think about:
How many GROUND LOOPS do you have if you place a whip antenna in the
center of a X Y grid of 10 wires by 10 wires where each wire is
connected to the crossing wire? 100?

Now we fill in the spaces between the wires with another ten wires so
we have a grid that is the same size but now has 100 wires by 100
wires. Do we now have 10,000 ground loops? Is this better?
What if we now fill in the spaces between the wires so that it is
solid metal. Do we now have an infinite number of ground loops? Is
this better? Yes, except for weight.

The answer is the current will divide among the many paths and follow
the path of least total impedance.

John Frerichs

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.

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

"Ron Wanttaja" wrote in message
...

The trouble is, to shield everything, I need to completely wrap around the
back end of the radio. It's 3.375" per side, so I'd need copper tape
about
16" wide, and about the same dimension long to have some overlap on the
back of the radio and still go along the wiring harness a bit."
McMaster-Carr's got copper tape 50 feet wide, but it's only 3" long. :-)


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

I'd start by spiral wrapping the harness, making sure to contact the
overbraid on the one end and end with solid contact around the connector at
the back of the unit.

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. If
there are cover edges that don't have good metal-to-metal contact or other
openings, put a strip of tape along the gap. (Note that a gap running the
length of one side (3.375") would be just shy of 1/4 wave at 1090 MHz.)

I've sealed up much larger air transport units (big ARINC boxes) with a
couple feet of 1/2" tape.

Gerry