STALKING THE ELUSIVE EMI BY JON SHARP AND JACK WELLS

L MAG

R MAG MAG GROUND

1. Chassis Box to enclose switches and wire

2. Seal gaps in box with copper tape GROUND WIRE+ BRAIDED SHIELD

HOT WIRES

TWIN BELDON 8412 WIRE (2 PLCS) GROMMETS

METAL CHASSIS

3. Attach ground wires and shield to Mag cap plugs 4. Attach "Hot" wires to Mag "P" leads 5. Break out shields and connect to chassis lug 6. Connect chassis lug to aircraft ground

7. Connect switch ground to chassis lug

— GROUND

EMI. .. electromagnetic interference ... is an elusive prey when one seeks to hunt it down and suppress its evil influence on our aircraft radio and navigation equipment. In our small airplanes, ignition noise is usually caused by unshielded spark plug leads and Pleads and the solution is to use shielded leads. Unfortunately, however, many shielded leads have a lot of resistance, which cuts down on the intensity of the spark and, in turn, the engine's performance. We accept a little performance degradation in ordinary aircraft in 86 APRIL 1996

INST PANEL

exchange for clear radio reception, but air racers are not willing to give up any performance. This was the dilemma faced by Jon Sharp and his Nemesis crew two years ago when they became the first in the Formula One class to use radio during competition. This year at Reno, radios will be required in all Formula One racers, so others will have to go through the same process Team Nemesis did to get clear transmission and reception. To help out, Jon offers the following suggestions, which may also be of use to homebuilders other than racers.

8. Ground metal box to grounded metal instrument panel

Ignition noise generated by unshielded plug leads. Go to your local hot rod/performance shop and get a set of noise suppression plug wires. We've tested the Accel 300+ brand wires that run S50-S55 for big block V-8s and Magnacore wires. Both were developed for NASCAR, Indy Car and NHRA, all of whom use radios in their race cars. Some also use onboard computers. These wires suppress EMI but have very low internal resistance. You lose the noise, but not the spark. We use the Magnacore wires and have the Aceels as backup. The magneto test

machine at AeroMag (our engine builder) finds very little difference between these two wires. We found the Accels at Pep Boys.

EMI NOISE FROM THE P-LEADS. Go to an electronics supply store and get Beldon 8412 (two conductors with braided shield wire, about 50 cents a foot) and a metal chassis box big enough to enclose the wire end of your mag switches (approximately 2" x 2" x 3"). Figure 1 shows how to keep those nasty EMIs from crawling up the mag switch wires.

COCKPIT NOISE. In a lot of racers there is not room for the pilot to wear a helmet and headphones, so other alternatives must be considered. I know of at least two companies that will turn ear molds into custom earphones for about $150. The companies are Racing Electronics (800/272-7111) and Racing Radios (800/646-7278). The initial ground work requires you to go to your local hearing aid center and get fitted for ear molds. The above companies will need these ear molds to make custom ear pieces that fit snugly in your ears and allow you to wear a helmet. If you watch auto racing on TV, you see the drivers removing these ear pieces after a race. They really work well. For about $150 extra you can send your helmet to these companies and they will attach a noise canceling mike and harness to connect the helmet to the ear plugs and radio.

THE RADIO. Handheld radios (firmly mounted somewhere in the cockpit, of course) work well in Formula Ones. We've switched from a handheld to a Becker AR4201, however. This German-made radio fits in a 2-1/8" instrument hole and packs seven watts of power. It is compact, light and reliable - a great setup. It is available from Aircraft Spruce (800/824-1930).

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THE ANTENNA. The standard "rubber ducky" hand held antenna

works well, but if you want to really nail down your radio reception, there is a super antenna fabricated by

Advanced Aircraft Electronics (800/758-8632). They just slip into your fuselage or wing and work great. They are currently available for $149 from the manufacturer or $ 120 from Aircraft Spruce. My suggestions for determining your specific needs are to start your testing with a hand held and a standard headset. Make a few flights to determine which of the above problems/solutions are applicable to your aircraft. Nothing substitutes for flight testing to narrow your focus. If you have questions or if our team can be of any assistance, please feel free to call Jon Sharp at 805/723-7636.

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