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#31
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Aaron Coolidge wrote in message ...
Cool, I thought that you might have kill-filtered me for some reason, because no one other than me seemed to be getting my messages! No, not at all, though I'm reading news through groups.google.com which is flaky on propegation BTW someone said they emailed me. I didn't receive it. This email address no longer works. If anyone wants to email me try hoeltzli at swbell dot net. Though this discussion seems on-topic and of possible general interest so I'd just as soon keep it on the net. On further reflection, this may be a red herring. Digital TV is in the 220+ MHz region. Can this kind of intereference be caused by subtracting frequencies? Channel 2 is 54 to 60 MHz, the 2nd harmonics of are 108 to 120 MHz. That's too low for the frequencies where we're getting interference (124.2 is clean, 126.5 is trash) This leads me to suspect one of the *NAV* radios. Can you physically remove them from your plane, one at a time, and leave them in your car? Not a problem. Can this really be caused by a radio which is *powered off*? Because I did try switching the navs to different frequencies and also turning them off, along with all the electrical power in the plane. Since I changed jobs I don't have my trusty HP 8591E spectrum analyzer anymore, if I did I'd consider a trip to St Louis! Oh, bummer! That would have been nice. : If this is a possibility, how do we safely remove the : marker beacon antenna for testing purposes? Do we need If you've got the bent metal rod kind Pretty sure it's a blade, though I admit the MB antenna is "out of sight out of mind" to me. If you wish to electrically remove the antenna from the plane while leaving it physically in place I'm perfectly happy to take it off and slap some 50 mph duct tape over the resulting hole, providing it's not going to hurt the MB receiver to be attached to dangling coax. If I were doing this, I'd probably terminate the RF input to the radio, as well. IIRC this would be a PITA -- the coax goes to the antenna with a standard BNC connector, but is soldered on to the radio tray. Please keep us (me) informed, we're trying to help the best we can! I appreciate this. It's a vexing problem and one which apparently lies outside our radio guy's experience Thanks, Sydney |
#32
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"Paul Sengupta" wrote:
"Mark Hickey" wrote in message .. . (Jay) wrote: In very high fields with amplitude modulation you can get what's often called "detection by overload". Some people have reported picking up radio on their fillings in their teeth. I used to be able to turn OFF my car radio and STILL get a loud "bbBBBRRRRrrrrzzzz" every time the long range radar swept past my car You can get that sitting in the car on top of one of the car parks in Heathrow. And to think there are people scared about the effects of cellular towers! Heh. Mark Hickey |
#33
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wrote in message
link.net... 54-60 mHz carrier at 55.125 (55.125 * 2) + (2 * 10.7) = 131.65 (55.125 * 2) + 10.7 = 120.95 76-82 mHz carrier at 77.125 (77.125 * 2) - (2 * 10.7) = 132.85 225.525 (digital? ham repeater?) - 77.125 - (2 * 10.7) = 127.00. Just playing... If it's intermittent then are there any ham repeaters or other comms repeaters in the area? And, two, a 'directional' antenna. If an AM radio station was causing the problems and you wanted to know which mast was causing it, you could tune in the ADF and see where the needle points! :-) If the source is outside the plane, you're conclusively dealing with front end overload intermodulation interference. Unless you're getting in-band interference from outside the plane, such as harmonics, intermod from rusty bolts or other transmitters as others have mentioned, etc. If that's the case: 1) high-Q bandpass filtering, to eliminate the out-of-band interference sources. wouldn't work. I'd say it was worth a try first though, relatively easy to stick in-line with an antenna and it would get rid of most of the out of band signal straight off. Not to leave the filter there but just as a trial to see if the interference is in-band or out-of-band. Paul |
#34
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#35
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On 5-Nov-2003, "Paul Sengupta" wrote: http://www.leeselect.com/shopping/pricelist.asp?prid=41 Maybe you could borrow one to try on the handheld. It could work if the strong out of band signal is causing problems with the receiver front end, but if the intermod/harmonics/ whatever are external and fall in-band then it's not going to help. The problem with devices like the one you suggest (see above link) is that they are not designed to carry the high current that will be present when the com antenna to which it is attached is used for transmission. (Remember, our com radios transmit as well as receive.) If the filter doesn't burn up under that current load it will at least significantly attenuate the transmit signal. The specs for this device show an in-band attenuation of 4 dB. That's a lot! It would be possible to rig up a transmit bypass using circulators or coax relays, but it would be messy. -- -Elliott Drucker |
#36
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Yes, I was only thinking using it as a troubleshooting tool
to see if the interference was in-band or out-of-band by the time it hit the receiver front end. My dim memory is telling me that there are setups using by-pass relays for transmit due to the adding of a receive pre-amp, though I can't remember where I saw it...I may be thinking of a different field entirely, I know they're used in amateur radio... Paul G1YJY wrote in message ... The problem with devices like the one you suggest (see above link) is that they are not designed to carry the high current that will be present when the com antenna to which it is attached is used for transmission. It would be possible to rig up a transmit bypass using circulators or coax relays, but it would be messy. |
#37
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#38
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: This leads me to suspect one of the *NAV* radios. Can you physically
: remove them from your plane, one at a time, and leave them in your car? : Not a problem. Can this really be caused by a radio which is *powered : off*? Because I did try switching the navs to different frequencies : and also turning them off, along with all the electrical power in the : plane. Yes, it can be caused by a radio switched off. With those ACK elt units that cause problems, they are off when their problems occur! : I'm perfectly happy to take it off and slap some 50 mph duct tape over : the resulting hole, providing it's not going to hurt the MB receiver : to be attached to dangling coax. It won't. : If I were doing this, I'd probably : terminate the RF input to the radio, as well. : IIRC this would be a PITA -- the coax goes to the antenna with a : standard BNC connector, but is soldered on to the radio tray. : It would be OK to terminate the coax where the antenna was connected. As someone else pointed out, you can short out the antenna at its BNC connector to electrically remove it from the plane. You could probably make a BNC shorting plug from parts found at Radio Shack. -- Aaron Coolidge (N9376J) |
#40
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You can stop playing with 10.7 as a source of the problem. I cannot recall an
aircraft navcom using 10.7 as the IF frequency. Jim "Paul Sengupta" shared these priceless pearls of wisdom: -Bear in mind that the radios involved may have different front -ends. It might just be a coincidence that your handheld and -panel radios are affected and nothing to do with the airframe. - -How about 127.00-10.7 (IF mixer) = 116.30 -116.30/2 = 58.15? -(i.e. mixing of the second harmonic of 58.15MHz) - -Ok, unlikely, just playing with numbers! - -Paul -G1YJY - -"Snowbird" wrote in message . com... - Update: I talked to a local DE who is also doing piles of - instrument instruction. She says she's flying in beaucoup - planes in that area, without the same problem. - 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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