Fixing the Transponder with Duct Tape and Aluminum Foil

On Tue, 09 Mar 2004 03:02:57 GMT, Richard Lamb
wrote:

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. :-)

This is just too cool for school, Ron!

After fifteen minutes fiddling with my multimeter, my guess is that the
aluminum layer is too thin to count on. Touching the probes on the shiny
exterior, the DVM readout never *did* stabilize...down to 40 ohms, back to
infinity, down to 100K, back to infinity, etc. The LCD screen spent more
time blanked than displaying any value.

I did a less-scientific test...I sliced open a balloon and inserted a cell
phone inside. The signal-strength readout did drop from full to just one
bar.

I may try it anyway, just because installation is fairly easy. But I
suspect the points of contact will rub away rapidly, losing the ground
connection and causing the noise level to rise again.

By now, I've collected material for several approaches, like the 1/2"
copper tape the RST winged-but-definitely-heterosexual-mythical-creature
left me and the aluminum window screening I shelled out $5 for last
weekend. I even got some of that "holy grail" copper tape I mentioned on
an earlier posting...it's 16" wide and 4 feet long, with a peel-off
adhesive backing. So I certainly don't lack options....

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 have noticed the clicking and chased the problem, mine comes from the
encoder..... if i turn the encoder off it coes away. Running a Microair
T2000 and ATK350

Thanks Michael

Jim Weir wrote:
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

Ron

More free advice from a long time Ham.

Have you tried a toroid? Radio Shack used to carry some that snapped
apart and then back together so you could put several turns on one
without cutting the wire(s). Price was a couple of dollars so not
expensive to try.

Also, we used to take a piece of aluminum foil from the kitchen and
just wrap around the wire(s) and then slide it back and forth to find
a spot where the interference stopped. Again, very cheap to test.

We also on occasion used some small (.02- .002) ceramic caps.

Also, any coax you use be sure it is double shielded. Some of the
cheap stuff on the market has very 'porous shielding.

Robert and my daughter are still in FL working on their boat. He could
have built another plane in the time they are spending rebuilding that
damn boat. G Haven't said if they are going to go by S-N-F this year
before heading back to WA?

Any words the spell checker didn't catch, I take full responsibiity
for G

And the best up there in the land of the every day rain.

Big John

On Sat, 06 Mar 2004 23:53:02 GMT, Ron Wanttaja
wrote:

I'd installed my new Microair transponder a month ago, but have been having
troubles with interference. The transponder was inducing bad clicking
sounds into the comm radio receiver, making the radio difficult to
understand.

I couldn't really blame the Microair, since the Terra transponder I'd had
before did exactly the same thing. But I'd been hoping the new transponder
and installation would eliminate the problem...especially since I'd
reworked all the transponder wiring and moved the antenna to a different
location. But the interference continued.

The main suggestion I got from the avionics guys around here is that the
transponder was interfering via the cables running to the headset jacks. I
installed shielded braid over those cables, though, and the problem
continued.

All the electrical connections to the radio (except the antennas) are made
via an unshielded flat connector. Since the wires had to spread out to fit
the connector, I hadn't been able to run the braid all the way to the
radio...it terminated about two inches away, and I ran a ground wire from
the braid to the radio chassis.

It seemed to me that the most likely culprit was that last two inches of
unshielded wiring and the unshielded connector...especially since they were
the closest part of the wiring harness to the transponder.

So, today I took a roll of aluminum foil out to the airplane and wrapped
foil around the back of the radio, leading it down the wires until it
contacted the braid. Duct tape, of course, to hold it in place.

I'm happy to report that this solved the problem. Radio signals were
perfectly clear.

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?

Ron Wanttaja

On Tue, 09 Mar 2004 08:27:50 -0600, Big John wrote:

Also, any coax you use be sure it is double shielded. Some of the
cheap stuff on the market has very 'porous shielding.

Actually, that's one thing I did wonder about. The coax I use is RG-58,
but I buy premade LAN cables from the local electronics emporium. I
shortened the one I bought for the transponder, installing a new BNC
fitting at one end. Still, the noise did exist with the old transponder,
before I went to the LAN cable stock.

But the store's bulk stock is probably the same brand, so I can't see the
LAN cable as being a second choice.

One thing that did occur to me was to take copper braid and slide some over
the transponder coax as an additional shield. Shouldn't affect the RF
characteristics, I guess, but didn't really think it would help the noise
problem.

Ron Wanttaja

Ron Wanttaja wrote:

One thing that did occur to me was to take copper braid and slide some over
the transponder coax as an additional shield.

Or spiral wrap some of that copper tape that RST's winged creatures delivered.

Russell Kent

Why can't you just continue to use what worked? Namely the aluminum foil.

Signed,
Not Understanding, Captain of the least resistance


"Ron Wanttaja" wrote in message
...
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:
The easy way to tell if the interference is coming from the radiation
of the antenna is to disconnect the antenna at the antenna and connect
a 50 ohm UHF rated dummy load in place of the antenna. If the noise
goes away it is for sure radiation from the antenna and not other
cable coupling paths or power pulse coupling through the 12 volt feed
wire.
The transponder antenna must have a ground plane that is at least 1/4
wave length in radius around the antenna. Avoid multiples of 1/2
wavelength ground planes until you have at least 5 wavelengths or
more of ground plane around the antenna since this affects the antenna
feed impedance and the amount of signal that is fed back on the
outside of the antenna coax.

If the problem is truly from the 1GHz radiation from the antenna one
of the clip on ferrites placed on the wire bundle(s) where the wires
enter the intercom and or where your audio sources come from may cure
the problem. If the coupling is from the power surges required by the
transponder from the 12 volt feed the ferrites will not do any good at
all since they only start to work above 100 MHz.
John


On Tue, 09 Mar 2004 15:10:32 GMT, Ron Wanttaja
wrote:

On Tue, 09 Mar 2004 08:27:50 -0600, Big John wrote:

Also, any coax you use be sure it is double shielded. Some of the
cheap stuff on the market has very 'porous shielding.

Actually, that's one thing I did wonder about. The coax I use is RG-58,
but I buy premade LAN cables from the local electronics emporium. I
shortened the one I bought for the transponder, installing a new BNC
fitting at one end. Still, the noise did exist with the old transponder,
before I went to the LAN cable stock.

But the store's bulk stock is probably the same brand, so I can't see the
LAN cable as being a second choice.

One thing that did occur to me was to take copper braid and slide some over
the transponder coax as an additional shield. Shouldn't affect the RF
characteristics, I guess, but didn't really think it would help the noise
problem.

Ron Wanttaja

If you do this, I GUARANTEE you that the noise will go away. This is a
trans-ponder. The trans part means that it responds ONLY to an input pulse. No
input pulse from a radar, no output pulse.

Wanna try again?

Jim



(John)
shared these priceless pearls of wisdom:

-Ron:
-The easy way to tell if the interference is coming from the radiation
-of the antenna is to disconnect the antenna at the antenna and connect
-a 50 ohm UHF rated dummy load in place of the antenna.
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

Humm, I forgot about that Jim. However most have a self test button
that causes the transponder to transmit some pulses. I know my 20
year old Cessna ARC transponder transmits at a low rate when you push
the test button.

On Tue, 09 Mar 2004 19:56:19 -0800, Jim Weir wrote:

If you do this, I GUARANTEE you that the noise will go away. This is a
trans-ponder. The trans part means that it responds ONLY to an input pulse. No
input pulse from a radar, no output pulse.

Wanna try again?

Jim



(John)
shared these priceless pearls of wisdom:

-Ron:
-The easy way to tell if the interference is coming from the radiation
-of the antenna is to disconnect the antenna at the antenna and connect
-a 50 ohm UHF rated dummy load in place of the antenna.
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com