Anticollision Lights and Electrical Radio Noise

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At some point in flying, the possibility of in-air collision should become an important consideration. The installation of strobes or pulsing incandescent lights will most certainly give you additional security.

In getting serious about this, let's first look at where one should put strobes. Certified airplanes usually have a rotating beacon on top of the rudder, but that is not visible from below so a strobe is often placed on the belly as well. These are turned on during the day. At dusk the navigation lights are turned on along with the anticollision lights.

We should install our anticollision lights so at least one light can be seen from any direction. If strobes are placed in the outer edge of the wing tips, that goal can be met with only two lights. Of course the other alternative is to place the lights above and below as on certified airplanes, but if that doesn't satisfy your concern you can do both.

Flashing lights of any type have to be a safety benefit. Tests that I have evaluated indicate that a pulsing quartz halogen light is more visible in bright daylight than a strobe. The best pulse appears to be about 1 second on and 1 second off in duration. Strobes appear to work best at night but we don't have to concern ourselves about that because ultralights aren't permitted to fly at night. Ultralight flight is permitted 30 minutes before official sunrise and 30 minutes after official sunset, provided the ultralight has anticollision lights.

One problem with these types of lights is the likelihood of radio frequency interference (RFI) - the result of the pulsed power that surges to the lamp or strobe. The pulse is in the form of a square wave incorporating some very high frequency components that can be picked up by the antenna on your transceiver. You will be subjected to a pop in your headset each time the light flashes on. The interference is easily avoided if some simple precautions are followed.

The circuit to your lights or lamps should not depend upon the airframe for its ground connection. The lights must be wired all the way from the pulse source to the lamp or strobe. There is no need to purchase some super coax cable for this purpose. Regular well-insulated tinned copper wire will do very well.

The trick in eliminating RFI is to twist the wires together. The amount of twisting is important. Ideally the wires should be twisted so there is about one twist per inch. This reduces the power of noise radiated as RFI by 10,000 at 100 Megahertz, and that is a lot of shielding. Higher frequency components of the pulse are attenuated even more. As compared to the noise from your ignition, it is essentially zero interference.

Now we come to our next subject - ignition noise.

This problem is eliminated on certified aircraft by shielding the ignition wires with metal braiding, and then grounding the braid. That may or may not work on your ultralight. I made up a set of shielded wires for my engine and found I couldn't fire the spark plugs. My engine is fired by a capacitance discharge ignition (CDI) system, which produces one pulse with high frequency components in its wave. The shielding, in conjunction with the wire insulation and the wire going to the spark plug, formed a capacitor. The capacitance, though small, shorted out the pulse to ground. This I found out a good deal later from an electrical engineer, who was versed in that sort of thing.

I had to find an alternative means of cutting down the fierce spark noise in my radio. I didn't want to use resistance wire because it has a way of deteriorating with use. Most resistance wire uses a nylon string that has been loaded with graphite as its current conductor. In time the spark current causes the graphite to sublimate (evaporate away), the resistance goes up and the engine will start to miss, especially at high power settings. Not good.

I tried various noise-suppression caps and finally I found one that would do the job satisfactorily. Of course it was the most expensive one so it is no wonder. The best cap I was able to find is a Bosch, part number 356-351-032 marked as having a 1K-ohm impedance. To be sure it doesn't cut out all of the spark noise but by turning up the squelch on my transceiver, it works just fine. The only time I hear any spark noise is when I have an incoming signal, but it is at an acceptable level.

If there is anything better out there I would like to hear about it.

Arnold C. Anderson has been flying ultralights since 1982, logging more than 300 hours in his Kasperwing. After 37 years in the engine and aerospace industry as a mechanical engineer, designing electro-mechanical equipment and solving reliability problems in equipment for unmanned deep space missions, Arnold is now retired. He lives in Bellevue, Washington, where he pursues his hobbies, including aerial photography and flying RC airplanes and gliders.
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