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#1
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Fixing the Transponder with Duct Tape and Aluminum Foil
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 |
#2
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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 If it ain't broke (no mo!) don't fix it... Richard |
#3
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"Ron Wanttaja" wrote in message news 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. SNIP 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? Use what the pros use -- copper tape. Call friendly neighborhood EMI engineer or go to McMaster-Carr; www.mcmaster.com. Search for "copper tape". Gerry |
#4
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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 |
#5
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"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 |
#6
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(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 |
#7
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"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 |
#8
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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 |
#9
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"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 |
#10
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Copper window screen can be soldered. The copper tape is also used by
stained glass hobbyists. "Gerry Caron" wrote in message . 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 |
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