Me too, Noswad. At VHF frequencies, the interference (which originates with the sparking of the motor's commutator) will be severely attenuated as it propagates through the premises wiring. So it's very unlikely that it's conducted from source to victim. Instead, the VHF noise will be radiated from the first foot or two of wiring connected to the brushes ... and this is where clamp-on ferrite chokes are very useful at spoiling the efficiency of the inadverdent antenna.
I have a hard time understanding how emissions from the motor got picked up by the wireless system from that distance. I was thinking more along the lines of power line noise.
This sounds way too oversimplifed. I work in the area of pro audio and have helped thousands of customers find all sorts of problems. What's hard to believe is that, because wireless mics operate at VHF frequencies where wavelengths are only a few feet, just insulating a motor (presumably from surrounding metal work) would have any effect at all. Generally, to reduce emissions caused by arcing (brush motor in this case), some filtering has to be applied as close to the brush connections as possible ... sometimes the filtering is as simple as snapping some ferrite choke cores around the radiating wires. I'd like to know more about the solution.
This is quite interesting, moreso when you considr that the "short wireless link" was probably FM. A correctly functioning link would have the receiver in a full quieting mode, meaning that there was enough signal to saturate the limiter stages, which is what allows FM to provide the much desired noise elimination. The signal from the sparking elevator drive motor should not have been strong enough to get past the limiter stages, leading to my guess that the radio signal amplitude was not nearly as high as it should have been. So perhaps there was a failure there some place.
One more question, which is, why didn't they check the wireless link first thing? LInks are almost always a source of noise in an otherwise silent system,
On the other side, what kind of company has an elevator that is only used by a security guard on Sunday? That is a bit puzzeling, I think. Must business elevators get used more during the weekdays.
Our church had a noise problem on some Sunday mornings years ago, before wireless mikes were used. It was a random, distorted squawking sound, in short bursts with long pauses between. Usually it was weak and ignorable, but now and then it was loud. Some of us started to recognize it as distorted speech. Turns out that one member's husband often drove her to church, but sat in the car in the parking lot during services. Then he got a CB radio, with a (illegal) power amplifier, to pass the time. Depending on how close to the sanctuary he parked, his calls got into the amplifier system! Just asking got him to stop (or move a block away), but we had to add lots of RF chokes and bypasses to make sure that other stray RF stayed out of the system.
This story shows that you can not give up when troubleshooting. Once you have eliminated all of the causes, check again because you may have missed something.
When I was in Technical Support in Michgan Bell, we got a series of Sunday morning crosstalk complaints from a neighborhood surrounding a particular church. Turned out the services were broadcast over a local radio station, they had leased a dry pair from Bell to get the audio. Investigation showed they had connected the pair to the 70 volt line terminals of the church's PA system. When the Rev got to the Turn or Burn part, everybody on the phone within a half mile got the message.
I guess you could call this a case of elevated noise. I love it when the Sherlock Ohms mysteries end up being some oddball element outside the immediate area of the problem.
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