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CB Radio RF Noise Suppression
 

The following dialogue occurred with John, a JU member that I was exchanging on-line posts with while he was attempting to reduce the RF noise in his TJ when he was installing his CB radio.  With his permission, I am including it here since I believe it is both valuable info and it gives others a glimpse at just what can be involved in trying to clean up a noisy RF environment in a vehicle.  Considering that entire books have been dedicated to the subject, this is just a short review of the things that were done.  Please note that some of this is as much magic as it is science....by that, I mean that what may work great for me does absolutely nothing for you and your Jeep's CB radio.....or worse yet, you do it and it makes things worse.  There are so many variables in the equation that you must understand that any one or a combination of these tricks (or others not listed here) may be just the cure for your noisy radio.


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John,

There are three main sources of noise on most vehicles that enter via the antenna.

#1. Fuel system - mainly the electric fuel pump and the injectors (the pump is easily detected as it causes the noise when you turn the ignition key to ON (not Start) and then if it is like most pumps, it will pressurize the fuel lines and the cut off).

I have seen people use bypass capacitors on the power leads for the pump, near the point where the pump wires enter the gas tank. I have heard of some vehicle companies providing RFI (Radio Frequency Interference) filters to car owners who have severe problems.

#2. Ignition system - the spark plug wires are like little antennas with a spark gap transmitter hooked to them.

I have read of people taking the braid off of coaxial cable and making sleeves for the spark plug cables, etc. MUCH work and no guaranteed results. I have heard that some HAMS have had good luck with some very pricey ignition cables....$100+ for a set of wires but they have suppression built into them and the owners claim they worked well. Sorry, I don't know the name but the search engines on the web should help here.

#3. Alternator - usually detected in the radio as a whistle or whine and follows the engine speed. Commonly enters the radio via the power leads but can enter via the antenna too. This does not appear to be your source of noise.


Concerning #2 above....I have had good luck reducing the ignition noise in my TJ by grounding the exhaust system. I put a ground strap (the shield from large size coax cable....but any good copper braid would work) about 10" back from the end of the tail pipe. I used a stainless steel radiator hose clamp to attach it to the exhaust pipe after thoroughly wire brushing that area of the exhaust pipe nice and shiny. The other end of the copper braid was attached to the frame.....wire brush it clean too and screw it down tight. I soldered a terminal lug on this end of the copper braid.

Well, that is about all I can pass on. There are books available at ham radio stores that deal specifically with vehicle RFI reduction, if you get serious about it. They will no doubt have more ideas to try than I can remember here.

Good luck!

Edit: John, do you have a single coil system or one of the new multi-coil ignition systems?

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2002 TJ sport.

I have a multiple coil direct fire system.

I have tried a noise supressor / capacitor on the alternator. No difference. Not a whole lot I can do about the injectors or the coils noise.

I will try to check out the fuel pump noise. Maybe I can get that sort of under control. I will also give the exhaust a shot.

The noise is *definately* proportional to engine RPM.

Edit:
I think the fuel pump is the culprit. I crawled under there while the engine is running and can hear a high-pitched whine. I can't imagine that is friendly to the CB radio.

Now, how to get rid of fuel pump noise.

There are a bunch of wires, which ones should I get put the capacitor on? I still don't have a FSM, that was on the list for this month.

Best regards

John

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Stu, thanks for the links. I checked them out and followed some of the directions.

I made myself a little sniffer out of my other piece of coax. I tried tests with both my battery and another car battery in a different car all together.

If I ground both ends of the shield while hooked up to another car battery, the radio is silent until I get the sniffer end close to the exhaust tip, top of the hood, or the rear door of the Jeep. I will get some ground straps to see about that. It cannot hurt.

When hooked up to my battery with the engine running, I get tons of noise no matter whether or not the coax is hooked up. The noise is louder with the coax hooked up, but the noise is still very loud without it being hooked up.

I have a cheapo noise filter, but I really do not think it is up to the task of power cleaning duties for a CB. I probably need to get a better one from a HAM shop or some place like that. Recommendations of a good quality line filter?

That under-hood area is *extremely* noisy. I may have to see about adding a little capacitor on top of the Jeep battery as an experiment.

I also tried out some aluminum foil on top of the direct-fire ignition and another piece over the injectors. It seemed to help even out some of the noise -- but not eliminate it.

Anyway, by itself, I am just getting too much noise in through the battery connections/ underhood area for the Cb to be much use except close on the trails. Once I have that sorted out, I will try quieting down other things too.

If you have any ideas, please post them for me. I have tried every idea I have been given, and I am closer now than I was. Now, I know which direction I need to go first.

Thanks for all the help.

Best regards

John

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Ok, got the filters in, no more dirty power. Now, I have a big honkin radio shack beast with the little cheapo piggy-backing in series. Mmmmm quiet power -- but only with a good routing of the power wire and a short ground.

Power wire:
The faster you can get it out of the engine compartment, the better, but hooking up to a power lead out of the fuse-box is bad like in the engine compartment. The direct fire ignition and fuel injectors makes gigantic noise. It is happiest with a length of RG-8 cable running the power. The quietest reasonable routing I could come up with was down behind the battery, along the floorboard, and back up through one of the drain holes. The shield is best left un-grounded -- It seems to cut down on the noise. It is much better than regular wire -- but only when the shield is not connected to anything. Weird.

Ground strap + muffler clamp on the exhaust pipe tip end seems to make it happy. No noise in the sniffer once that is hooked up. Good call Stu.

Fuel pump noise -- terrible with rear door open. Rear door shut seems to cut most of that noise -- though I still get a little hi-pitched whine. I have a 470pf capacitor to put on the fuel pump leads. I am going to run out the gas tank and then drop the tank and put in the cap on the pump. It can't hurt.

Ground location -- Cleaned off all the engine, body, and head ground locations. Brushed them off real good and hooked the wires and straps back up. That did seem to help some too. CB is happiest with a close ground to the front roll bar close to the CB mounting. It is *very* unhappy with a direct battery or underhood ground -- even when the lead is exiting the engine compartment quickly.

One point of weirdness that I found. When I have the sniffer coax hooked up...... For some odd reason, it is happy when the center lead of a length of shielded TV (rg-6u) coax is hooked up to the body ground point. Only 1 end though, and without the shield grounded. A jumper cable worked too -- hooked up on only 1 end to the CB ground point. Neither regular wire, nor RG-8A worked so well, though. I will try a cheapo spark plug wire tomorrow. That makes me think I have some weird ground plane issues -- and not real sure what to do about that. Food for thought for you HAMs out there. Maybe it will ring a bell with someone.

Overall, this is a real learning experience, but knowing what I know now, I would have bought a *much* more expensive CB with good noise supression. I have probably paid close to the cost of the CB again over in misc stuff to get it working right.

Thanks for the help. If anyone has some ideas, or any of this rings a bell, give me a suggestion.

Best regards

John

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Hey all

Finished up grounding the frame directly to the battery today. Yesterday night, I moved the ground strap that connects the head, hood, and body to connect directly to the battery/body connection.

That quieted down the engine noise *substantially*

Now, the major noise source is a high-pitched whine from the electric fuel pump. To give you an idea, I could not run the gain up enough to pick up the fuel pump before -- I was getting so much engine noise radiating through *everything*else that it drowned out everything including the incoming signal.

I will try out the 470pf ceramic disc capacitor trick on the fuel pump leads to see if it helps some.

This is a totally different CB radio. Still cheap and I would not buy this model again, but I have made *considerable* progress from the beginning.

I will put money on the fact that the Cell phone head-set will work much better too.

Best regards

John

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Hey all

Thanks for all the suggestions, links, and moral support.

Finally got the 0.1 micro-farad capacitor in the fuel pump power lead.

The final piece of the puzzle.

This thing is *much* better with the engine running.

I can actually hear things with the gain all the way up and the squelch all the way down *other than noise*

I still have a *little* engine noise and fuel pump noise -- but only with the gain all the way up and the squelch all the way down. but I am happy to say I am done. Now, I have to turn down the gain a little to get rid of the power line noise and stuff like that.

Best regards

John

 

 

 

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