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Mobile Noise

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jon_k

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Greetings,

I've got an antenna mounted via an ¬ mount to the side of my trucks aluminum toolbox. I've also got a portable BC246T scanner, the handheld variety.

I plug my scanner BNC into the antenna mounted on the toolbox. I then connect the earphone plug using a MALE-MALE cable to the INPUT jack on my stereo headunit. This is done to get sound via my car speakers.

When I start my vehicle, I hear a sort of noise. When I rev the engine, the noise increases. When I flip my blinker, the noise oscillates when the blinker draws current.

If I disconnect the antenna and apply the rubber ducky, there is no noise.

Is this noise that is being radiated via the metal body of the vehicle into the antenna, through the antenna ground to the mobile ground, and up into the radio from the scanner?

Any ideas on this? Ways to eliminate?
 

Don_Burke

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Joined
Jan 16, 2007
Messages
1,180
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Southeastern Virginia
Is the noise present with the scanner connected to the antenna on the toolbox, but not connected to the car stereo?
jon_k said:
Greetings,

I've got an antenna mounted via an ¬ mount to the side of my trucks aluminum toolbox. I've also got a portable BC246T scanner, the handheld variety.

I plug my scanner BNC into the antenna mounted on the toolbox. I then connect the earphone plug using a MALE-MALE cable to the INPUT jack on my stereo headunit. This is done to get sound via my car speakers.

When I start my vehicle, I hear a sort of noise. When I rev the engine, the noise increases. When I flip my blinker, the noise oscillates when the blinker draws current.

If I disconnect the antenna and apply the rubber ducky, there is no noise.

Is this noise that is being radiated via the metal body of the vehicle into the antenna, through the antenna ground to the mobile ground, and up into the radio from the scanner?

Any ideas on this? Ways to eliminate?
 

jon_k

Member
Joined
May 7, 2008
Messages
269
Location
Fort Worth, Republic of Texas
Don_Burke said:
Is the noise present with the scanner connected to the antenna on the toolbox, but not connected to the car stereo?
Without the connection to the stereo, there is no noise.

Is it possible I've got a ground loop problem? Could I isolate the circuit with a balun with the audio cable to the stereo?
 
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af5rn

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1,059
Location
N. Tex / S. Fla
I have a feeling it may be those plastic/nylon stand-offs you are using on your antenna mount. You're losing ground with those and I bet that is affecting you. As an experiment, remove those stand-offs, open your tool box, and mount the antenna and bracket directly to the box and then go check for the noise again. If it's gone, you've found your problem with little effort. If it's there, then you'll have to keep looking.

Where does your power wire run, and what size wire are you using for it?
 

Don_Burke

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Messages
1,180
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jon_k said:
Without the connection to the stereo, there is no noise.

Is it possible I've got a ground loop problem? Could I isolate the circuit with a balun with the audio cable to the stereo?
Ground loop would be my first guess, although af5rn may be onto something.

I would try a jumper to make sure the antenna mount is grounded.

You could use an isolation transformer in that audio connection to beat a ground loop, if that is what the problem really is.
 

jon_k

Member
Joined
May 7, 2008
Messages
269
Location
Fort Worth, Republic of Texas
af5rn said:
I have a feeling it may be those plastic/nylon stand-offs you are using on your antenna mount. You're losing ground with those and I bet that is affecting you. As an experiment, remove those stand-offs, open your tool box, and mount the antenna and bracket directly to the box and then go check for the noise again. If it's gone, you've found your problem with little effort. If it's there, then you'll have to keep looking.

Where does your power wire run, and what size wire are you using for it?
Power wire is isolated. It's a handheld scanner I'm using at the moment with batteries, so any suspicion of scanner power being the culprit aren't counted.

The noise must be a ground loop of some sort. The only two items is 1) the Antenna 2) The car radio. They are grounded in separate locations and the difference in potential must be leaking in somewhere (probably from antenna.)

The plastic standoffs could very well be the source of a flakey ground connection. I will try mounting directly to the toolbox and see if this helps. I'll also try grounding the base of the antenna mount to the metal shell of the car radio, which should remove any ground loop since the antenna and vehicle radio will have same ground potential.

We'll see what happens. I'll give it a shot tomorrow and report back here.
 
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