X ft pipe resonance and consequences

rnsc

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I chose this forum because of an excellent thread titled "Antenna Length & Resonance" that seems to be highly related, though I cannot apply what I read there. I know the forum name does not quite match, please redirect me if appropriate.

I need to run audio cables through a set of 1-1/4" electrical conduits (EMT) under a stage for physical protection. The conduits will be under a stage about 200 ft from the base of a 3KW 150ft FM broadcast tower (I assume that the frequency is just another parameter and so not important to the discussion). I am concerned that certain lengths will resonate and that this might be a problem either for the radio station or for the audio cables or for someone touching the conduit! I also expect that this will be an improved situation with respect to shielding, though right now they pick up three stations other than the one being broadcast from this tower.

Question 1: Will this happen (I think yes)? What specifically will happen... will there be a stationary voltage profile along it, or what? How can I calculate the voltage?

I am thinking that it might be good to ground it at the transmitter for safety reasons, through a ferrite toroid that is lossy at the transmitter frequency to discourage interaction with the transmitter or the transmission pattern or it does not result in transmission frequency current to ground while not building up for example a static charge as a floating metal object. I realize that the wire from the conduit to the transmitter will not be an RF ground because of its inductance and huge length, but the primary purpose is to suppress static charge while not becoming a counterpoise of sorts with unknown consequences.

Question 2: Please comment on the conduit floating vs. grounded subject.

Question 3: How can I calculate the frequencies at which it will resonate, and the frequencies that are maximally not resonate? How precise do I need to be? How do I convert this to lengths? How can I measure what is going on so that I can confirm that the lengths that I choose are doing what I want (Namely nothing!)?

Thank you for helping me to understand!
--Ray
 

prcguy

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Generally speaking, when you run cables into a metallic conduit it will choke an amount of RF from the cables no matter what the conduit length. A resonant 1/4 wavelength or odd multiple length might give more RF choking compared to a resonant 1/2 wavelength or even multiple but grounding the conduit and/or its mounting hardware, lengths of attached Unistrut, etc, will change the calculated resonant length of the conduit. Either way the conduit should be grounded per NEC for human safety.

However I think it would be better to treat the audio cables with ferrite as its a more predictable way to remove common mode RF currents off the cable. For the FM broadcast band 88-108MHz I would use three snap on ferrites of a #43 mix or Laird 28 mix at both the output of the audio device and the input of the next audio device. If the bundle of audio cable is small then I would pass all cables through the two or three snap on ferrites with large center hole at each end rather than a smaller set of ferrites on each individual cable.

If the audio cables are within close proximity to the antenna and RF is induced onto the cables there will be voltage peaks and nulls along the cable at 1/2 half wavelength increments along the cable or about every 4 to 5ft and I don't think the locations of the peaks and nulls can be calculated and must be measured. I also don't think that matters much as the ferrite treatment should reduce RF currents on the cables by a good 20dB or 100X.

If AC or ground common mode currents are a problem on shielded balanced cables you can lift the shield at the destination side which will keep most of the shielding properties but sever the ground currents flowing on the shield which can sometimes induce hum. Or use a good 1:1 audio transformer at the destination side to break up grounds in the shield and isolate the equipment.
 
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rnsc

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prcguy, thank you for your good input. I am pulled away from this and will return in a few days.
 

rnsc

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I apologize for the long delay. I have not learned anything further, and driven by "the show must go on" principle metallic conduit has been installed. I don't know how long threads are left open, but if possible, I will capture any news. (This is not my project, rather one that I was asked to comment on)
 

prcguy

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Ground both ends of the conduit.
Since this is an FM broadcast thing a 1/4 wave resonant conduit would be a manageable length and you would want to only ground the equipment side to make it a "bazooka balun". For that to be effective all the cables inside would have to be floating down the center of the conduit and not touching its inside surface. You can make insulated doughnuts out of thick hard foam shipping material that cuts easily with a knife. I still think ferrite is the best solution here.
 

rnsc

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merlin, Re "Ground both ends of the conduit": Any ground wire would be multiple wavelengths and so would be anything but a ground, right?

BinaryMode, Re "Don't they make...fiber?" Probably, but pre-existing equipment uses XLR connectors.

FKimble, Re "PVC ?" Yes, for mechanical support. If metalic conduit is causes more trouble than good or is not helpful, and either ferrites or nothing is the right answer, PVC for mechanical support would be invisible to the magnetic fields and harmless to the electric.

PRC Guy, Re "Since this is..." I will have to read about "Bazooka balun". Getting limp mic cables to stay down the center would be very difficult at best - these are many dozens of feet long. I just read a little. Very interesting, needs more study. I need to understand what happens if the cable is not supported. I am thinking that the propagation velocity would be different, de-tuning it, but I don't want to give up that easily. By now it may be done and I can find out empirically what is happening. Thanks for the heads up, it sounds interesting.
 

BinaryMode

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I have an M-Track Solo which takes XLR to USB. Surely the same may be made for fiber. Though the cost could be higher, though technically it shouldn't.
 

rnsc

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Thanks for the info. I will forward it to the sound guy, which is doing the project. Interest will be commensurate with the size of the problem when it is fired up!
 

prcguy

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Ferrite should fix the problem but I would also be concerned if there is any consumer grade equipment in the chain with a plastic case where the FM broadcast RF can blow right through the case into the circuits. Is the FM broadcast antenna up 100ft or more on a tower or is it lower?
 

rnsc

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Near top of 140 foot tower, let's guess 140 feet. The transmitter sits at one end of the stage but about 10 ft higher than the stage floor. The audio equipment is at stage level. The transmitter has a 2" wide perhaps 1/32" thick copper strap going down into the ground which is roughly at stage level. It drives a 2" diameter hard-line that goes up to the second floor ceiling and travels horizontally from the transmitter about 150 feet at the second floor ceiling level and out a wall and up the tower 120 feet to the antenna. The conduits run around the width of the stage to various XLR jacks in the walls, front of the stage, and floor, varying in length from 10 to 50-70 ft (Guess). So the conduit and audio equipment would be about 200 ft diagonally down at a 45 degree angle from the antenna. Thanks.
 

prcguy

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Is the transmitter 3kW feeding the antenna? What type antenna, single bay or multiple? Or is the station 3kW ERP including antenna?

Either way its not a lot of power and I would not expect any major RFI problems from the description. Its always important to use good RF/grounding practices and run low level audio cables in metallic conduit or transition to fiber for long runs between studio and transmitter if that's what's going on here. If this was a 30kW transmitter it would be a different story.
 
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