The noise is an efficient "jammer", and there is one common thing about RF jammers, they work best when placed between the receiver antenna and the direction of interest. Hangflyer has clearly demonstrated that the "blocking" is ahead, i.e. forward of the antenna, which leads me to suspect that the "spark" transmitter is somewhere on the engine block.
Again I ask anyone proposing an "external" RFI source (spark plugs or whatever) how that explains these observations:
1: Relative "badness" of interference is the same at 200 meters (hangflywer stated direct line of sight to base station antennna) or at a significant distance, 20km would result in 40db Rx level difference.
Hard to see how a fixed level source would always have same impact.
2: Interference present -only- when tuned to an active transmitter.
3: The extreme dirictional sensitivity, even assuming bad shadowing the RX level would only change 6Db or so as the aircraft turned.
4: The strong correlation with RPM - not there at all then fully on just 100 rpm higher.
Yes DERG feel free to answer a direct question or stop complaining about those who dont.
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BTW On the natue of the noise: Hangflyer is it all all the same as what a distant "on the edge" of recpetion signal sounds like?
Possible the "white noise" without the chop?
Above with interference not active of course.