Engine RPM noise on CB radio

[secretspy711]

You can do the exact same test but instead of using two whips as a dipole, take about 6in of insulated wire, wrap it into 2 or 3 small turns and attach the wire across the coax connector center pin and ground. You then have a small pickup loop that you can probe around the engine compartment and hopefully pinpoint the specific noise maker.
prcguy

I actually tried that earlier and was not able to pick up any noise...

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[Fowler]

I have the same problem with my old Datsun 620. If its popping noise changes with engine RPM its ignition. I changed the spark plugs (making sure they were resistor plugs and no cracks in the ceramic) Changed the wires with Magnecor wires. New distributor cap, coil ...still no joy...Finally looked at the wiring from the coil and found the wire going to the Tach. The tach uses pulses from the coil to determine RPM....The molex connector on the tach had dried and rotted causing a bad (arcing) connection to the coil!. Replace connector..no noise! YMMV

 

[FiveFilter]

Wow Fowler, great detective work!

 

[secretspy711]

OK, I might be finally getting somewhere in determining the source... sort of.

I decided to re-do the test with the sniffer coil (aka H-field probe), except this time use an isolated power source.

Long story short, I'm still not able to pick up any noise with this H-field coil with leads attached to the center conductor and shield. However, if I disconnect the shield part of it, which I believe makes it an E-field probe, I'm able to pick up noise all over the engine compartment.

An interesting finding is that there were 2 distinct types of noises. When I put this new probe near the alternator, it's definitely a "whine" noise. Same thing near the water pump behind the bumper. But when I put it near the battery, or on top of the manifold, or virtually any other piece of metal, it's a buzz type of sound, or maybe a "fast popping" at about 60 Hz. If I put everything back together and listen to the noise on the CB more carefully, I can hear both types of noise at the same time.

I also noticed that if I took the coax and laid it across the engine, or along the fender, with my new "E field probe" attached to only the center conductor and held somewhat away from the engine compartment, I can pick up some noise that way too.

So, with that info, what can I do??

I had another ham friend helping me this weekend (before I discovered the above info), and he asked his other Jeep friend, who said that the CAN BUS system in modern vehicles actually sends signals through ground. Could this be what's happening? I was only able to find one reference on the web to such a thing, which described a "Digifant engine management system" for Audi's that switches the ground on and off to control the duration of the fuel injectors.

 

[mtindor]

When I was a young punk with a CB in my car, I always enjoyed being able to "hear" my car through the CB as it accelerated, since my father never allowed me to throw glasspacks or cherry bombs on it.

M

 

[secretspy711]

Haha, well, maybe when I was a kid I would've enjoyed that too, but now it's just plain frustrating!

I recorded the buzz noise with my phone and analyzed the audio in some video editing software by slowing it down and determining the time span for 10 peaks. I came up with 34Hz.

I did some more math. The idle speed is 700rpm. There are 6 cylinders. This equates to a firing rate of 4200 firings/minute, or... 35Hz!

 

[secretspy711]

For clarification. that should have been 4200 strokes per minute. Since the spark plugs only fire every 2 cycles of each piston, you have to do (4200 strokes/min) / (60 seconds/min) / 2 (strokes/spark) = 35 sparks/second, or 35Hz

 

[mtindor]

Haha, well, maybe when I was a kid I would've enjoyed that too, but now it's just plain frustrating!

I recorded the buzz noise with my phone and analyzed the audio in some video editing software by slowing it down and determining the time span for 10 peaks. I came up with 34Hz.

I did some more math. The idle speed is 700rpm. There are 6 cylinders. This equates to a firing rate of 4200 firings/minute, or... 35Hz!

I have to admit that was some pretty clever analyzing. Good for you, and good luck. I have a feeling you'll spend a little money, replace one or two things and will be good to go.

Mike

 

[secretspy711]

Here's a video to show what I'm hearing, with the probe, using a completely separate battery. It's the same noise I'm hearing when everything is re-connected to the jeep and using the firestik antenna on the tailgate.

https://www.youtube.com/watch?v=EDE4FubBt78

 

[KC4RAF]

Man, that is some serious noise! That type of interference won't give any leeway for some humor, (like buy a new vehicle).
Fuel injection is the only thing that comes to my mind, (very limited info up there).

 

[secretspy711]

So this problem definitely seems like radiated RF noise, but just for grins I did 3 things yesterday:

1.) re-made the power cable with 10 gauge wire. I left a connector close to the CB for ease of removal, but replaced the tamaya connector with an XT60 which is rated for 60 amps. Also put a heavy duty fuse holder (with a 3 amp fuse) close to the battery instead of close to the CB.

2.) Re-routed the cable to go behind the battery, down the back of the fender, along the frame and up through a drain hole. Stu-offroad said this was the fastest way to get the wire to exit the engine compartment. This left more excess wire than I intended but allowed me to do #3 below.

3.) Coiled the wire several times around a rather large mix-31snap on choke. This coil lives under the passenger seat. The CB itself is mounted to the side of the footwell.

I think it helped a little, but didn't eliminate it. This morning on my way to work I could leave the squelch on 4/7 instead of the usual 5/7 without having the noise break through. If I could get it down to 3/7 I'd be happy. With the engine off I can put it on 2/7.

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[secretspy711]

Went to a CB shop today. Mostly a waste of time...

There was a very nice elderly fellow working there. At first he said "this isn't a CB radio." Guess he's not seen some of the newer ones. After I convinced him it was, he hooked it up to his bench, looked at the modulation on a scope, checking the power output (4 watts as it should be), and did a voice check both ways. Said it seemed to be working normally. I told him I was getting engine noise on it, and also that I could hear noise from power lines while driving to the shop. He seemed stunned and said he had absolutely no idea why it would be doing that. Of course there was no noise when he was testing it on the bench.

When I took it back out to the Jeep and hooked it back up, it was immediately flooded with noise, even before I turned on the engine. I assume this was from the overhead power lines. As I drove away it got better, but then the engine noise became dominant again. The fact that there was absolutely no noise when he had it hooked up to the bench leads me to believe I just don't have a good enough ground plane...

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[secretspy711]

I borrowed a DSO Quad oscilloscope from work and hooked it up at the alternator.

Not sure what settings are "normal" for diagnosing an alternator problem, but when I set the time division very short (50 microseconds) and set it to AC, I pick up the spike shown in the picture. I wonder if this has something to do with it.

 

[popnokick]

Re-read this entire thread and noticed two things:
1) The pic of your antenna and mount... and the rust / corrosion at the base and near the connections
2) I could not see that you have tried another antenna. And by that I mean not only another antenna, but also a completely separate coax run and connectors as well. Should be easy to do, even if only with a K40 mag mount or similar mobile antenna stuck on the hood or somewhere in front of the windshield. If something improves (less Rx noise) then the noise is starting to point to your regular antenna as the problem... and as you noted, maybe not having a proper ground plane (although not sure if Firestiks need one). But the mag mount is easy to try and may reveal something. If there is improvement then think about a 102" whip on a body-side ball mount. Or a hood lip mount bracket.

 

[secretspy711]

Re-read this entire thread and noticed two things:
1) The pic of your antenna and mount... and the rust / corrosion at the base and near the connections
2) I could not see that you have tried another antenna. And by that I mean not only another antenna, but also a completely separate coax run and connectors as well. Should be easy to do, even if only with a K40 mag mount or similar mobile antenna stuck on the hood or somewhere in front of the windshield. If something improves (less Rx noise) then the noise is starting to point to your regular antenna as the problem... and as you noted, maybe not having a proper ground plane (although not sure if Firestiks need one). But the mag mount is easy to try and may reveal something. If there is improvement then think about a 102" whip on a body-side ball mount. Or a hood lip mount bracket.

The corrosion around the connections is just that... *around* the connections. Under the bolt heads and washers is fine. A couple days ago I removed the entire mount, wire-brushed everything and put it back together, but no improvement.

I have 2 firestik antennas (one is a FL, the other an FS) and the issue occurs with both. I have a 3rd antenna, a Wilson Flex, and I am not able to get the SWR low enough to use it at this location. I ordered a fender mount bracket and will try again at that location.

I have a length of RG8X coax that I ran straight from the CB, out the back window to the antenna mount. No improvement.

I just came back in from trying my old $30 Uniden 505, hooked up to vehicle power. With the engine off, I adjusted the squelch to just barely make the radio silent to atmospheric noise. Then I started the engine, and I hear some noise but it's more like static. I barely have to touch the squelch knob to make it go away. If I take the squelch back down and listen to the audio very carefully, I think I can hear the same engine noise over the static, but it's nowhere near as bad as the $150 CMX760 is. Of course, I have no idea how the receive range of these 2 radios compare. It could be that the 760 is just much more sensitive I suppose.

 

[secretspy711]

I should note that both of my firestik's are 3-foot antennas, and the wilson flex is 4-foot, so theoretically it should be better. Weird that I can't get the wilson to tune properly.

According to firestik, both the FS and FL require a ground plane, as does the Wilson.

 

[secretspy711]

Went back and read the reviews on the 760 on Amazon. A couple of them say the performance is not what they expected. One person says:

"receive is too noisy, even with nb/anl on, you still have to turn up squelch so high you cant receive transmissions .... overall, good idea, extremely bad design. was using in city for test. might work in the middle of nowhere without any type of electrical utilities present. but still prone to rf interference, almost like there isnt any shielding in handset at all."

I'll try the fender mount, but if that doesn't work, I think I'm done trying to solve this and will probably try to sell it and get the Cobra 75WXST or something else.

 

[secretspy711]

For completeness, here is what it sounds like while driving.

https://www.youtube.com/watch?v=JErhOxpkWHA

 

[mmckenna]

Went back and read the reviews on the 760 on Amazon. A couple of them say the performance is not what they expected. One person says:

"receive is too noisy, even with nb/anl on, you still have to turn up squelch so high you cant receive transmissions .... overall, good idea, extremely bad design. was using in city for test. might work in the middle of nowhere without any type of electrical utilities present. but still prone to rf interference, almost like there isnt any shielding in handset at all."

Take Amazon reviews with a grain or two of salt. Especially with consumer radios.

I'll try the fender mount, but if that doesn't work, I think I'm done trying to solve this and will probably try to sell it and get the Cobra 75WXST or something else.

Would be interesting to see if moving the antenna closer to the likely noise source would actually improve things.

I have the CMX-560 in my F150. None of the other radios have any issues with ignition or other noise. The CB picks up a tiny bit of noise. The radio is grounded, power from the battery, permanent mount NMO antenna on the roof, antenna tuned right on the mark, no corrosion, good coax, everything done right. I think that maybe these radios are missing some filtering. If your other CB works better, I think that's your answer.

 

[secretspy711]

I installed the fender mount, routed the coax temporarily through the window, and went for a drive.

Hard to say if it's an improvement or not. I feel like maybe there's less "buzz" but more "whine"? Still have to keep the squelch on at least 4 when the engine is running. I feel like ANL and Hi Cut are more effective than before, but I think my conclusion is that this radio just does not have very good filtering.

I noticed that when driving around my neighborhood it seems to be worse than when I'm out on the main roads. It seems to be especially bad when driving between 2 houses directly across from each other that are close to the curb, as if the walls of the houses are reflecting the noise my jeep is emitting.

I tried tuning my Wilson Flex antenna at this location and was able to get 2.0 across all channels. Not quite as low as the Firestiks but it's a flatter curve.

Not sure how I feel about having the antenna in my field of view. The firestik's just barely touch the garage door. The Wilson hits it quite a bit but seems to take it just fine.

I may install the CB in a friend's jeep on Monday just for grins. His jeep is nearly identical, just 1 year older than mine. If the noise is still there, then I think I'll be looking for a different radio. If it goes away, well then I might just bang my head against a brick wall.