RFI interference--How to eliminate???

ozzie · #1 (**permalink**) 08-25-2012, 4:39 PM

I recently purchased a new gas powered truck and find that after installing my radios I am getting RFI interference. It appears to be coming through the antenna and coax. The placed that installed the system determined this and determined that the spark plugs or coil pack are the culprit. I am looking for ideas on how to get rid of the noise. It is a snapping or popping static that does not seem to vary with RPM. I have a number of ferrites that I am willing to try. Where would these be placed--on the coax by the radio,by the antanna on the coax, or on the wires going to the coil pack. Is there possibly a better spark plug than the OEM. Any help is appreciated.

Chauffeur6 · #2 (**permalink**) 08-25-2012, 4:43 PM

Without knowing the details of what make/model truck it is, what spark plugs are currently installed, etc., it's tough to give definitive answers. A general suggestion would be to find out what plugs the truck takes, go to your local auto parts store, and see if they have a resistor model. NGK "R" seres plugs have resistors to prevent RFI.

Frequently Asked Questions

ozzie · #3 (**permalink**) 08-25-2012, 6:21 PM

The truck is a 2012 Ram 2500 with stock or OEM plugs.

zz0468 · #4 (**permalink**) 08-25-2012, 9:35 PM

Ignition noise from the spark plugs or distributor/electronic ignition will change with changes in engine RPM. There's a lot of information missing... radio type? Frequencies and modes you hear the noise on? Does it occur with the engine off? Does it occur with the ignition on, but the engine not started? Does it occur if you power the radios from a source external to the vehicle power? Ferrites are not effective on coax if the signal is coming in through the antenna.

ozzie · #5 (**permalink**) 08-25-2012, 10:27 PM

The radios are Vertex vx4204 on vhf. The mode is analog. When nthe truck is shut off the rfi goes away. No interference with just the ignition on only. We have not thus far tried to power from an external source. I actually did hear it vary with rpm but only once. The freq range in question is 154.00--155.00. We have the power direct to the battery and the ground to a grounding bolt in the base of the mounting console.

tomfassett · #6 (**permalink**) 08-26-2012, 3:24 AM

At that frequency, don't forget how dirty the power coming from the alternator is. It wouldn't hurt to put a filter on the power leads to the radio. Even in a truck that new, if you put a scope on the battery terminals you will see a huge amount of low frequency noise (I know--I tried it). Also, make sure you are well grounded. Finding the nearest piece of metal in a new vehicle is no longer a guarantee that you have the shortest path to the battery ground. The metal console frame of my Jeep has a ground strap to the system ground (directly from the battery) because everything else it attaches to is either plastic or some composite material and without that strap there is a pretty good resistance to the system ground. Just for giggles (and to start narrowing down what it isn't) I would run a wire directly from the radio ground plug to the negative on the battery terminal and see if the noise changes. For test purposes, you can run the wire out the open door and do this while the truck is sitting there idling.
Since a truck that new does not have the old points and rotor system, the most likely culprit (in the old days) is not a factor anymore. I would also try using shielded spark plug cables. That solved a similar problem I had years ago. Cheap or cracked spark plug cables can play havoc with VHF radios.

Good luck.

Tom F

jfhtm350 · #7 (**permalink**) 08-26-2012, 10:01 PM

Pull of the distributer cap if it has one and check it out. I had a problem with the carbon ball in the center of the cap that rides the rotor button. The ball wore out and was actually loose and fell out of the cap when I took it off. The carbon ball should not rotate any if you try to move it around with your fingers. If it does it will cause a bad interference issue from it rotating around with the rotor button. Actually I believe the ball was loose when I bought the cap new but I didnt know any better and didnt put a scanner in until later on.

zz0468 · #8 (**permalink**) 08-27-2012, 12:51 AM

Check the routing of the power and antenna cabling. Make sure it doesn't get near any ignition related wiring, the electronic ignition, or spark plug wires. Generally, running along the factory wiring bundles is ok. Check the operation of the vehicle's AM radio and see if that has interference as well. If so, there may be a warranty issue that the dealer can resolve. Manufacturers of new vehicles do take some care to insure that factory radios are interference free, so if the factory AM radio is experiencing interference, that might be an indication of something wrong with the vehicle that the dealer needs to repair.

ozzie · #9 (**permalink**) 08-30-2012, 7:02 PM

I checked the am radio and no RFI at all. How are they keeping RFI out of the radio?? This may be a place to start. Is it because of the freq used or does it have to do more with the radio or somthing on the antenna or cable. Also, if I want to make a ballun what would the diameter need to be and how many turns would I need to make? It would be for 154.000-155.000mhz.

tomfassett · #10 (**permalink**) 08-30-2012, 9:24 PM

I would assume that your business radio is FM if it is in the 154.000 MHz range.
Have you tried filtering the power to the radio? My problems with commercial radios in the past were almost always power related (especially with older radios). It is common for old capacitors inside the radio to "de-rate" (lose capacitance) over time and often the original filter caps on the power systems inside the radio just don't work as well as they did when new. Only once did I have to isolate the antenna from the vehicle ground.
Another thing to try is to hook up the radio and connect an external antenna (not connected in any way to the vehicle). Then try hooking up an external power source with the radio still in the vehicle. This way you could start narrowing down where the interference is getting into the radio. It is better to isolate each system one at a time to narrow down the culprit as opposed to taking the "shotgun" approach and start "Fixing" things until you know what the problem is.

Tom F

ozzie · #11 (**permalink**) 11-18-2012, 6:42 PM

I am continuing my quest to eliminate the RFI in my radios. I now know that the RFI is entering the radio through the antenna and not the power line. I am using double shielding on my coax. I have installed ferrites on my coax near the antenna and near the radio with no change.It appears that the cab is "charged" with RFI. When we use a quarter wave mag mount and move about a foot away from the cab the RFI disappears. As we move to the bed rail the RFI also goes away. I have recently put in a request with Chrysler to contact the EMI/RFI group for input. Any other suggestions.

jfhtm350 · #12 (**permalink**) 11-18-2012, 7:08 PM

Well did you check your distributor cap and rotor button? When is the last time you changed them?

Wait until night time, open the hood and crank the truck. If you have a leaking spark plug wire you will see the spark. Dont be touching the plug wires with your hands. Use a long piece of wood if you have to move the wires. It probably not the wires or your truck will be missing.

Like I said before I had severe interference in my truck mounted scanner becuase of a bad cap and no interference in my am/fm radio at all. That distributor cap wasnt all that old either

ozzie · #13 (**permalink**) 11-18-2012, 7:40 PM

This is a new truck with no cap or rotor. It has no spark plug wires either. It does however have a coil pack mounted on top of the spark plugs. Would it help to switch to platinum or iridium plugs possibly.

jfhtm350 · #14 (**permalink**) 11-18-2012, 8:43 PM

Quote:

Originally Posted by ozzie

This is a new truck with no cap or rotor. It has no spark plug wires either. It does however have a coil pack mounted on top of the spark plugs. Would it help to switch to platinum or iridium plugs possibly.

I really dont know if the plugs would help or not, but its possible.

majoco · #15 (**permalink**) 11-18-2012, 9:05 PM

Changing the plugs won't make any difference, if it was plugs or ignition noise, you'd hear it on the AM radio when tuned off station and it would vary widely in frequency with RPM.
.

Quote:

It is a snapping or popping static that does not seem to vary with RPM.

Is the antenna cable outer firmly connected to the vehicle ground at both ends and the radio firmly grounded too. Don't want any circulating current passing down the coaxial outer.

A thick wire joining the radio chassis to the vehicle frame may be required. Don't rely on any wiring under the dash to carry radio power or ground.

You could try making a direct connection for both power and ground from the radios right back to the battery so that only the radios are drawing current down the wire, not anything else that may be joined to a noise generator - windscreen wiper motors for instance.

I have had problems with electric fuel pumps making a noise that stays at a constant rate until you really boot it and the rate then increases.

ozzie · #16 (**permalink**) 11-19-2012, 7:01 PM

I have both the ground and the hot going directly to the battery.

k3cfc · #17 (**permalink**) 11-19-2012, 7:31 PM

Quote:

Originally Posted by ozzie

I recently purchased a new gas powered truck and find that after installing my radios I am getting RFI interference. It appears to be coming through the antenna and coax. The placed that installed the system determined this and determined that the spark plugs or coil pack are the culprit. I am looking for ideas on how to get rid of the noise. It is a snapping or popping static that does not seem to vary with RPM. I have a number of ferrites that I am willing to try. Where would these be placed--on the coax by the radio,by the antanna on the coax, or on the wires going to the coil pack. Is there possibly a better spark plug than the OEM. Any help is appreciated.

Well if it does not vary with rpm i guess you can leave out all the plugs wires coil alternator ..... what did i miss?

Missed the fuel pump. it's speed is constant.

W2NJS · #18 (**permalink**) 11-19-2012, 7:50 PM

So far I don't see that anyone has mentioned possible interference from the electric fuel pump, which is a common source of RFI.

k3cfc · #19 (**permalink**) 11-19-2012, 8:17 PM

Quote:

Originally Posted by W2NJS

So far I don't see that anyone has mentioned possible interference from the electric fuel pump, which is a common source of RFI.

I think you should look at all of the post. i just said about this at 6:38 pm this date.

ResQguy · #20 (**permalink**) 11-20-2012, 9:05 AM

Quote:

Originally Posted by ozzie

I have both the ground and the hot going directly to the battery.

That right there should be changed. Connect the ground to chassis ground as close to the radio as possible, not the battery. Connecting anything directly to the battery's negative terminal invites not only noise but a possible return path thru your radio should the main ground lead become loose.