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Re: [ARSCLIST] RF problems - used to be - - Effect of vibrations on audio tape

From: Patent Tactics, George Brock-Nannestad

David and interfering persons,

----- I have a few comments of a technical nature. I would like listen to the 
audio band footprint of your noise.

Jim Lindner wrote:

Radar is WAY up there with very short wavelengths and harmonics go up not
down. The inverse square law applies so the power falls off very quickly
relative to the distance from the source and you are modulating through the
air if it is the kind of RF that you are suspecting. 

----- the inverse square law only applies if you radiate 360 deg. A radar is 
by its design a beam, and here the rule does not apply (you would not be able 
to bounce a radar beam off the moon, for instance, if the losses were that 
high). But measure the "sweep" frequency of the disturbance, and then go look 
at airports. Before you are picked up by the police you may have been able to 
count the sweep frequency of the radar installations you observe.

----- several spoke about mu-metal. It is a good material, but extremely 
difficult to use in practice, because it does not tolerate any mechanical 
stresses. It has to be shaped to the final form and then annealed, or its 
good properties will fall by 60dB at least

----- another source of cyclic noise that I have experienced is similar to a 
radar, because it is also microwaves, and that is indeed a household 
microwave oven. When used at a low setting, it is actually just applying full 
power at a suitable duty cycle. All high frequency (HF) energy can create 
problems in audio circuits, because any transistor pn junction, and some 
soldered connections will act as rectifiers and push the modulation frequency 
of the HF into the audio circuit. The hum bucking coil found a great and 
logical use in pickups for electrical guitars. The functioning is that one 
coil receives string information plus hum, the other only hum, and the second 
coil is wired in counterphase so that the two hums cancel. I have a Source 
Engineering black record preamp, and it had some hum. I was able to reduce 
that hum by taking one of the internal audio leads just on the right path 
across the small mains transformer, injecting inductive hum in counterphase, 
so to speak.

Ho, hum,


> Wow. Thanks to everybody (Ron, Jim, Jerry, Tom, Mike...) who gave 
> suggestions about our RF problem. I should have known there were experts 
> in such things on this list.
> All equipment in the studio is on balanced AC from an isolation 
> transformer, so I assume we have clean power. Don't know anything about 
> building ground. Once we put in the isolation transformer (which didn't 
> help this but reduced the noise floor in general) I assumed that it was 
> induced.
> It's a very strange interference that fades in, "wanders" (I can't think 
> of how else to describe it) in frequency for a few seconds and then 
> fades away again. I'm not good at guessing frequencies, but it's 
> somewhere around the "bumble bee" frequency range. I really only notice 
> it if there is no signal and the speakers are fairly high. Because it 
> was cyclical we thought it was the sweep of the radar. But from what Ron 
> said, there are clearly many other possibilities. I could record a bit 
> of it next week and post it online if anybody cares to make a diagnosis.
> Anybody do consulting and need a vacation in Santa Barbara this winter? 
> I've got a spare bedroom.
> Lots and lots of leads to check out now...
> David Seubert

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