Antenna coax shield ground loop problem

[515]

I've recently installed a RadioShack PRO-2096 scanner in my 2001 Silverado Duramax, and it appears I've got a bit of a ground loop problem with the antenna coax shield, resulting in pretty bad alternator whine noise on the scanner's receive audio.

The scanner has actually been in the truck several months using a mag mount antenna, and it worked fine with that... But when I installed the permanent NMO mount antenna on the roof, I get alternator whine from the scanner speaker, squelched or un-squelched.

As soon as I disconnect the permanent mount antenna from the scanner, the whine goes away completely. I've also have a VHF dash mount and UHF remote mount Spectra installed in this truck using the same power source (direct from the battery), also with rooftop NMO antennas, and they have always worked great without a trace of alternator whine...

I've got good DC continuity between the shields on all three NMO mounts on the roof, and good DC continuity between any NMO mount shield and the mounting bracket inside the cab with all antennas disconnected from the radios.

The scanner and Spectras are basically grounded in three places: the DC power ground (direct to the battery), the mounting bracket, and the antenna coax shield.

Disconnecting the negative power lead to the scanner and just letting it ground through the mounting bracket and coax shield reduce the alternator whine some, but it's still there... Connecting the scanner's negative power lead to a ground point near the mounting bracket doesn't make any additional improvement.

I'm sure reinstalling the BNC connector on the antenna coax without connecting the shield would eliminate the whine, but I'd rather not do that. It is just a scanner, though... Any thoughts?

Would a DC power filter on the scanner's power lead help at all? If so, what filter would work well?

I'm not sure how I'd do it, but would installing some kind of ground strap/cable connecting the roof to the rest of the body be likely to help?

Any other suggestions?

[Jim202]

There are a number of reasons your getting the alternator
whine. First and foremost is that most scanners have about
as cheap of a power supply system in them as they can make
it. There is little or no filtering that was used to prevent
problems like this.

You could try installing an inline DC filter on the +12 volts going
to the scanner.

You could try installing some RF bypass caps inside the scanner.

You could try installing some more DC power filter caps (large)
inside the scanner.

You could check your alternator for possible failure of one or
more of the diodes. Use a digital voltmeter for all measurements.

Best way to check the alternator is to measure the battery
voltage before you start the engine. It should be some place
around 12.2 to 12.6 volts or so. Start the engine with no
electrical load. Measure the voltage at the battery terminals.
You should see 13.8 to 14.5 volts.

Turn on the headlights on low beam. Measure the voltage
again. It shouldn't have dropped very much. Turn on the high
beams. Check the voltage again. You might see some drop.
It should stilll stay above the 13.6 volt point.

Next turn on the heater fan on high. Again look at the voltage.

Last trun on the wipers. If your idle is set to the right RPM and
the alternator is working correctly, you should still maintain a
battery terminal voltage above the 13.6 volt point. If it has
dropped down, see if just a slight increase in engine RPM
doesn't bring it back up.

Now while under this max load, change the voltmeter to the
AC volts scale. Put it on the lowest range. I am looking for
millivolts of AC voltage. Again place the meter across the
battery terminals. You should see very little AC at the battery.
This would indicate a good operating alternator.

If you see much for AC voltage, I would look at replacing the
alternator. If the battery voltage doesn't maintain the 13.6 to
14.0 volts, replace the alternator.

Last of all, how old is your car battery? If it is near the 3 year
old mark, your on borrowed time. As a battery gets older, the
internal resistance goes up. As this happens, it becomes less
and less of a good filter to keep noise off the DC supply. It also
will fail you when you need it the most.

Cold weather is when old batteries show up the ghost the
most. A clue to their demise is the car clock starts to jump or
reset it's time when you start the engine. If you see this
happening, replace the battery before you get stranded.

Bottom line is go do your homework and locate the problem
to your noise. I would place my bets on a poor scanner design
first.

Jim

I've recently installed a RadioShack PRO-2096 scanner in my 2001 Silverado Duramax, and it appears I've got a bit of a ground loop problem with the antenna coax shield, resulting in pretty bad alternator whine noise on the scanner's receive audio.

Any other suggestions?

[Wowbagger]

First, connect JUST the coax ground to the scanner, and see if you hear the noise. It could be that you are actually receiving the noise, and that the presence or absence of the ground is not relevant to the problem.

Second, since the scanner grounds to the battery directly, verify the ground from the alternator to the battery - it may be that the battery ground to chassis is bad. Try running a ground braid from the alternator directly to the battery, rather than relying upon the ground path through the engine.

You may also check for a bad alternator to battery ground path using a headphone, and connecting one side to the alternator, and one side to the battery ground.

[thebigphish]

You may also check for a bad alternator to battery ground path using a headphone, and connecting one side to the alternator, and one side to the battery ground.

{thread hi-jack} Wowbagger, can you explain this a little? I've never heard of this trick before...sounds good!??

[Wowbagger]

You may also check for a bad alternator to battery ground path using a headphone, and connecting one side to the alternator, and one side to the battery ground.

{thread hi-jack} Wowbagger, can you explain this a little? I've never heard of this trick before...sounds good!??

OK, what you are looking for is a voltage drop between the alternator ground (which is electrically connected to the frame via the engine brackets and engine ground straps) and the battery ground terminal to which the radio is connected. If there is a bad ground from the battery to the frame, or from the frame to the engine, there will be a voltage potential between the battery ground and the alternator case.

Now, how to detect this? If you just put a voltmeter between the battery negative and the chassis, that will not show millivolt signals very well - plus, since those signals are likely to be short pulses, the sampling rate of the meter will be too short to really capture them.

Now, you *could* try to put an oscilloscope probe across the frame to battery negative, but then, unless your scope is battery powered, you are just introducing a whole new variable (the AC mains power) - not really what you need.

So, what we need is a device that:
a) can detect and respond to millivolt signals
b) can detect and respond to short pulses.
c) is not connected to any other circuit.
d) is relatively insensitive to RF.

Like a headphone. So, you take the ground side of the headphone, and connect it to frame ground. You take the tip of the headphone, and connect it to the battery negative terminal. You then listen to the headphone as you run the vehicle. You should hear nothing in the headphone - the two terminals should be very close in potential. If you hear noise, you know the two are NOT at the same potential.

You can then move the center of the headphone from the battery negative to the alternator frame. Again, this *should* be at the same potential as the frame, but if you have a bad engine ground it won't be.

You can also then test what happens if you take a heavy ground strap, and run it directly from the frame of the alternator to the battery negative.

[515]

Thanks for the responses.

Jim, I'd agree the scanner is pretty much consumer grade junk... Like I said, the Spectras that are wired the same have no alternator whine problems. So I'll probably end up trying to add some filtering.

Wowbagger, I get the noise by just touching the BNC antenna connector shell to the scanner chassis, so I'm not receiving it. Also, this noise appears to be affecting the scanner's audio amp more than anything else, as I hear it even when the scanner is squelched.

I'll try the headphone trick when I get a chance, but my guess is that the scanner is just lacking DC filtering.

[Tom in D.C.]

No one has mentioned installing ferrite beads on the
power cable to the scanner but that might work to
solve the problem either partially or completely.

I agree with the consensus that the design of the scanner
probably leaves a LOT to be desired when compared to
commercial receivers we're all accustomed to using.

[kb0nly]

I would guess that the scanner is more at fault than the vehicle and it's charging system.

I used to have a Bearcat scanner in the mobile, horrible noise on that thing no matter how i tried to cure it, bypass caps, extra grounding, ferrite beads for the power wires, etc.

I pulled that scanner out to further investigate and ended up buying an old Uniden MR8100, had one years ago and always loved it so when i saw a really nice, like new condition, one for sale on the fleabay i snatched it faster than a donut at the PD. (no insult intended guys, i'm guilty of eating plenty of those round pastries myself)

I put the MR8100 in the van, using the same antenna, same power source, etc.. And guess what? No noise. Scanner design is an issue since some are good and some are down right cheesy cheap.

I had a Pro 2051 in here last week, just helping the owner program it, looks a lot like the 2096 but i think internally it's fairly different. That scanner went nuts in the radio room with the computers running, you couldn't keep it squelched for nothing. But the MR8100 sitting on the test bench before install never cracked squelch and it's a super sensitive receiver, a scanner built back when quality still mattered i guess. That scanner now operates off a glass mount antenna, yeah not the best but it was quick and cheap and served the purpose for monitoring local stuff, and it still manages to pick up all the surrounding counties. For comparison i put the 2051 on the same antenna in the van and it didn't hear near as much, i even put a BNC T fitting in line and put both receivers on the same antenna, the 8100 would hear a lot of signals that the 2051 wasn't even hearing with the squelch open. Not very promising but it will serve the owners purpose using a base antenna to monitor local coms.

Anyway, i'm getting off subject a bit. Bottom line, it might just be the scanner itself if none of the other radios are exhibiting troubles with the noise in either their receive or transmit audio.

Here is one more thing you can try.. If you have a battery pack around to power the scanner while in the vehicle and connected to the antenna then you can find out if the noise is coming from the power source or the antenna. You mentioned that grounding the antenna side of the BNC allowed the noise to continue even without antenna, so it's probably power related, but still worth checking out.