Some basics on electromagnetic interference
The by-product of producing an ignition spark is the creation of waves of electromagnetic energy within the radio frequency spectrum (above 20,000 Hz). Radio frequency interference (RFI) includes radio interference caused by electrostatic discharges (ESD) from static electricity. Electromagnetic interference (EMI) encompasses interference to computers and other solid-state electronics.
EMI including radio frequency interference radiates or conducts undesirable voltages into victim circuits. Radiated interference is that which is transmitted by electromagnetic fields and received by the antenna effect of other equipment. Conducted interference is interference that is introduced into a circuit by coupling. Coupling can be either resistive, capacitive, or inductive. Conductive interference exists most often where common return circuits such as power supplies and grounds exist.
Conducted interference can be identified by temporarily operating the affected equipment (victim) from a separate power supply. If the interference stops then it is conducted. If the interference remains then it is radiated. Some equipment is more prone to EMI than others. Conducted interference can be reduced by grounding back to the power supply using twisted pair wire, by using filter circuits, or by distancing the power supply line from the sources of interference. Radiated interference is best reduced at the source by shielding.
Use of shielding
Shielding sets boundaries for radiated energy through reflection or absorption. The shield surface reflects interference energy because of the impedance discontinuity at the shield boundary. Shields also intercept radiated noise and return it to its source through a low impedance. Thus the need for a low resistance ground back to the source using the shortest possible path.
Distancing the power supply line from sources of interference reduces the capacitive and inductive coupling between circuits. Capacitance coupling transfers electrical energy between the two circuits. Inductive coupling occurs where magnetic field links the circuits and magnetic energy is transferred between the two circuits. By distancing wires the circuit coupling is limited to radiation of electromagnetic waves. All conductive problems are either grounding or cabling.
While the by-product of the ignition spark is high frequency RF at the harness, the "P" lead circuit is a potential source for low frequency (1 Mhz and below) interference. It should be shielded to prevent low frequency (AM band) radio interference caused by voltage fluctuations within the coil's primary circuit.
The "P" lead has voltage, but because it is an open circuit when the magneto is operating, it 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 end being the most important.
Ground the "P" lead shielding to the magneto to keep the ground path back to the magneto as short as possible. Don't twist the shielding braid into a pigtail and solder it to a ground lug. It is better to connect the braid around the entire circumference so that the center conductor is surrounded. Not grounding the "P" lead shielding turns the shielding into an antenna. The shielding radiates energy back and forth to the ignition harness shielding causing extensive RF radiation.
The "P" lead connection at the ignition switch can act as a small antenna. A grounded metal cover installed over the back of the switch shields the connection.
Ignition filters are used to suppress radio interference from the "P" lead. These filters are capacitors that short circuit RF currents to ground while maintaining an open circuit to ground for direct currents. Ignition filters are more frequently used on Bendix S-20 and S-200 series magnetos since these magnetos use a conventional capacitor that is not as effective in suppressing higher frequency interference. Bendix S-1200, D-2000/D-3000 and Slick magnetos use a coaxial capacitor (also called a flow-thru capacitor). Coaxial capacitors are more effective at radio frequencies.
Conventional capacitors, 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 should be installed as a flow-thru capacitor.
The ground path for the ignition 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.
Ignition filters are more effective if installed on the magneto rather than on the firewall. If mounted on the firewall, the interference signals have to travel back through the firewall, engine mount, engine, and magneto. There is considerable opportunity to radiate energy along the ground path. If it is installed on the magneto, the ground path is considerably shorter with almost no opportunity to radiate interference energy. 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. Ignition filters change the capacitance of the coil's primary circuit. The change in primary capacitance alters internal magneto timing and this reduces spark energy.
Slick magnetos are designed so that the breaker point and the breaker cam wear at the same rate. This reduces "E" gap timing drift. Cam wear is caused by friction between the cam and the cam lobe. Breaker point wear is determined by the arc suppression ability of the capacitor. Changing primary capacitance by the addition of a noise filter, changes the arc suppression at the breaker point. Doubling the capacitance makes the points last twice as long - but 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 "E" gap timing drift.
Inspect the harness for broken or frayed shielding wires. Ignition harnesses require a good ground at both ends which reduces the shield's mutual reactance to ground and prevents the shield from becoming an antenna. A good RF-shield connection is below 3 milli-ohm resistance. This is a low resistance value and emphasizes the importance of a good electrical connection to ground. Resistance above this level transmits noise. EMI from the ignition harness is usually radiated. If the harness is the source of EMI, it should disappear when the antenna is disconnected from victim.
Harnesses with plastic covering over the braided shielding may prevent the shielding from grounding to the magneto cap. Make sure that the ferrule used to install the lead into the magneto cap has pierced the plastic covering and makes contact with the steel braid. Early Bendix ferrules (pre-1985) part number 10-620009 are notorious for their poor grounding and associated harness induced EMI. Bendix improved this ferrule in 1985 with a new part number 10-620109 (reference Bendix Service Information Letter JP00325). A shielded lead, if it is not properly grounded, can resonate at radio frequencies and become a dipole antenna. The shorter the lead the higher the frequency, the longer the lead the lower the frequency.
There are several design features used in ignition harnesses to lower EMI. These include using a conductive shield, increasing copper content of shield conductor, increasing lead inductance, and increasing lead resistance.
Shielding effectiveness is directly proportional to the conductivity of the shield material. If we take annealed copper as having a conductivity of 1.00, the relative conductivity of aluminum is 0.61, iron 0.17, and stainless steel 0.02. Stainless steel therefore, is a much less effective shield material as is copper. However, if stainless steel and copper are used you get high strength and high shielding. Aircraft ignition harnesses are usually shielded with tin coated copper.
The chief drawback of a conductive shield is increased lead capacitance. Capacitance causes a sharp voltage pulse to the spark plug electrode which increases electrode erosion. The sharp voltage pulse also creates more EMI since the noise component of the pulse is the rise-time. Although shielding absorbs electrical radiation, it creates more electrical radiation to absorb. The spark plug incorporates a series resistor to absorb the capacitance shock and extend negative electrode life.
The fast rise-time pulses from 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 lead uses a straight conductor with 1/3 ohm per foot while the Slick 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.
Any current carrying conductor has a magnetic field surrounding 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 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.