Sport Aviatiuon - 4/95
By Tony Bingelis
When you install a radio in an engine-powered aircraft you will be subjecting it to an environment that can be quite hostile to good radio reception.
This hostility is manifest not only by the audible noises heard in the aircraft’s headsets and speakers but also by the effects such electrical disturbances have on the outputs of navigation receivers and other navigation equipment. In effect then, noises are there whether we can hear them or detect the visual effects in our navigation receivers. Of course, when they are audible to our ears that condition can become quite annoying and personally unacceptable.
Anyway, it might surprise you to learn that many of these troublesome radio noises often result from the poor wiring installations sometimes incorporated during the construction of an aircraft. In other words, the most troublesome noise is that caused by stray electromagnetic waves that emanate from installed electric equipment, such as magnetos, alternators, relays and the like, and are not necessarily the fault of the avionics equipment installed.
Contrary to hangar tales you may have heard, the elimination of radio noise has nothing to do with black magic. Nor is it only something a highly skilled radio technician can do.
Troubleshooting a Noise Problem
You can, for example, solve a fair share of the typical radio noise problems you encounter by first locating and isolating the source of the noise problem. Once this is done, the necessary corrective action often becomes obvious and is readily accomplished. Troubleshooting any kind of problem is based on common sense and logic.
Tracking down and curing an unwanted radio noise problem is no different. Begin by examining the obvious and the easiest to check potential trouble sources: poor ground connections, loose terminals, broken or frayed wires, inadequate shielding. Then, check the operation of the various electrical systems (lights, strobes, fuel pump, rotating beacon, intercom, etc.), one at a time, listening for noise. Do this with the radio(s) ON and with the engine running - then again with the engine shut down. After you pinpoint a noise producer, you can then try one "fix" at a time.
Always check the results of that effort before trying something else. Otherwise when you try several corrective actions before checking the results of each, you might never learn which of the "cures" really solved the problem.
Since radio noise can originate almost anywhere in the aircraft, it should come as no surprise to learn that a particular radio noise problem is most often traced to something no more mysterious than a poor battery ground connection or a loose accessory ground connection.
What To Do About Ignition Noise
On the other hand, noise-free radio reception depends, in great measure, on a properly shielded ignition system. This means the entire magneto system, including the "p" leads (the wires connecting the ignition switch to each magneto), MUST be shielded type leads with the external shielding carefully grounded, both at the magneto and at the opposite end, at the ignition switch. In other words, in an ignition system the shielding is most effective when the shielding pigtails are grounded at both ends of the "p" leads.
It is equally important that the ignition harness be in good condition with no breaks in the shielding. If you made up your own ignition leads, keep in mind the possibility that the source of an aggravating engine noise in the headset might be traced to a poorly fabricated ignition lead. Of course, your spark plugs must be the standard shielded type and in good condition (I hope you didn’t drop one).
I have often found it unnecessary to install magneto filters on each magneto when shielded "p" leads are used - provided the shielding was properly grounded at both the magneto and ignition switch. This can be good news as the cost for two magneto filters now totals an astronomical $40 or so. My advice? Don’t install magneto filters in anticipation that they will be needed. They may not be! If, however, you do find that your shielded ignition system does not effectively reduce the ignition noise, you may have to install a filter between the magneto and magneto switch. Even so, it is just as easy to add a magneto filter later if you have to.
Incidentally, wouldn’t you expect ignition noise to disappear when the engine is shut down? If it doesn’t maybe it’s not ignition noise
Filter Out Alternator Noise
Among the other frequently encountered radio noise makers, I would consider alternator (generator) noise to be second only to ignition noise. Alternator noise is generally recognized by sort of a whining sound . . . a sound that rises and falls with changes in the throttle setting. Alternator noise is most frequently eliminated with the installation of a filter mounted directly to the alternator. The noise filter is connected in parallel to the alternator’s output lead and to the case-to-ground. The higher the output of the alternator, the larger the capacity of the filter may have to be to reduce RF interference and noise in the system. Most wiring diagrams show the alternator installation is to be made using shielded wire . . . does yours?
Strobe Lights As A Noise Source
Here’s what you can do:
1. Install a reliable wingtip position/strobe light installation like Whelen’s A600-PG-PR installation kit. Be sure to follow the recommended installation instructions. Homemade installations may be somewhat less expensive but often fail to meet the regulation standards governing anti-collision lighting systems.
2. Install your strobe light circuit breaker at the end of the electrical buss closest to the battery using a 14 or 16 gauge wire.
3. Be sure to terminate the interconnecting cable shield to ground ONLY at one end or the other. Do not terminate both ends of the shield to ground. Usually the power supply end provides the quietest ground . . . but not always.
4. Check to see that your radios have a good central ground point.
Electric Fuel Pump A Noise Source?
Other Potential Radio Noise Sources
Intermittent Radio Noises
That means you should make every effort to determine if the noise source might be in the aircraft’s electrical installation before blaming the radio and having someone tear into it.
This intermittent kind of noise problem is likely to occur because of a poor ground connection, frayed shielding, a corroded or loose connector, and possibly a poorly soldered (cold) joint . . . although soldered connections are more commonly encountered inside avionics equipment than in the aircraft’s electrical system installation.
Examine each shielded ring terminal to assure yourself that it is securely crimped to the wire.
Switch Controlled Noises
As always, your best assurance for noise-free radio reception is a properly wired installation.
Especially important are good connections between the switch, unit (accessory), and the aircraft metal structure, to forestall the generation of noise and the pickup of the noise.
Some power cables (wires) may not be shielded at all when they really should be to minimize the noise potential. In other instances, additional shielding over that already provided might solve an otherwise vexing noise problem.
When radio noises start with the activation of a switch and continue as long as the unit is being operated, the noise source is obvious, isn’t it?
What About Noise Filters?
For maximum reception sensitivity you would ordinarily turn the squelch control all the way clockwise at which point you will hear a hissing sound from your receiver (also ignition and alternator noises, if present). This setting, with the squelch open, allows you to receive the weakest signals.
It also causes that hiss when no signal is being received . . . it is not an atmospheric noise being picked up by the antenna. As you turn the squelch control counter-clockwise, there will be a point where the hissing noise disappears. This is the correct setting for your squelch control. Turning the squelch any further than this will prevent the receiver from picking up weaker signals.
And Finally . . .
Some of the noises you hear in flight may be due to somebody’s stuck microphone button. Incidentally, you might check your own mike button to see that you are not the offender.
Noise, who needs it? And that goes for unnecessary radio transmission, too.
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