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#51
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rip wrote in message .com...
Can you hear this interference through the cabin speaker? Don't know. Haven't tried that one -- frankly I can hardly hear anything through the cabin speaker Or perhaps your using active noise-cancelling headsets? 'rip', we are -- but we have done the obvious experiment of seeing if it's still there when the ANRs are turned OFF and we're listening on a passive headset hooked to the handheld radio. We have had problems with RF noise from a bad ANR in the past -- however, it was just noise, not the audio part of some TV or radio program. However, the switched OFF ANRs are still sitting in the plane if anyone thinks that makes a difference I'm still trying to understand how a powered OFF radio with the power OFF in the plane might be contributing to this problem and hoping someone will explain this to me. Cheers, Sydney |
#52
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#53
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#54
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-In your case, I assume that you are flying either far enough horizontally -away from or high enough above the antennas for safety. Let's say a minimum -of 1/2 mile horizontally. Now lets assume that the strongest broadcast -signal is 100,000 watts "effective radiated power" and a frequency somewhere -around 100 MHz. In a metropolitan area 100kW ERP would rank you in the lower third of broadcast signals, but for grins and giggles, let's make that presumption. At a distance of 1/2 statute mile the power from that -signal that would be picked up by an aircraft com antenna would be about 10 -milliwatts. More like 5 milliwatts, but let's not debate how many milliwatts can dance on the head of a dipole. This is a spectacularly strong signal for purposes of -reception, and certainly more than enough to make it impossible for the -attached receiver to pick up any other signal, but nowhere near strong -enough to cause the receiver, if not otherwise powered, to generate and -re-radiate an intermodulation product. Oopsie. 10 milliwatts is about 0.7 volts RMS, or about a volt peak. If for whatever reason that front end were wide open to the interfering signal, a volt is sure as little green apples capable of turning on the B-E diode of the RF amplifier. Now any lower power signal is perfectly capable of being mixed with our newfound "LO" and being reradiated. - -So, if the signal WERE strong enough to provide enough power to cause an -otherwise unpowered receiver to generate and re-radiate an interfering -signal, the same powerful signal would pretty much wipe out operation of any -other nearby receiver that IS powered on. Ah, no. NOt if the other receiver had enough filtering in the front end to get rid of it. - -By all accounts, what you have is plain old garden variety intermodulation -interference. The intermodulation products doing the interfering are being -generated in the same receiver that is being interfered with. In an earlier -post I offered a relatively simple way to prove this. I'm not debating that it is intermod. I don't know. I'm not there. But front end reradiation is a phenomenon that should be investigated also. And, for that matter, that is exactly why two brands of ELT have had factory recalls...the C-B junction in the output transistor was a wonderful intermod generator. Jim Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#55
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Ah, ok, thanks for that! What do they use? Is it standard?
Paul "Jim Weir" wrote in message ... 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. |
#56
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The rate at which the attenuation increases away from the centre
frequency is affected by the "order" of the filter (first order, second order, etc). It would be difficult to filter out nav frequencies from the com band. Even if the nav radios are the cause, it's unlikely to be a signal actually in the nav band causing the problem...the thinking is that it would produce interference in the com band. The filter from the link would only be used really to see if the cause of the interference was due to a TV band (50-80MHz?) signalling causing problems in the receiver, or whether it was something external causing problems within the com band. Question (Jim?)...I would expect the image channel rejection to be pretty good in aircraft radios, is this the case? How about with handhelds? (and what IF is used?) Paul "Snowbird" wrote in message om... Yes, that looks like the correct kind of product, but my concern is it includes the nav radio frequencies. If Aaron is correct that the problem might be via the nav radios, we need a narrower freq. range. |
#57
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#58
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Sydney, Here are a few ideas concerning your interference problem (worth at least as much as they cost... *** You mentioned 127.0 might have the strongest interference. This could be the result of mixing between the TV channel 2 visual carrier (55.25 MHz) and the channel 4 aural carrier (71.75 MHz); alternatively, the ch. 2 aural carrier (59.75 MHz) could be mixing with the ch. 4 visual carrier (67.25 MHz) to produce the same result. I looked at all the high-power broadcast services (VHF-TV & FM, as listed in the FCC databases) within 10 km of the location you gave; trying all the 2nd & 3rd order intermod possibilities I could think of, the above result seems the most likely. *** FCC databases: FM: http://www.fcc.gov/mb/audio/fmq.html TV: http://www.fcc.gov/mb/video/tvq.html You can scroll to the bottom of these and enter a lat-long and radius. *** According to the TV database, channel 2's transmitter (100 kW) is the closest to the location you gave. If the intermod is being produced EXTERNAL to your airplane, IMO this is the most likely source; a ch. 4 signal (from a different, but nearby, antenna) is received by ch. 2's transmitting antenna, travels back into the high-power output stage where it mixes with the ch. 2 signal, and the resultant 2nd-order intermod product then radiated from ch. 2's antenna. *** You mentioned in another post that on *one* radio, the controller becomes "very faint" under the intermod, while on the other radio and the handheld the controller is still heard nearly normally. This may be a clue that the *one* radio may be the SOURCE; the intermod is perhaps being generated in it's input circuit (wiping out ATC on that radio) and also re-radiated from it's COM antenna into the other radios (but not as strong in the others due to the attenuation [12-18 dB or so] between the COM antennas.) As others have mentioned, it's possible for the radios to generate intermod even when powered off (the "crystal radio" principle is applicable here.) The easiest way to check this would be to simply disconnect the antenna from the *suspect* radio(s), ideally at the antenna end (IMO, no need to bother "terminating" the open connectors) and fly out to the Antenna Farm and note the result. *** Take your handheld in your car and drive around near the Farm, note the results. If no intermod, the problem's most likely in the plane; else, the problem could *still* be in the plane (and the handheld), but *could* also be the fault of one of the TV station. If you suspect a TV station (Channel 2 most likely IMO) is at fault, or even if you don't, I would suggest getting in contact with the station's Engineering Department and (diplomatically) explaining the situation. They're technically qualified folks, are well-equipped with test equipment (spectrum analyzers, ect), and will likely be VERY interested in finding and correcting any such problems in their transmitting plant. If there's an RF emissions problem they aren't aware of (that could possibly result in the station being fined), they'd be most appreciative of it being brought to their attention. *** Hope this helps. Have fun... Mark Snowbird wrote: OK, we're still having our RF interference problem and our avionics guy pleads 'stumped'. Meanwhile we're going nuts whenever we need to get radar vectors for the ILS at our local Class D or when we depart IFR to the SE. Here is what we know 1) the problem is intermittant. occurs both at night and during day. 2) when it does occur, the problem occurs in a specific area -- heading towards a local antenna farm 3) legitimate radio transmissions come through loud and clear 4) the interference isn't just random noise, but sometimes has voices in it (like a radio or TV show) 5) we have disconnected the ELT from its antenna (but left it turned off in the back seat of the plane) -- problem persists 6) marker beacons on, marker beacons off, nav radios different freqs, nav radios off, no effect 7) swapped our KMA 20 audio panel for a loaner KMA 20 no difference 8) we have tried turning off the airplane's entire electrical system and listening for interference on a handheld radio with its own "stick" antenna. Problem persists (!!!!) 9) we have tried different frequencies while experiencing the interference -- not exhaustively. here is a list (- means no interference + means interference) 124.00 - 124.20 - 124.52 - 125.00 - 126.00 + 126.50 + 126.50 mb on, mb off, nav 111.9, nav 110.8, nav off 126.50 handheld w/ alt off, airplane electrical system off 127.00 + 127.10 - 127.25 - 127.27 + 127.30 - 127.50 + 127.97 - 128.00 - 129.00 + 130.00 - 131.00 - 132.00 + (126.5 is the local tracon frequency where the interference is problematic for us, which is why I focused there. 127.0 might be the strongest interference) geographical location where interference seems strongest (there's an antenna there, and when we were directly over it interference stopped) 38 31 90 90 21 75 Can we figure out the frequency and maybe the station which is causing the problem from the above info? Ideas? Other tests? Things to check? Help! If we still get the problem with the plane's entire electrical system off and using a radio/antenna which is not connected to the plane, is there ANYTHING we can do or must we just grit our teeth and bear this? Plane's equipment: Sigtronics SCI-4 intercom KMA20 audio panel/mb King KI-170B nav/com TKM 170B nav/com Apollo 2001 IFR GPS King KN-75 glideslope receiver King KT-76 Transponder no ADF or DME THANKS! Sydney Grumman AA5B "Tigger" |
#59
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Nope. They are all over the map. I chose a random book from the library...it
happens to be the King KX-170B. The high COM IF is 9.0M and the low COM IF is 861.25k. The high NAV IF is 15.1875M and the low NAV IF is 1.1857M. I could pick half a dozen books from the shelf and no two would be the same. Jim "Paul Sengupta" shared these priceless pearls of wisdom: -Ah, ok, thanks for that! What do they use? Is it standard? - -Paul - -"Jim Weir" wrote in message .. . - 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 Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#60
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Blimey, thanks. Any idea how they decide to use these figures? Just out of
curiosity. Paul "Jim Weir" wrote in message ... Nope. They are all over the map. I chose a random book from the library...it happens to be the King KX-170B. The high COM IF is 9.0M and the low COM IF is 861.25k. The high NAV IF is 15.1875M and the low NAV IF is 1.1857M. I could pick half a dozen books from the shelf and no two would be the same. |
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