Metal RF shielding for plastic cases

[N_Jay]

4) the part you were sheilding was a powersupply transformer. A device, that by its very nature is a source of MAGNETIC fields, and typically not a source of EMF.

I'm trying to figure out how a transformer could radiate any EMF more than the wires connected to it. Maybe it's been too long since school, but I'm at a loss.

Its not a "perfect" device.

I would bet it can radiate a little bit, but nowhere near the magnetic field that comes off if it.

Maybe he has never heard a transformer hum due to the magnetic field?

 

[kb2vxa]

Hi guys,

Let's quit the guesswork and get down to brass tax fax. If a receiver needs shielding it's already there, keep those little fingers out of the box. A metal case isn't for shielding RF, it's for shielding impact and cheaper than those high impact plastics commercial speakers and microphones are made of.

If you're plagued with RFI most likely it's coming in through the antenna. The AC mains supply usually won't pass RF, check it out by unplugging the antenna and most likely it'll disappear along with the wanted signals unless it and thay are extremely strong. Still it's not likely to come from the mains, it's being picked up along with those strong signals by the front end wiring, particularly by that short jumper between the antenna jack and the PC board and it's foil track on the board itself.

The only real cure is track down the source and eliminate it if possible, that means eliminate the offending device. Modifying, shielding and filtering on either end is difficult to impossible, especially when the interference comes from outside the home. Yeah, your neighbor will tell you where to go and just how to get there. Then it becomes an FCC matter, another can of worms but sometimes it works and sometimes he'll ignore the FCC and unless you press the matter "the buck stops here", his waste basket.

60Hz radiation? This guy must have seen it on Fox. (;->)

 

[John_M]

Hi guys,

Let's quit the guesswork and get down to brass tax fax. If a receiver needs shielding it's already there, keep those little fingers out of the box. A metal case isn't for shielding RF, it's for shielding impact and cheaper than those high impact plastics commercial speakers and microphones are made of.

If you're plagued with RFI most likely it's coming in through the antenna. The AC mains supply usually won't pass RF, check it out by unplugging the antenna and most likely it'll disappear along with the wanted signals unless it and thay are extremely strong. Still it's not likely to come from the mains, it's being picked up along with those strong signals by the front end wiring, particularly by that short jumper between the antenna jack and the PC board and it's foil track on the board itself.


The only real cure is track down the source and eliminate it if possible, that means eliminate the offending device. Modifying, shielding and filtering on either end is difficult to impossible, especially when the interference comes from outside the home. Yeah, your neighbor will tell you where to go and just how to get there. Then it becomes an FCC matter, another can of worms but sometimes it works and sometimes he'll ignore the FCC and unless you press the matter "the buck stops here", his waste basket.

60Hz radiation? This guy must have seen it on Fox. (;->)

This guy? The name is John. Fox, I hardly ever watch TV. :lol:

 

[John_M]

Okay, the 60hz radiation from the power supply that we used on our project certainly was an issue that we had to deal with. We used
steel sheet metal to stop it. Now whether you want to beleive me or not is
up to you. Would this be a likely source for inference for a scanner
no. I simply brought it up because the discussion was interference.


That's it for me on this thread :lol:

 

[Al42]

Maybe he has never heard a transformer hum due to the magnetic field?

It sounds as if he's, at most, a technician, so he probably has no idea of any of the theory, emf or magnetic. He saw something that was getting 60 Hz interference, assumed it was em interference, and that settles it - in his mind.

Like the CBers whose explanation for not coiling up coax was that the electrons would get lost and jump turns instead of staying in the wire. The effect is real (a 12" diameter coil of 100 feet of RG-58U is pretty lossy at 27 MHz - he did see something being interfered with), but the explanation, in both cases, leaves much to be desired.

BTW, if I'm guessing correctly, the device was a bubble chamber, which is extremely sensitive to magnetic interference, and would have been extensively shielded.

But we'll never know - he said that he won't participate in this thread any longer.

 

[John_M]

.
If I am guessing "assuming" it was a bubble chamber. No sir it was not, it was a Bubble Dectector to detect bubbles in emulsion
lines. The other engineer that helped us with the project had 25 years in
He knew what the problem was right off 60HZ raditon 60hz all over the signals of board B which I already desrcibed. He was just assuming. I worked at Kodak and spent a year there before they even hired me as an Engineer. I have a whole book case full of textbooks and other
engineering books so I can look up any theory I want to that I don't remember. I along with 20,000 other people no longer work at kodak do to Digital Photography.Everything is going digital there now. I spent 2 years in production at another company testing circuit boards. I wanted to get into R&D so I then started at Kodak as a contracter. I spent 12 years there in R&D.

They sent me to Los Angelas to learn about a new developement tool call VEE put out by HP (Who makes alot of good test equipment for all facits of engineering.) Three months later Kodak layed off 15% of all R&D units. I was lowest on the totem pole so I was "Tapped" a term they use for saying bye bye to people. I along with another person in my unit. Since then about 20,000 people have been layed off there. I was upset when I first was layoffed. Now I consider it a blessing. Like I have said before in another post, I have no desire what so ever to reenter the field.

Try jungleing up to 10 projects at a time. Then you have to keep track off all the time spent on each project and charge your time to the different projects that you worked on for the week. Time put in by computer. No, you cant just charge your 40 hours to
an account like the supervisors can.

Al42 if you are going to call somebody something then say
it. Don't use assuming to get your opinion across.

BSEET 1989.

I have stated before that I will no longer continue to post on this thread. After a few days I thought that the childish discussion would
abate some. Appartently I was wrong to assume this. "Have fun guys"