[SBE] Aircraft radios.. cross posted

[Dan Rapak]

Ed,

Just a thought here. The fact that the aircraft are all "on approach" would indicate that they were all operating on one specific aeronautical frequency which helps narrow the search.

I'd be curious as to what radius you looked at for contributing signals when running the numbers for a potential intermod problem. If the aircraft are at an altitude of greater than 10,000', a contributor to an intermod product could be coming from pretty far away. At that altitude, the horizon goes out pretty darned far and the contributor's power level wouldn't have to be terribly high.

Also, don't forget that the center radius for the intermod search has to be where the aircraft were located when the interference occurred, not centered on the broadcast station or the airport. At 10,000+ feet, that could easily put the aircraft and the center of your search fifty or more miles away.

These factors would dovetail in with why you and the ground controllers are not hearing the interference.

The good thing is, there are standard approach patterns that aircraft use that will help limit where the center of your search might be. If possible, it would be good to learn what particular approach route was in use (and if possible, precisely where the aircraft were) when the interference occurred to help narrow your search.

It would also be good to know whether the interference was relatively constant (two constant carriers producing an IM product) or whether it comes and goes (the broadcast station and a two-way radio transmitter, perhaps one with a spur causing the problem.)

My two cents worth.

Dan Rapak
CPBE, 8VSB, CBNT



 ----- Original Message ----- 
  From: Edwin Bukont 
  To: radio-tech at broadcast.net ; pub tech ; sbe 
  Sent: Wednesday, September 26, 2012 7:56 AM
  Subject: [SBE] Aircraft radios.. cross posted


  For those familiar with commercial aviation... question.
   
  Am curious if those who follow commercial aviation, especially the radios used in commercial planes, are aware of any recent significant changes to radios, perhaps something added.  Or a new make/model of some receiver that might be recently deployed.
   
  Have a strange interference complaint which makes no sense.  I have been all over the client's transmitters with scanner and spectrum analyzer.  Driven the major airports and a bunch of the smaller ones.  Drove more or less as the crow flies a line between the major airports in the area, with scanner and analyzer running in the car.  After six hours I heard many very clear 5/5 communications of aircraft, and whole bunch of noisy ones, but none of the interference, and no pilot in that time complained.  The problem is not there on the ground.  But sporadic complaints do come in.  The FCC and FAA are all of a sudden being coy as well.
   
  One interesting angle is that the complaints are coming from aircraft at >10,000'.  Even FAA admits, no ground controller has heard the interference that the pilots report, when on the same freq.  Most of the complaints have been 'on approach', if that sheds any light.  This adds to my suspicion that the problem is aircraft receiver based, not broadcast transmitter related.
   
  Also, the comment was made by someone that sychronous AM from FM transmitter could generate VHF spur in aircraft band.  Ok, perhaps, But in this case, the interference reported is NOT an IM product.  The nearest product is 300kHz away, and unlikely to occur.  I have done that math, including allowance for HD carriers mix, and no number equates.  Furthermore, the complaint is such that were said synchronous AM product occuring, I would for sure see it on the analyzer, and again would expect to hear it on the ground and see its appearance in transmitter readings.  
   
  I am suspecting something has changed in what is commonly used in aircraft.  Note that proximity to the broadcast tower is not a factor.
   
  Reply off list is fine.
   
  Thank you


  Edwin Bukont CTS, CSRE/DRB, CBNT 
  V- 240.417.2475 (DC/Baltimore)
  V- 615-357-7390 (Nashville)
   












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