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RFI and Faraday cages

GOYA

GOYA

Super Member
Does anyone know the best way to block CB interference in audio equipment? For me, it used to be a minor problem but has recently become a big one. Maybe "good buddy" has recently got himself a more powerful base. I thought of building a big Faraday cage but I'm not sure what size mesh I would need.

The interference is strange. It is very minor in my mid and tweeter amps but is now very strong in my woofer amp. I tried different speaker wire lengths and different amps. No change.

When I tried an amp with meters, even with the amp turned off, the RFI would make the meters jump.
 
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HI,
Your proposed Faraday cage holes only need be smaller than the wavelength you are trying to block. You're in luck! CB wavelengths are in the 11m band, and I believe that chicken wire with holes that big are not available.:D

Ever wonder about the strange hole pattern in a microwave oven door?:scratch2: You guessed it- it is a Faraday cage for the wavelengths associated with microwave radiation and protects the user from that radiation.

I do think that you'd need something thicker than chicken wire though. And don't forget to ground it.

Good luck!
 
I would suggest winding appropriate chokes on the input/output cables before going the route of a Faraday Cage. (see pic) I believe you will want chokes with type 31 mix ferrite. Some ferrite mixes are more effective at say, 10MHz and others at 300MHz. You need something with very high impedance in the 27MHz range.

Check out www.radioworks.com and see page 73 of the current catalog. (no affiliation)

picture.php
 
I should add that I am a ham radio operator and had my fair share of solving RFI issues in my own home as well as neighbours. It can be a process, but it usually can be done.

If the source of the RFI is "clean" it makes it easier to solve. If your neighbour is using a "dirty" CB amplifier, it might be more difficult.
 
DC said:
I would suggest winding appropriate chokes on the input/output cables before going the route of a Faraday Cage. (see pic) I believe you will want chokes with type 31 mix ferrite. Some ferrite mixes are more effective at say, 10MHz and others at 300MHz. You need something with very high impedance in the 27MHz range.

Check out www.radioworks.com and see page 73 of the current catalog. (no affiliation)

picture.php
Click to expand...

I was just thinking of trying some DYI ferrite chokes. Thanks. :-)

Edit: I just saw that Radio Shack sells some cheap chokes and I may try them first.
 
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Based on my experience, and in an effort to be systematic, I would start at the speakers (speaker wires at the actual speakers) and work toward the amplifier from there. However, if you are able to determine that the interference is worse with a particular component (or is non-existent with another), that will indicate the path your should take.

Good Luck! It can be maddening ...
 
And while I am thinking about it, you can build a very simple Faraday Cage for the component in question simply by wrapping it in aluminum foil and attaching a ground wire to the foil. Crude, yes, but quickly indicative of whether or not that solution will be effective. :)
 
It would help to know how the RF is entering. Up the power cord? Through the signal cable? Through the speaker wires?

I think you've determined that the speaker wire had no effect. Most power amplifiers (except maybe Spectral) use an output choke and high frequency load, just so speakers wires, acting as antennas don't feed the signal back into the amplifier. Ferrites on the AC cord may help. A small cap on the input jack's hot connection to ground might help?
 
I'd suggest you use Type 43 ferrite chokes. The good news is that these are quite available. Here's one spot: http://www.allelectronics.com/make-a-store/item/FB-80/FERRITE-SNAP-BEAD/1.html

Google "Type 43 ferrite" for more hits.

I, too, am a ham and have dealt with lots of RFI. The biggest problem with farraday cages is they're expensive and difficult to install. If you have mis-diagnosed the problem, then they'll be uber-expensive! Best to invest in the chokes where possible.

Now, if the guy is running his amp hard - very likely - or using lots of compression - again, very likely - and does not have solid antenna grounding - very very likely, then no amount of chokes on your stuff will help.

Cheers,

David
 
Easy way to solve it! Ferrite chokes on all audio cables. And if very prevalent, a slingshot into the miscreant's antenna works miracles. One big issue is that most of these illegal CB Radios emit more harmonics than the Chicago Symphony under Fritz Reiner (Big Grins)
 
Well, I still haven't fixed it but I've pinned down where it's entering my system. It's my crossover. With all the patch cords that lead to my power amps unplugged, there's no RFI over my speakers. With my preamp not plugged into the crossover, there IS RFI. So it has to start in the Rane AC23.
 
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Yep. I made them myself. Microphone cable with Switchcraft plugs and the shield is only attached on the source end.

I was in the midst of trying a grounded foil cage when CB guy stopped transmitting.
 
You might want to drop the FCC a line, they don't take kindly to illegally overpowered transmitters. :D
 
I see the Rane is balanced, are your other components also balanced? Might want to change your pinout on your TRS connectors.

(http://www.rane.com/pdf/old/ac23man.pdf)

Might want to make a wire that fits into the 3'rd (GROUND) opening in a standard wall outlet with a test clip on the other end.

When the guy's transmitting, clip this wire to the Rane chassis.

Since you made your own connectors, you know what part of the wall outlet is the ground.

-Edit- I see your other equipment is unbalanced, I'd experiment with your TRS pinout, it's likely that ring and shield are tied together now.

http://www.m-audio.com/products/en_us/Delta1010LT.html
 
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Sam Cogley said:
You might want to drop the FCC a line, they don't take kindly to illegally overpowered transmitters. :D
Click to expand...

I had this problem about 15 years ago at a different location. I complained to the FCC. They sent me a brochure on how to use shielded cables.
 
CruzBay said:
I see the Rane is balanced, are your other components also balanced? Might want to change your pinout on your TRS connectors.

(http://www.rane.com/pdf/old/ac23man.pdf)

Might want to make a wire that fits into the 3'rd (GROUND) opening in a standard wall outlet with a test clip on the other end.

When the guy's transmitting, clip this wire to the Rane chassis.

Since you made your own connectors, you know what part of the wall outlet is the ground.

-Edit- I see your other equipment is unbalanced, I'd experiment with your TRS pinout, it's likely that ring and shield are tied together now.

http://www.m-audio.com/products/en_us/Delta1010LT.html
Click to expand...

This is what doesn't make sense. The Rane chassis is already supposed to be earth grounded. I've confirmed good ground at the outlet. I guess I should pop the lid off and check the connection.

You are correct that my equipment is not balanced. I'm not using TRS plugs. TS makes good connection.

Need to do some more trial and error. Waiting for him to broadcast.
 
I am concerned about your comment that you could see the power meter needles move on your amp with the power off. I would suggest that there could be still some charge on the power supply caps in the amp. Watch the meters for awhile with the amp off and him transmitting and see if they stop. If not, he is putting out one heck of a field and I would worry about your health more than the noise in the system.
 
The allowable max wattage for CB is 4 watts and any linear amp is illegal per FCC part 95 subpart D. You have a legit claim to the FCC if you care to navigate the red tape. The CB op could be causing interference to others as well.
 
dr*audio said:
I am concerned about your comment that you could see the power meter needles move on your amp with the power off. I would suggest that there could be still some charge on the power supply caps in the amp. Watch the meters for awhile with the amp off and him transmitting and see if they stop. If not, he is putting out one heck of a field and I would worry about your health more than the noise in the system.
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Exactly what I was thinking. Bouncing meters in an amp withOUT power being supplied, through airborne waves, takes a lot of energy -enough to increase cancer risks, etc...!

Try draining off those power-supply caps to be sure (they should naturally "bleed out" fairly quickly, anyway, in most amps), and see if the meters still bounce when he broadcasts. If so, he has some dangerously high power levels going there, definitely illegal.
 
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