Filtering Magnetic Fields: A Discussion with Electrical Engineer Al Hislop

Filtering Magnetic Fields

A Discussion with Electrical Engineer Al Hislop

Recording artists know that electrical “noise” can disturb their sound quality. Geologists and other scientists whose work requires sensitive meters have noticed that nearby elevators, computers and solar power inverters can render their electrical equipment dysfunctional.
In 1934, when the U.S. created the Federal Communications Commission, the agency began encouraging the public to invent and market electronics–as long as they did not create “harmful interference.” They defined harmful interference as anything that interferes with existing radio, TV (and, now) Internet broadcasts.

The FCC’s definition has never included biological harm.
Today, many scientists, physicians and laypeople recognize that long and short-term exposure to electromagnetic radiation (EMR) emitted by electronics can harm human health and wildlife. (For more info about the biological effects of EMR exposure, visit, and; read An Electronic Silent Spring by Katie Singer.)
To protect ourselves from EMR emitted by electronic devices, first steps would include turning off the offender; correcting wiring and grounding errors, and not using a new technology until it’s proven harmless.
Next steps start with measuring for EMR (magnetic fields and radiofrequency fields) in your home, school or workplace.
Thirdly come filtering and shielding. To clarify, we filter lower frequency field currents that emit magnetic fields. We shield from radiofrequency fields.
To measure, filter and shield EMR, you need helpers who know how to use meters, how to install filters and shielding safely and properly. You need electricians and plumbers–and homeowners with open and experimental minds!
Here, electrical engineer Al Hislop describes how filters work and how they can increase magnetic fields. Al designed and built parts for the WMAP spacedraft, the first spacecraft to make precise measurements of cosmic background radiation. He also designed and built parts for the Rosetta spacecraft, currently orbiting comet 67P/Churyumov-Gerasimenko. The documentary “Broadcast Blues” shows Al using a meter to demonstrate that antennas broadcasting radio and television from Lookout Mountain near Golden, Colorado emit radiation beyond what FCC guidelines allow.
Al Hislop Before we discuss filtering, let’s review briefly the difference between Radio Frequency (RF) and the fields associated with “dirty power” that we may want to filter. Most RF fields are deliberately produced in order to transfer information from one place to another without the use of wires. Examples of these fields are AM and FM radio, cordless telephones, television broadcasts, cell phone signals and microwave transmissions. In all of these cases, the transmitted power is intended to “detach” from the transmitting antenna and radiate away. If one wishes to avoid these fields, one must either be far from the transmitting antenna, or one must employ “shielding” to either absorb or reflect the radiated signals from wherever you are.
Moving charges produce magnetic fields. For the purposes of this discussion, the moving charges we will consider are the electrons moving in the wires of your house wiring. The moving electrons are called the current. If no current is flowing in the wires (no electrons moving), then there is no magnetic field caused by the wires, even though there may be voltage on the wires
We have electrical wiring in our homes because we want light and energy in many places in our homes. When we turn on a light or use an electrical appliance, currents flow in the wiring, and there is the possibility to create magnetic fields around the wiring. Just having some devices plugged into an electrical outlet (even though not turned “on”) can allow the flow of some current in the electrical line.
We’ll call anything that is attached to the electrical wiring in your home a “load.” Loads can be broadly categorized as resistive (using power), capacitive or inductive, or a combination of all three. Capacitive and inductive loads (called reactive or non-resistive loads) can allow current to flow without actually using power. Even though these non-resistive loads do not use power, the current that flows through them creates magnetic fields just as well as the current from loads that use power. (Power companies don’t like these kinds of loads, because they must supply the current, but you don’t pay for any power, because these loads don’t use power.)
Most modern homes have wiring that comes in cables with three (and sometimes four) closely spaced wires. A typical three-wire cable has a “hot” (for example 120 volts alternating current [120 VAC] wire), a neutral (or return) wire, and a ground wire. The ground is for safety purposes and in normal circumstances will have no current flowing through it.
In a wiring system with no wiring errors, the current in the hot wire flowing into a load will be the same as the current in the neutral wire flowing back out of the load, and the currents in the two wires of the cable will be flowing in opposite directions. Each wire will produce a magnetic field proportional to the current flowing in the wire. Since the wires are very close to each other, the two magnetic fields created by the opposite currents cancel each other everywhere except very near the wires.
In some cases, one might want to control the lights in a room from any one of several switches. The wiring schemes for these circuits leave open the possibility of un-matched pairs. That is, the hot and neutral wires may not always run in the same cable, and thus the magnetic fields for the two wires do not cancel, because the hot and neutral wires are not very close to each other.
I think we can now get to the topic of filtering. Ideally, we would like to have (in the U.S.) only 60 Cycle Per Second (60 Hz) currents and voltages on our wiring. Voltages and currents other than the desired 60 Hz frequency can come in on the electrical lines from sources outside our homes, or they may be generated by items inside of our homes. For the purposes of this discussion, let’s call the undesired, non-60 Hz voltages and currents “noise.”
We may want to add filtering to our power lines to reduce the magnetic fields within our homes by blocking noise currents from entering our home at the meter or by keeping noise currents generated by the various items within our homes from propagating into the wiring system of our homes.
A single filter at the meter can block noise coming from outside the home. In an apartment building, a single filter at the electrical input to the building may not be effective, because noise generated by devices in some apartments can get to other apartments without flowing through the input filter. Each apartment would have to have its own filter at the meter to keep out noise generated in other apartments.
Many loads in our homes and businesses create voltages and currents with frequency components other than 60 Hz. Circuits like light dimmers often create harmonics of 60 Hz (120 Hz, 180 Hz, 240 Hz…). Other appliances such as computers, radios and televisions often use Switched Mode Power Supplies (SMPSs) to provide the operating voltages necessary for the internal circuits. These power supplies convert the alternating current (AC) input voltage to direct current (DC). Then, at high frequency, they turn on and off fast switching transistors to create high frequency AC signals to be transformed to the desired operating voltages. SMPSs are used because they are cheaper, lighter and more efficient than older “linear” regulated power supplies. The drawback of these SMPSs is that they can produce noise currents and voltages that “contaminate” the power lines, thus “contaminating” the power lines with their noisy signals.
Let’s take a look at filtering out noise generated by devices you may have in your home. Ideally, we would like to keep the noise from getting to the house wiring. A filter is required BETWEEN the device generating the noise and the house wiring. You would plug your device into the filter and plug your filter into an outlet.
Remember that the magnetic fields generated by the noise are proportional to the noise current flowing in the wires. A filter that looks like an open circuit to the house will be best, since it will not allow noise current to flow into it. Filters can also be designed to block noise by appearing as a “short circuit” to the noise. This type of filter usually has a capacitor as its first element. This type of filter, however, will maximize the noise current flowing between the device and the filter. In this case, the power cord from the device that is plugged into the filter will have the maximum possible noise currents, and therefore the maximum potential to produce magnetic fields. The consumer has no way of determining whether the filter is designed to be an open circuit or short circuit unless the manufacturer states so in the specifications. Filters that involve ferrite beads as the first element are more likely to be the desired “open circuit” type.
At 60 Hz, the filter should be “transparent.” That is, it should not block or otherwise affect the 60 Hz power that flows to your device.
Some “filters” are sold that just plug into unoccupied outlets, and are not placed between the offending noisy device and an outlet. If these “filters” are of the “open circuit” type, they will do nothing, as an unoccupied outlet is an open circuit anyway.
If these “filters” are of the “short circuit” type, they may reduce noise currents (and magnetic fields) in some portion of the wiring; but they will maximize noise currents in the portion of the house wiring that is between the “filter” and the offending noise-generating device.
Let’s look at an example: A power cable goes from your breaker box to an outlet (outlet #1). This cable continues on and is connected to outlet #2. The cable continues again and ends at outlet #3. Suppose you have a noisy device on outlet #1, and you plug a “filter” that appears as a “short circuit” into outlet #2. Noise currents generated by the device plugged into outlet #1 travel to outlet #2, where they see the short circuit presented to them by the filter. Magnetic fields are maximized in the section of wiring between outlet #1 and outlet #2, and magnetic fields are minimized in the section of wiring between outlets #2 and #3.
Now suppose you also have a noisy device plugged into outlet #3. The noise currents from the device plugged into outlet #3 travel from outlet #3 to outlet #2, and the short circuit of the “filter” maximizes the currents and magnetic fields on the section of house wiring between outlets #3 and #2. In this case, the “filter” at outlet #2 has maximized the noise currents and noise magnetic fields in the whole section of wiring between outlets #1 and #3 due to the noisy devices plugged into those outlets.
We can conclude from this that a “filter” that is just plugged into an unoccupied outlet and that has no load plugged into it has the potential to actually increase magnetic fields in the home.
In conclusion, in order to be most effective, a filter must be placed between the noisy devices and the house wiring. The filter should appear as an open circuit (not a short circuit) to the noise.
I would NOT recommend either the Stetzer or Greenwave filter. These filters are not designed to be placed between the noisy loads and your house wiring. The Greenwave filter webpage states, “The filters utilize state-of-the-art EM filtering technology and are made with the highest quality components and materials to ensure maximum dirty electricity filtration. The circuitry in the filters deploys classic capacitance technology designed to short circuit (emphasis added) (or shunt) dirty electricity, while allowing standard 60-Hertz electrical current to pass through the filter unimpeded. The filters are most effective at reducing electrical noise (EMI) between 1kHz and 30,000kHz (i.e. 30 MHz).”
This is exactly the type of filter (short circuit type) that maximizes the noise currents flowing into it.
According to the website for Stetzer filters, it, too, is capacitive and therefore maximizes the noise currents.
Neither manufacturer has posted any technical specifications that can be tested.
I have looked online for filters to be placed between offending devices and the outlets, but have not seen any with specifications that I can evaluate. Some power strips claim to have EMI filters as well as surge protectors. It seems that all are intended to provide protection and noise reduction to audio and video equipment plugged into the power strip, not to keep noise off the house wiring. The filtering on a power strip, however, should work both ways, so even though the noise currents would be maximized on the cords between the devices and the power strip (assuming a capacitive “short circuit” filter), noise currents in the house wiring would be minimized. Some reviews I read said that the noise reduction afforded by these power strips is minimal, but others claimed that audio noise in their amplifiers was diminished. I could see no published specifications for the noise reduction of the EMI filters in these power bars, so I have no recommendations here either.

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