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post #1 of 24 (permalink) Old 05-24-2005, 08:45 PM Thread Starter
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Default Brake interference with AM reception

Has anyone else had this problem? When listening to AM radio (Air America) in certain areas (for example near lots of power lines), when I apply the brakes I get a static burst on the radio.

Can anyone explain to me why this happens and has anyone else had this happen to them. It occurs on AM, not FM channels and is more noticeable on the higher frequencies. One thing I do remember from Freshmen radio class is that the lower the frequency in AM, the stronger the signal. Air America broadcasts on 1190, so I have this problem with this channel and less problem with 710.

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post #2 of 24 (permalink) Old 05-24-2005, 09:12 PM
 
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Air America-- that's your problem!!! (just Kidding)
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post #3 of 24 (permalink) Old 05-25-2005, 08:28 AM
 
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i agree air america causes a lot of unecessary static.BUTT from my old stero days it was somewhat common to get static when applying brakes when using am. seemed to happen when turned on wipers as well. the answer was always stated as a bad ground.so my guess is thats the case here unless your signal is just too weak . also, you might try switching to stations on america right. they seem much stronger (alright already, just kidding) but im not kidding about a bad ground
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post #4 of 24 (permalink) Old 05-25-2005, 08:29 AM
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AM is amplitude modulation. The broadcast is encoded by changing the strength (amplitude, "loudness") of the carrier wave. AM at any frequency will be prone to interference. AM radio is at a fairly low frequency for broadcast, so any ambient electromagnetic wave source could cause interference, especially with the harmonics, not to mention causing jamming even if not in a harmonic.

FM however, is frequency modulation. Here, the broadcast is encoded by shifting the center frequency a bit, correlating the shift with the amplitude of the source information. The receiver tracks this shift, and uses the "error" to reconstruct the original message. This is less prone to interference, because the interference generator (such as power lines) will not alter the instantaneous frequency of the broadcast, where with AM modulation, the interference generator can change the amplitude of the original broadcast signal.
However, an interference generator could produce a harmonic close to the center frequency of the broadcast, and the receiver tries to track that, rather than the real broadcast. This false tracking may be very short, thus appearing as part of the "message", but really comes out as interference noise. This is easier to prevent.
Also if enough of the broadcast amplitude is altered, the receiver could still be confused, even though it doesn't otherwise use amplitude to decode the message. The ability to resist amplitude changes in the received FM signal is known as AM rejection. The Prius radio seems to have poor AM rejection as well as other interference rejection.
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post #5 of 24 (permalink) Old 05-25-2005, 08:33 AM
 
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dan! p l e a s e ! arghhhh.!!i just think im going to slit my wrists (well, maybe not) i think my explanation was more fun although i do defer to my colleague big dan on ALL technical grounds
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post #6 of 24 (permalink) Old 05-25-2005, 08:36 AM
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OK, re-read your post, so let me answer your question more closely.
Now that I explained how AM and FM works, and how AM is more prone to interference, the question becomes: why do the brakes cause interference?

Because you have half a dozen computers, each operating at radio freqency speeds. Now they do have shielding to minimize broadcast radio interference, but there still will be some.
Then the bigger problem becomes the inverter. The inverter is shifting large amounts of current at very high speeds. Ever notice the inverter "whistle" when you first accelerate off the line, or brake? That's the power transistors turning on and off very rapidly. Again, the radio frequency radiation is minimized by shielding, but still isn't perfect. A weak AM broadcast will easily be overwelmed by the interference from the nearby inverter, especially as you accelerate, or perform regen braking.
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post #7 of 24 (permalink) Old 05-25-2005, 08:44 AM
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MrFredSporty, it's time to pass around the grape Kool-Ade and the 45!

What's wrong with the good old-fashioned "because"? Always worked for Mom!

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post #8 of 24 (permalink) Old 05-25-2005, 08:59 AM
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I remember a long long time ago that there was a watchamacalait you could attach to your car to help prevent am radio static.

It connected something metal to something else that was also metal and prevented static electricity from building up and discharging...which is what was causing the radio static in those days.

Hope this helps

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post #9 of 24 (permalink) Old 05-25-2005, 09:02 AM
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MUCH better ... more like the vague general "because" and "twiddle the doohickey" we're used to.

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post #10 of 24 (permalink) Old 05-25-2005, 09:15 AM
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Sorry Fred. Your post came shortly before my first one.
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