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Author Topic: My Van Generates 146.52 RFI  (Read 7927 times)
N0EQ
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« on: August 12, 2012, 12:40:06 AM »

Very strange. My Chev Astro van generates a low power signal ONLY on 146.52 MHz. Move off freq to 54, 56 or down to 50 or below and it's clear. 6m, 440, HF, 144.39, 27MHz all clear Does it on radios installed in the van. Does it on HTs held within about six ft of the van. Does it any time ignition is on, motor running or not. Doesn't vary with engine speed or road speed.

I've tried millions of things. Filter the power supply, no joy. Bring a separate battery into the van passenger compartment to run the radio(s), no joy. And again, it does it to any radio near the van, doesn't have to be connected to the van at all.

Replaced ignition components, vehicle speed sensor, shielded this and that, grounded everything-everywhere.

Not particularly hoping for magic bullets, I've shot all the usuals. If anyone recognized the situation as something they've heard of or experienced, I'd be happy to hear about it. But I really can't think of anything left to try.


Craig 'Lumpy' Lemke

www.n0eq.com
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NO2A
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« Reply #1 on: August 12, 2012, 09:00:37 AM »

Hello Craig,being that it happens away from the van with batteries,I`m willing to bet it`s your on board computer. Especially with the engine off but ignition on. Is it a "hash" type of noise? The fuel pump makes more of a pulsating noise. But a hash noise would indicate the computer.
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SIGINT11
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« Reply #2 on: August 12, 2012, 11:31:56 AM »

My 97 saturn produces a signal on 146.52 as well making monitoring it impossible , When I enter a pl tone it will mute it.   With the increase of computer generated RF pollution I would like to see the amateur community adopt "universal PL's" for all VHF/UHF simplex operations.  A nice easy one to remember would be 100.0hz.      I have not been able to eliminate it at all yet it does not seem to affect operation other than scanning and not having a PL.
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N0EQ
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« Reply #3 on: August 12, 2012, 01:14:45 PM »

Interesting suggestion, 2A, about the onboard ECM (computer). I wouldn't call my RFI "hash". It's more like there's a low power transmitter in my engine. It wants to capture the receiver and interfere with regular stations in my area.

I can't imagine a PL tone would be any help, either in the social or the technical setting. 52 is, of course, simplex. PL tones on simplex would be counter to the concept. In a technical sense, it wouldn't make a difference. The interference is there, whether it's squelched out or not either with CTCSS or traditional audio squelch.

Seems very odd to me that it occurrs on 52 only. I run virtually every legal ham freq from 160m to 440MHz. None of them, including AM/SSB frequencies are bothered by this gremlin.


Lumpy

You Played on Lawrence Welk?
Yes but no blue notes. Just blue hairs.

www.LumpyMusic.com
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AA4PB
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« Reply #4 on: August 12, 2012, 03:05:14 PM »

Probably the oscillator for the microprocessor in the on-board computer has a harmonic that falls on 146.52 MHz. Not much you can do about it unless you can shield the onboard computer, and possibly bypass connecting I/O leads. Since the oscillator generates a steady CW signal, a receive PL tone would keep it from breaking squelch on the receiver. However, any received signals with the proper PL tone would also need to be strong enough to overcome the oscillator signal in order to open the squelch and be received "interferrence free".

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AJ8L
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« Reply #5 on: August 13, 2012, 05:29:31 AM »

Try removing the airbag fuse and then turn on the ignition or start the van. My 98 Saturn generates a carrier at 146.52 also and it is the airbag system causing it. Geoff, AJ8L.
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NO2A
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« Reply #6 on: August 13, 2012, 04:55:53 PM »

Probably the oscillator for the microprocessor in the on-board computer has a harmonic that falls on 146.52 MHz. Not much you can do about it unless you can shield the onboard computer, and possibly bypass connecting I/O leads. Since the oscillator generates a steady CW signal, a receive PL tone would keep it from breaking squelch on the receiver. However, any received signals with the proper PL tone would also need to be strong enough to overcome the oscillator signal in order to open the squelch and be received "interferrence free".


Some auto makers use a color burst crystal as an oscillator in the on board computer that operates on 3.579545mhz. The 41st harmonic falls in 146 mhz,and it can be anywhere in the band. It often can be heard from 146.760-.800.
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N0EQ
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« Reply #7 on: August 13, 2012, 05:29:07 PM »


Some auto makers use a color burst crystal as an oscillator in the on board computer that operates on 3.579545mhz. The 41st harmonic falls in 146 mhz,and it can be anywhere in the band. It often can be heard from 146.760-.800.

That's pretty interesting. Is the fact that it's generating this RFI anywhere within about 6 feet of van's front end (computer is in the R front cabin) suggestive that there's some kind of shielding or other problem with the ECM (computer)? I've had the ECM out. It's a typical GM ECM, metal case intact etc.

Could that xtal overtone be transmitted along the various data and other wires that connect to and from the module?

Thanks for all the various new angles on this RFI gremlin. I've been through all the stock solutions.

AJ8L thanks, but no airbags in this 89 Chev Astro.


Craig 'Lumpy' Lemke

www.n0eq.com
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NO2A
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« Reply #8 on: August 13, 2012, 06:10:27 PM »

It can very well be transmitted that far since you`re hearing it. Some ecm`s will even transmit without the key turned on,such as opening your door. At least you found the problem though.
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NO2A
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« Reply #9 on: August 13, 2012, 06:15:34 PM »

Your ecm is probably always on,even when the car is off. It`s in a "sleep mode",so that when you go to start it, it starts. Maybe you could shield it somehow.
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WD5G
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« Reply #10 on: August 14, 2012, 11:04:13 AM »

I bought a new 1992 GMC pickup that had also a very pronounced signal on 145.52.  I found that GM has a group of engineers that deal strictly with RFI issues.  I contacted the GM engineers at the Millford proving grounds regarding the problem and what a solution might be.  They indicated the fault was in the module that controls the ABS braking.  They were kind enough to provide me with a custom replacement ABS module that lives behind the glovebox.  I fixed my problem on 146.52, but it moved the birdie up to 146.55 or so.  I could live with that.

Dave de WD5G
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WB2WIK
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« Reply #11 on: August 16, 2012, 08:01:48 AM »

I realize it's radiating sufficiently to be heard on a hand held six feet away, etc; however have you tried different 2m antenna locations (mounting positions)?

That won't make the signal go away, but you might find a "null" point where the directly radiated signal is out of phase with a reflection off the roof or elsewhere, and when those two signals are nearly the same strength and 180 degrees out of phase, the signal will disappear unless you move the antenna again.

Easiest way to experiment with that possible solution is to use a "mag mount" with a long enough cable on it to allow moving the whip all over the roof to see if the signal nulls anyplace.
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AJ8MH
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« Reply #12 on: August 16, 2012, 03:39:13 PM »

In the late 70s, a digital clock in a new police car emitted a carrier strong enough to lock up and block their primary frequency used in that vehicle.

When I was a cellular tech in the 80s, I found that the local oscillator in my GM truck radio would completely block 800 MHz cellular channels depending on what FM station I was listening to.

Ya never know...
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WB6DGN
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« Reply #13 on: August 27, 2012, 10:34:01 PM »

This is no doubt the ECM radiating its usual garbage.  First one I had that did it was a 1981 ElCamino with a 267ci V8.  Had a couple GM diesels that didn't do it but, for the most part, the gas engine ones did.  In my case, since it was a handheld, I just used "nuisance delete" on .52 when I was in that car.  Cheer up folks, in this computer-obsessed world, its only gonna get worse!
Tom
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N0EQ
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« Reply #14 on: October 12, 2012, 07:20:37 AM »

Well, here's an update with some resolution.

Based on my guess (and everyone else's) that the RFI in my van was caused by the onboard vehicle computer (ECM), I simply moved the 2m antenna to the REAR of the van. My Chev Astro has an offroad roof rack with several Moto mounts here and there. So I moved the 6m antenna from the rear to the front, and the 2m antenna from front to rear. That seems to have resolved the gremlins on 146.52. I'm back to being able to hear grumpy old men whine about politics and the current state of their gall bladder.

The quarter six and 5/8 2m antennas are essentially the same antennas, of course, but they are cut slightly different for resonance in their appropriate bands.


Craig 'Lumpy' Lemke

www.n0eq.com


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