How A WireGuard Works

Using ferrites for suppression of cell phone headset radiation

One of the most used yet least understood techniques for reducing RF interference is the application of ferrite sleeves to cables and at interfaces. This webpage is meant to shed some light on the use of ferrites for shielding high frequency cell phone radiation from headsets, and also presents a description of how ferrites work.

NOTE: When using a Shielded Headset, radiation remains at your phone instead of partially being transferred through the headset wire to your brain. For your safety, when the phone is held near the body always use radiation shielding between you and the cell phone handset itself! 

The effect of magnetic material on a conductor
Current flowing through a conductor creates a magnetic field around it. Transfer of energy between the current and the magnetic field is effected through the "inductance" of the conductor - for a straight wire the self-inductance is typically 20nH per inch. Placing a magnetically permeable material around the conductor increases the flux density for a given field strength and therefore increases the inductance.

Ferrite is such a material; its permeability is controlled by the exact composition of the different oxides that make it up (ferric, with typically nickel and zinc) and is heavily dependent on frequency. Also the permeability is complex and has both real and imaginary parts, which translate into both inductive and resistive components of the impedance "inserted" into the line passed through the ferrite. The ratio of these components varies with frequency - at the higher frequencies the resistive part dominates (the ferrite can be viewed as a frequency dependent resistor) and the assembly becomes lossy, so that RF energy is dissipated in the bulk of the material and resonances with stray capacitances are avoided or damped.
 

Size and shape

There are two rules of thumb in selecting a ferrite for highest impedance:

• choice of shape, longer is better than fatter;
• get the maximum amount of material into your chosen volume that you can afford.

The impedance for a given core material is proportional to the log of the ratio of outside to inside diameter but directly proportional to length. This means that for a certain volume (and weight) of ferrite, best performance will be obtained if the inside diameter fits the cable sheath snugly, and if the sleeve is made as long as possible. A string of sleeves is perfectly acceptable and will increase the impedance pro rata, though the law of diminishing returns sets in with respect to the attenuation.

To show the attenuation in a 10 ohm circuit for three sizes of clip-on core in a rectangular box, same material, same manufacturer. They are all the same length but of different cross-sectional area. In fact, the smallest, yet longest performs the best

Number of turns

Inductance can be increased by winding the cable more than one turn around a core; theoretically the inductance is increased proportional to the square of the number of turns, and at the low frequencies this does indeed increase the attenuation. But it is usual to want broadband performance from a ferrite suppressor and at higher frequencies other factors come into play. These are:

• the core geometry already referred to; the optimum shape is long and snugly-fitting on the cable, and this does not lend itself to multiple turns
• more importantly, inter-turn capacitance, which appears as a parasitic component across the ferrite impedance and which reduces the self resonant frequency of the assembly.

The normal effect of multiple turns is to shift the frequency of maximum attenuation downwards. It will also increase the value of maximum attenuation achieved but not by as much as hoped. The source and load impedances are critical in determining the effect: the lower the impedances, the less the effect of parasitic capacitance.

Capacitance

Because a ferrite material is in fact a ceramic, it has a high permittivity as well as permeability, and hence will increase the capacitance to nearby objects of the cable on which it is placed. This property can be used to advantage especially within equipment. If the ferrite is placed next to a grounded metal surface, such as the chassis, an L-C filter is formed which uses the ferrite both as an inductor and as a distributed capacitor. This will improve the filtering properties compared to using the ferrite in 6.00 space. For best effect the cable should be against the ferrite inner surface and the ferrite itself should be flat against the chassis so that no air gaps exist; this can work well with ribbon cable assemblies. Taken in a 150 ohm system shows the improvement for the geometry depicted. Around 5dB can be gained at the higher frequencies (note that this plot extends into GHz)

Resistance

A ferrite material is also slightly conductive. This is rarely a disadvantage unless you intend to place the ferrite over a bare conductor, in which case you should be aware of the possible hazards, such as leakage in high-impedance circuits, it might bring. Volume resistivities of 10⁵ to 10⁸ ohm-cm are typical with 10⁹ achievable.

Saturation

As with other types of ferrite, suppression cores can saturate if a high level of low-frequency current is passed through them. At saturation, the magnetic material no longer supports an increase in flux density and the effective permeability drops towards unity, so the attenuation effect of the core disappears. The great virtue of the common-mode configuration is that low frequency currents cancel and the core is not subjected to the magnetic field they induce, but this only happens if the core is placed around a cable carrying both go- and return- currents. If you must place a core around a single conductor (such as a power supply lead) or a cable carrying a net low frequency current, be sure that the current flowing does not exceed the core's capability; it is usually necessary to derive this from the generic material curves for a particular core geometry.

Here is more information on the history of the ferrite and their usage

1) A reusable radiation suppression clamp-on device similar to a clamshell and named a WireGuard. This device clamps over the headset wire above the plug that is inserted into the phone. It incorporates a ferrous composition based material which interacts directly with the high frequency energy and suppresses it effectively while allowing the voice signal to pass through to the earpiece unimpeded. The housing is a white or black composite manufactured using nylon resins.  FITS ANY HEADSET!

	Sold on other sites for 29.99ea
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Just pay our Fixed Rate S&H of $6.00ea - Delivered to your door!
6.00 WIREGUARD

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2) Four (4) 5/8 O.D shields made from silver coated RF Shielded Fabric. They are designed to eliminate the chances of stray radiation from penetrating deep into your ear canal, Earbud Shields are simply inserted under the earpiece's foam cover. They fit all in-ear headset models with foam or gel bud covers.

	Sold on other sites for 17.99ea
	RF Safe has the WORLDS Best Price!

Just pay our Fixed Rate S&H of $6.00ea - Delivered to your door!
6.00 EARBUD SHIELDS

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