since 1940

Other related articles: Aircraft Strobe Noise Radio interference. Radio noise. Radio noise or interference with deicers on Aircraft Magneto Filters Aircraft Electrical Bonding to reduce radio noise by John Schwaner

We stock two different magneto filters

Electrosystems MF-3A

Lonestar LS03-01010-04

When the magneto is operating, the P lead doesn't carry any current. It is a high impedance source for near-field electrical radiation. (Current carrying conductors are low impedance and produce magnetic near-field radiation). This means that P lead coupling is capacitive and the shielding attenuates the field by reflection. The best shielding material for reflection is high conductivity materials such as copper or aluminum. The shielding needs a low impedance path to ground at both ends with the ground at the magneto to keep the ground path back to the magneto magneto's since these magneto magneto magnetos use a coaxial capacitor (also called a flow-thru capacitor). Coaxial capacitors are more effective at radio frequencies.

Conventional capacitors as used in the older Bendix magnetos, because of their higher inductance, are less effective at attenuating interference than coaxial capacitors. Inductance results from the capacitor's internal inductance and the inductance of the capacitor's lead-in wire. The higher the inductance the lower the frequency range where the capacitor is useful. Inductance can be decreased by shortening the lead-in wire.

The coaxial capacitor eliminates the lead-in wire, thereby lowering inductance and increasing capacitor effectiveness in suppressing higher frequency interference. The MF-3A aircraft magneto filter should be installed as a flow-thru capacitor.

The ground path for the magneto filter is through the filter's housing. Make sure a good electrical contact is made. Scrape away any paint and sand the contact area. You can also apply a conducting grease (Penetrox) to the contact surfaces.

Aircraft magneto  filters are more effective if installed on the magneto rather than on the firewall or remotely from the magneto . There is considerable opportunity to radiate energy along the ground path. If the magneto  filter is installed on the magneto housing there is almost no inductance from the hot side to ground. The result is excellent high frequency performance.

Ignition filters shouldn't be used on Slick magnetos. The Ignition filter changes  the  magneto's internal timing by 10 degrees. The capacitor is designed so that point wear compensates for cam wear. The addition of an ignition filter changes the breaker point's wear rate causing timing drift.

Inspect the harness for broken or frayed shielding wires. Ignition harnesses require a good ground at both ends with the ground at the magneto cap. Make sure that the Harness ferrule used to install the lead into the magneto produce electrical noise starting at 0 Hz and extending up in frequency. Because the pulses have a finite rise time there is an upper cutoff frequency where the energy starts to drop off. If the pulse rise-time can be slowed down then the cut-off frequency can be lowered, preferably below interfering frequencies. Pulse rise-time can be slowed down by adding damping elements (resistance and/or inductance) to the ignition circuit. Increased resistance or inductance lowers EMI.

Lead resistance lowers the voltage rise-time and is a very effective method of suppressing lead generated interference. The Bendix magneto lead uses a straight conductor with 1/3 ohm per foot while the slick magneto lead uses a helical conductor at 1 ohm per foot. To be electrically quiet, resistor leads need a resistance of at least 3,000 ohms per foot.

Resistor leads are universally used in automobiles but not on aircraft. It has been stated that the reason is that resistor leads are not as durable in the high vibration aircraft environment. Although the British Spitfire used resistor leads in World War II (although it had a short life expectancy).

Any current carrying conductor has a magnetic field surrounding the conductor. If the conductor is straight the magnetic field radiates out in space at right angles to the length of the conductor. If the conductor is curved into a helical then some of the magnetic field returns to the conductor inducing a back current into the conductor. This back current opposes the current flow and lowers the voltage rise-time.

A harness with a helical conductor also reduces the level of RF radiation by returning more field energy to the conductor. If you coil a conductor the energy field surrounding a conductor is spread apart on the outside of the curve and squeezed together on the inside of the curve. The field spread on the outside has less energy and the field squeezing on the inside increases energy. Less energy is radiated and more energy is returned to the conductor.

Alternators rely on the low impedance of the aircraft battery for filtering. Alternator output wiring to the battery should be tight and not corroded.

You're out of your plane, clear of it, you've pulled the rip cord, the canopy is open, and you're sitting comfortably although without your pipe and a good book. Parachute Sense, US NAVY 1944

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.