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#21
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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 |
#22
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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. |
#23
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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 |
#24
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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 |
#25
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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 |
#26
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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 |
#27
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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 |
#28
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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 |
#29
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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 |
#30
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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 |
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