Help! Need to build a 60hz shield

I hope someone here might know something about building a 60hz electromagnetic shielded cage for a electronic device.

The shielded cage is to prevent electromagnetic interference with a critical medical device and 60hz is the magic number I need to shield. I have found info for higher frequencies like cell phones but nothing at the 60hz level. The size would be approximately the size of a VCR.

Anyone here have any experience in this area or steer me in some viable direction?

Since you are designing a medical device there will be a very specific procedure for design approval. Be sure you understand the EMC (Electromagnetic Compatibility) certification and testing requirements. For military spec products it's MIL STD 461, most likely there is one similar for medical electronics. There are two components to any electromagnetic wave, an electric component and a magnetic component. At low frequencies the shielding requirement is usually a magnetic problem. Shielding is designed using high permeability metals. One product is mu-metal. See:

http://en.wikipedia.org/wiki/Mu-metal

Frequently the electronics is housed in a mu-metal box. The magnetic component is effectively shunted through the mu-metal.

Be sure you understand the physics of an electromagnetic wave and how different techniques are needed for shielding the magnetic and electric components of the wave. Also understand the difference between conducted and radiated interference. As Apple has found out with it's phone antenna problem, it's not a simple design problem. Sometimes is just more cost effective to hire an expert consultant.

[http://en.wikipedia.org/wiki/Faraday_cage]

A Faraday cage will likely not be the solution. Faraday cages only are effective at frequencies many, many decades greater than 60 Hz. In addition it only shields the electric field portion of the wave. At low frequencies it's almost always the magnetic field portion that causing a problem. You want a mu metal shield, like what was on some vacuum tube TV set deflection yoke coils. It shuts the magnetic field around the circuit to be protected.

Is this a factory made unit or are you designing something?

Hi guys, thanks for your insight. This is not a medical device that I am developing but instead building a shield to prevent or very significantly reduce both electric and magnetic field interference at the 60hz frequency.

Consider for example a cardiac pacemaker. It needs to be shielded from magnetic interference to prevent adverse effects to the patient. Unknown to most patients is that they also need to be shielded from electric fields generated by small electric devices. In my research, a digital cable box is adversely affecting a pacemaker patient. When the box is disconnected there are no problems suffered by the patient. Reenergize the box and the symptoms return. The wattage consumed by the box is small (<3 watts) but the electric field has great effect on the pacemaker performance up to about 25 to 30 feet. This has been a perplexing problem. Household walls are helpful in reducing the field somewhat but not enough.

Cable companies have begun requiring digital cable boxes and shut down analog services (satellite may have always had this problem). That is why this problem now arises. It's unfortunate, but I believe that there are a great many pacemaker patients who are unknowingly affected by this change and problem. The pacemakers are designed go into a default mode when it encounters unusual problems. Once a patient leaves the offending electric field, operation returns to normal. One would assume that the medical community would be aware of this but they aren't.

The obivious answer is to not watch TV, but a better solution is to build an effective shield and hence my purplexing problem. The local "nationally large" cable company is clueless and has ended analog cable to this person and has left them "behind" with the digital cable change. Consequently this person cannot enjoy a technology most everyone has access to.

Any additional advice would be helpful.

The interference may come from the power cord on the cable box. Sometimes this can be cured with a choke.

A choke is a ferrite bead on the power line near the cable box or the electric cord might be wrapped onto a ferrite ring.

You may have seen chokes on computer monitor cables.

It might be possible that a theoretically perfect shield may be made for the cable box and interference may still escape because of the power cord must exit the shield.

The interference may not be 60hz, but some other frequency used by the cable box.

A frequency counter or oscilloscope might be used with a resonant antenna of suspected interfering frequency for detection.

There might be data available showing what interference is made by looking up the FCC id number of a device like a cable box.

A spectrum analyzer is what you need to see what the box is giving off. Digital cable box is probably made by Scientific Atlanta or Motorola. If the digital cable box is actually causing a problem, contact the FCC with all the information and get a doctor to witness the effect, and then contact the cable company as they may have a defective box or unknown problem on their hands. Try going to satellite tv, different box by different maker. If you are just trying to invent something, good luck.

Not to digress from but I called the FCC regarding the device (which is by Pace America's in Taiwan and also had a lawsuit brought to the FCC). I don't think the lawsuit is going anywhere especially after I called the FCC hot line (1800callfcc) and was transferred after selecting option 1, to a voicemail stating "Thank you for calling the FCC...Goodbye!" Really instills confidence in Federal agencies. I will be resorting to snail mail but have little confidence in this as well.

The choke is a good lead and I try that option. I've tried other digital boxes that are offered and they are different designs and manufacturers but they all initiate the same problem. I have a strong suspecion that the pacemaker is acting as a small loop antenna similar to pager device and or electromagnetic induction. Using an on line calculator for antenna design at 60hz and using some assumptions I was able to obtain an indicated design 43 volts that can be transferred to the pacemaker which would somewhat validate the pacemaker going into default mode and resulting in physical weakness. That is a scary thought.

The reason I'm doing this research is that a good Neurologist friend indicated to me that Medical research is more focused on biochemical and metabolic rather then bioelectrical research. So essentially on on my own here, cracking the physics and medical research documents to try pull together some type of solution. As a non electrical engineer it gets pretty ugly tying all of these concepts together. The only confidence my Neurology friend gave me after reviewing my preliminary research is that "I'm on to something here."

I thought I'd find some good intellect here and you haven't let me down, so any additional thoughts are appreciated.

Take a look at the software products sold by Ansoft see:

http://www.ansoft.com/

What you need for an initial analysis is a field solver program. You need to solve Maxwell's equations with boundry conditions that apply to your hardware. This is very difficult to do since Maxwell's equations are four sets of partial differential equations.

See: http://en.wikipedia.org/wiki/Maxwell%27s_equations

The Ansoft product that solves these equations is "Maxwell". See:

http://www.ansoft.com/products/em/maxwell/

It will be expensive, have a steep learning curve but it does do the job. When I was still working an earlier release of "Maxwell" was used in conjunction with a lot of custom software to design aircraft phased array radar antennas.

I wish you a lot of success in your efforts, but it will take a lot of hard work in understanding the problem, even more finding a solution and it will all be very expensive.

Yikes, Yanche!

I have nine months in to this already, what's another 36 months to go through Maxwell's equations. Looks like I have a three pronged path: try to review, understand and apply Maxwell; attempt blind experimentation; and create a thesis paper and seek a engineering graduate program willing to investigate the phonemomena.

I thank you very much for your help. If you have any additional thoughts please let me know. If I get more traction in this I'll certainly post it here.

Snuffy

P.S. Got our four tons of Superior rice today. Six of nut comes next week. Thank God for gravity to the storage site. From there its all up hill hauling till next May

snuffy wrote:In my research, a digital cable box is adversely affecting a pacemaker patient.

Have you simply replaced this converter box? I work in a large cable system here in Utah and have never heard of this problem. We have many many thousands of converter boxes (including Pace models) in-service at this time and even though we have gone through analog/digital conversions recently we've had huge numbers of converters in the system for decades.. Not saying it's not possible but just never heard of anything like this in my 24 years experience in this industry.
If you haven't replaced this converter box, I would do that before doing anything else as it could simply be a bad one. All these converter box are designed in accordance with FCC guidelines.

Rockwood,

I've tried every one of the digital converters offered by the local cable company over the last three years. Disconnect the power source and the difficulties subside. Repower and same symptoms with the converters. Even went so far as to disconnect power to the rooms located in that wing and run an extension cord to the box. Same set of symptoms.

The proprietary design dosen't permit a peek inside the box to understand its design (I'm not about to pay the penalities for doing so). So far I have been able to determine it to be an electromagnetic design problem simply because I can separate a magnetic field symptom from the electric field symptom simply by repositioning for a perpendicular position. Recall, in physics, that the electric flux (or field) is perpendicular to the magnetic flux (or field). I have a strong suspecion that the proprietary descrambling technology is set to some Hertz level below 50MHz and most likely closer to the 50-60 Hz frequency to deter piracy.

Ironically, the cable firm is completely unhelpful which surprising given it's top market position and access to top notch researchers.

What really steams me, is in this technological age we shouldn't be leaving people behind from using such technology.

snuffy wrote:attempt blind experimentation;

Having little knowledge on the subject I would go with blind experimentation. I'd build a copper box to put the offending box into. If it needs cooling, a double box, vent holes on opposite sides and a fan to move air. I don't know squat about how to stop interference, but I do know that the copper box trick is basically how the government keeps computers and monitors (At least the old style monitors) from "radiating information".

Freddy,

Thanks for that thought but I already tried that trick. I was able to secure a metal decoder box which had two steel shells, left most of the electronic guts including a large old transformer as well as grounding all of it. Even placed it in the basement. SOB electric field blasted right through the floor. Guess that's why 60 Hz has a wave length of better than 800 miles (in fact I think it's 3,100 miles) even at five watts AC. I may next have to try to define a polarized shield (whatever that might entail).