RF problem

D

dccate

New member
I am in a basement underground near a radio station. (quarter of a mile away)
I have a focusrite pro 26io. If I have a 100 foot snake attached to the xlr input, I suddenly get the local radio station loud and clear. I have tried 2 different snakes, several different channels per snake. If I run a 50' mic cable, daisy chained to another 50 foot mic cable I still get interference, just not as bad (still way past acceptable). Since my signal is balanced I am not sure how I am getting so much RF. I have tried a different interface, a different amp, and over 6 different cables. Any ideas?
 
My first guess would be that the snakes and/or cables are miswired. It's common for cables to be wired with the metal jackets ungrounded under the assumption that they might be used to connect between pieces of gear with different ground potential (though this really doesn't solve that problem). The problem is that in the presence of large amounts of RF, ungrounded metal jackets basically turn into antennas....

Add a jumper wire in all your cables from the ground pin to the ground lug for the shield and make sure all the set screws are tightened thoroughly. You might also need to ground the box at the end of your snake, but if you ground the jackets on both ends of all your cables, that will probably end up grounding the box without further intervention.
 
I made my studio mic cables with Canare cable, and used a diagram that was in the Markertek catalog on how to wire them. It was a method called StarQuad wiring that offers additional protection against RF by using a twisted pair of signal wires instead of a single wire.
The wire looks like this:
l4e6s-1.jpg

You twist the blue wires together and used that for one connection, and the same for the white. Apparently it makes for much better RF rejection. I read once that a twisted pair of wires, a ground and a signal, was almost as good as shielding with a ground.

Another thing to make sure is that you have a really good ground. I pounded a 6' brass rod into the ground, and then ran a bare 1/4" diameter bare copper wire from that to my fuse box.

Just thinking out loud but maybe the RF is coming in through something that's powered that the snake is plugged into and that some sort of ac line conditioner might help. I have this Tripp Lite box that's supposed to help that. And sometimes lifting the ground shield on one end of audio cables can help things too, but that's primarily for ground loops that cause hum.

good luck
 
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Is it really nessassary to have that long of cables? Use the shortest spans of cable possible. Is your house old enough to have any aluminum wireing? Aluminum wire is an open invitation to radio interference. Think about geology for a moment, if your house is located above an iron deposit you are providing the top of a big undergroung antena. This can be especially bad if the radio station's broadcast antena is located over the same deposit. If all else fails you could shield the entire room with grounded copper mesh, but that will cost a small fortune.
 
Maybe a dumb question, but are you sure your picking it up from the mic cable. Could it be the cables between your computer and your monitors contributing? Just something else to explore... Good luck :). That sounds frustrating :mad:.
 
With more work I have learned a few things. With the exact same set up, I ran an extension cable to my neighbors building and plugged in. Studio still in the basement, same snake, etc. and the problem went away. I have tried every outlet in the building (which is brand new). On the top two floors the Radio Interference is no where near as bad, but still not workable. It seems like that rules out the snake, is there something I could run by power through (I have wrapped audio and power cables around several Ferrite Chokes and that seems to make 0% difference) Thanks!
 
Given that a different power source eliminated the problem, I agree with dgatwood that you probably have a ground problem.

I'm curious, is it an AM radio station? AM towers, themselves, are the annteneas. They are usually some fraction of the wavelength, so 100' is getting into that range.

Seeing as how your in the basement, I wonder if it also points to (if it's AM radio) a bad ground screen at the tower. I believe the ground screen affects the impedance of the tower and also the shape of the radiation pattern. I think among other things that a good ground screen helps reduce energy directed underground, by squashing the radiation to something more horizontal (like a pancake as opposed to a ball - additional towers can further change the horizontal shape).

Or if it's FM, I guess it can make it into a basement with relative ease.
 
Just for kicks you could plug into one of those 2 prong adapters that has a ground terminal to run a wire to a cold water pipe... not great but in a basement you might not have many options.

or maybe one of those line conditioners like Furman makes.
 
It's also remotely possible that the higher resistance to ground caused by such a long extension cord caused some noise in some device somewhere to flow a different way instead of flowing through the shield in some cable. Hard to say. These sorts of things can be a royal pain to debug. Does this happen if you have nothing hooked up but the computer, the interface, the mic through the snake, and a pair of headphones to monitor it? (Disconnect speakers and any other devices that are plugged into an outlet.)
 
Faraday Shield

Dani Pace said:
Is it really nessassary to have that long of cables? Use the shortest spans of cable possible. Is your house old enough to have any aluminum wireing? Aluminum wire is an open invitation to radio interference. Think about geology for a moment, if your house is located above an iron deposit you are providing the top of a big undergroung antena. This can be especially bad if the radio station's broadcast antena is located over the same deposit. If all else fails you could shield the entire room with grounded copper mesh, but that will cost a small fortune.
Click to expand...

My studio is not far from a cell phone tower, I have been shielding my building a little at a time over the last 10 years. My control room has heavy duty aluminum foil under the wall sheathing. Just recently I needed a new roof so I installed 3' wide aluminum flashing under the shingles. I also replaced 2 walls so I again covered the walls with the same flashing. All of this Aluminum is bonded to an isolated grounding electrode system (many ground rods) This is seperate from the Electric Companies ground. It is great no more annoying cell phones ringing all the time!
VP
 
Thanks for the help and sorry for the delay. I built a wooden box around my snake and that seemed to take care of the RF issues. With a little more investigation though, it turns out while everything was wired correctly, the grounding post was in too much rock and not really grounding.

Thanks!
 
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