Fixing Radio Noise
by Robert Victor
Montreal, QC H2S 3J5
Copy and print the following, and put it somewhere you can find it - you'll likely need it one day.
Here's my hit list for dealing with noise in your avionics. Read it carefully, follow the instructions, and you will solve your noise problems once and for all...
1) You have more than one noise problem. This is not a question, it's a fact. You'll have to address several issues to cure the problem.
2) Identify the noises...
Use your ears and this info to determine what's going on. Here are the various noises you will hear...
a) Ignition noise - you know that "crack" or "snap" type noise you hear when you reach for a doorknob after walking on carpet? Imagine that repeated at the rate of 20-200 per second. Rises and falls with engine speed. Very distinctive. Listen for it - hear it.
b) Generator-alternator whine - sounds like more of a musical tone than 1 above. Rises and falls with engine speed, from a low hum up to a mid-range note (like middle c on a piano - b string on a guitar). Listen for it - hear it.
c) Power regulator hash - a continuous scratching, scraping noise - sounds mean, rude, aggresive. May increase in volume or change character somewhat with engine speed, but does not rise and fall with speed like 1 and 2 above. Similar to radio static but much more harsh- sounding. Listen for it - hear it.
d) Radio Static - varies in intensity depending on how strong is the signal you're listening to. On a very strong signal, you hear none - on a weaker signal, some - sounds like someone saying "shhhhh" in the background. On a very weak signal will compete in volume with the voice you're trying to hear. In all cases except the very worse, sounds relatively smooth and non-aggresive. Listen for it - hear it.
e) EMI - stands for electromagnetic interference. This is coming from your other equipment like GPS, EIS, strobes, etc. The sound varies from one piece of gear to another. Will often sound like a buzzing, hissing noise, like a small blowtorch, with multiple, harsh tones mixed together, like a dial tone from Mars. May have a rhythmic pattern, like there's an underlying clicking or cycling nature. Can sound a lot like c) above, but will have some steady or cycling tones mixed in.
f) RF feedback. Heard only when transmitting - sounds like a howling, single note each time you transmit. May vary in frequency and or volume when you do things like touch the radio, grab or move your headset cord around, or even just move your head (with the headset on). Listen for it - hear it.
g) Ground-loop effects - Hum or noise, poor audio quality, clicks, pops, and scratches when you operate other equipment - a general grab- bag of crap. If you've fixed everything else and still have all kinds of garbage sounds and effects, this is likely what's going on.
h) Cabin sounds in intercom or radio - You or the people you transmit to hear the ambient sounds from the cabin / engine at a higher-than-acceptable level.
i) Mother-in-law - sounds like a constant, nagging whine.
a) Install resistor-type plugs. In 99% of cases, this will reduce this noise to the point where it's no longer an issue. Hard to get rid of the last 1% but it won't bother you. Shielding your plugs and plug wires is a lot of work, and may not improve things much.
Your mag-kill wiring may be conducting a lot of this noise. If the resistor plugs don't fix it, you'll have to use shielded cable on these wires, and or separate them from other wiring. See below for materials
b) Make sure your power wiring is not running close to your starter cable from the battery back to the engine. Install a whine filter on the charging line from your regulator to the battery, or alternatively, on the isolated power line for your radio-intercom circuit. See below for materials
c) Same as b) above.
d) Either a defective radio or (more likely) a defective, poor, or poorly installed antenna. A rubber duck antenna is a poor choice. Get a proper 1/4 wave antenna, use a proper ground plane, install it and TUNE IT in place, or get a no-tune antenna like the Air Whip.
A decent comm system should work easily from 50-100 miles. If yours doesn't, the antenna is likely the culprit. If you're getting only 5-10 miles, your antenna or radio is bad.
e) You have to either shield or move the offending equipment. You can experiment by moving the gear temporarily - if the noise diminishes or stops, that's the culprit. This problem is not caused by your radio gear, it is caused by the offending equipment. Manufacturers should be looking after this - unfortunately many don't. Shield or isolate (move) - the only cure. Also make sure to wire the power to the offender on a separate circuit.
f) This is caused by the transmitted radio signal feeding back and getting picked up by mic, headphone or power cables. Unlike audio feedback, which goes in the mic, out the speaker, and back around the loop again, RF feedback can enter the radio by ANY wire connection to the radio or intercom - not just the mic wire.
The easy cure is to use snap-on chokes, placed on all cables associated with the radio and or intercom. Place the chokes first on all radio wiring, as close to the radio as possible. If it's still happening, place chokes on the headset and intercom cables. See g) below.
Your antenna can be causing this. A poor antenna can leave most of the radio power floating around the cabin rather than getting radiated out. Rubber ducks are bad for this. Get a decent antenna and MAKE SURE IT'S TUNED!
g) Every ground from every piece of gear should be attached to the same physical, single ground point. The airframe is not a single ground point. A single physical location is the same point. And everything means everything - battery, voltage regulator, EIS, strobes, cable shields, intercom, radio, gps,,,,, - everything!
(well, ok, with the exception of the starter motor.)
While you can often get away with compromises to this principle, often you can't. Plan, build and upgrade with this in mind. If you're trying to cure a problem, keep doing this until the problem goes away.
Any cable shields (with the exception of the antenna cable shield) should be grounded to this point only -leave the other end ungrounded.
h) Either your mic gain, set in the mic itself, the intercom or in some cases, the radio, is set too high, or the headset you're using is not suitable to the environment. A more-sensitive mic will not pick up your voice better, it'll just transmit more cabin noise into your system. In most cases, lower gain is better. If the intercom has too much gain and you can't adjust it, you need a better intercom.
i) No legal solution.
Filters - Alternator whine, hash, or voltage regulator hash can be cured by putting the appropriate filter in-line. I've had trouble finding these filters so (guess what) we're going to make them.
Contact me at the address below for info.
RF chokes - these need to be made of the appropriate material. You cannot use chokes you've taken off other gear - they may not work. We
have them, - if you'd rather not buy from us tell me, and I'll point you to another supplier.
Shielded cable - for power supply wiring, mag kill wiring, etc. You can find this at many electronic supply houses. We have it – if you'd rather not buy from us tell me, and I'll point you to another supplier.
Antennas - Contact us for the Air Whip. Forget rubber ducks for anything other than temporary use. Many other antennas will work fine, but most need to be tuned at installation. Beg, borrow or buy the gear to do this. Try finding a local ham - he or she may well have the gear and be happy to help, especially if he can go through the field and do a bunch on the same day for a couple of bucks each.
Note - if you've installed a quarter-wave whip without an appropriate ground plane and without tuning it, it's probably not working as well as it should, maybe hardly at all.
Final note re wiring. You need to wire up your gear, including radio and intercom, according to some rules. Do a professional job on this - use proper fusing, isolated circuits for various subsystems (like comms), proper grounding techniques, etc. Trying to avoid noise problems by running comm gear off batteries is a kluge (IMHO). Bite the bullet and do it porperly - you'll operate trouble free and safe for ever thereafter.
All of this is like cleaning the garage. It's a big job when you've let it go for a long time, but get it cleaned up once and for all and keep it that way, and it's a real pleasure. Having good, clean comms and intercom is a real asset to pleasurable operations of your little baby. Get it fixed up and - enjoy!
And most finally, have faith! Do the above, and you will fix these problems. Some may suggest that you can never really cure all these things, but I heartily disagree - do it right, clean, and thorough, and you'll be trouble-free. I HAD all these problems, fixed them up, and I love the results. Good comms add 100% to my flying pleasure.
-Robert - http://www.miracleantenna.com - 866 311 6511