Mobile Noise Abatement

(HF Mobile - Vehicle Generated Noise)

by Stephen T. Reynolds WA4CNG

Wow how time goes by really fast. Here I am a General Class Ham for the second time in two centuries, and I am also HF mobile again. New rig, new amplifier, new antenna(s), new vehicle and all that goes with it. Back in the mid 1960's HF mobile presented the same problems as it does now, with the exception that getting rid of the Computer Mobile Generated Noise that is a bigger challenge than ever before.

Back in the 1960's we had a spark generated ignition system using an ignition coil triggered from a contact point driven by a rotary cam assembly on the top of the engine. To get rid of the noise back then you only had to bond the hood to the front quarter panels, ground the engine block, ground the tail pipe, put a coaxial capacitor in line with the alternator and install resistor plugs and resistor wires. Today we have 1-4 computer driven Black Boxes that determine everything from emissions of the engine to the inside comfort level of the vehicle that we travel in. These Black Boxes cause a great deal of Wideband Noise Blanketing Effect with or without the Noise Blanker in your radio and will cover up HF communications from 3-30MHZ.

Ok, where do we start to find and remove these Computer Noise Generators. First let's look at the known noise generating factors. The current vintage of vehicles produced in the last 10 years share the same group of issues.

Total electronic engine control from the ignition spark, fuel injection, fuel pump in the gas tank, and the monitoring of multiple sensors attached to the engine. All of these items must be tied together with multiple conductor wiring harnesses, none of which is shielded. The overall design of the vehicle amplifies these issues with the following items:

RG8 Coaxial Cable Shield stripped from RG8 Coaxial Cable, soldered on the ends, and attached with self-drilling sheet metal screws beats these Clips 10X. So much for the Engineers at the Big3.

Ok, so how come none of this affects your car radio, CD player, and other consumer items. Look at where these items operate. AM radio frequencies are below the computer frequencies, the FM radio will not here the AM noise, the CD player is often hardwired to the radio (shielded wires) and it uses Audio Frequencies. As a result it is relatively easy to not have problems with any of these items. However receiving equipment operating in the 3-30Mhz AM/SSB frequency spectrum is very different. We will hear any noise generated in the vehicle and a lot of it covers up those weak signals in the band. A quick test is to turn your HF rig on, tune a weak signal on 40/20 meters and then crank your engine. If the signal goes away and the S-Meter is moving in the S5-S9 area, you've got noise. Most often you crank the vehicle and then turn your HF radio on, so the noise is already there covering up the weak signals. Of course there is also the power line noise and other external man made noises that we encounter also, but usually these go away as we move away from the source. You stay the same distance to the noise generator in your vehicle and it does not go away.

Think you have a Noise Free Mobile. Take this test.

One way to get rid of this Hash is to go to Radio Shack â„¢ and get several packages, (start with a minimum of 3), of the Rectangular RF Chokes and do the following to the wires coming out of the Engine Control Computer. There may be up to three or more sets of wires coming out of the computer. One set controls the ignition, spark, and fuel injection. The second set is remote sensor inputs. The third set goes to your gas tank for the fuel pump system. This set of wires is the longest and runs under the vehicle and to the rear where the gas tank is located. You may find that if you add more chokes you have more reduction of noise. (I have a total of 18 chokes placed on the two harnesses coming out of my Engine Computer. The first group got rid of 1 S-unit of noise and the next two sets have reduced it down to where it is not a factor any more.)

Apply the Square Chokes around each group of wires that come out of the Engine Control Box. Start with a minimum of 3 sets of Chokes per group of wires. You can space them out, but putting them together in bunches works best. Doing this can reduce HF Noise Radiation by up to 30DB, that's 5-6 S-Units for you Non-Engineer types, and it will get rid of most if not all of the HF Computer hash Mobile Noise. Grounding your tail pipe will reduce additional engine noise in the 10/12 meter band as the tailpipe is a good ¼ wavelength at those frequencies. Ground it under the rear bumper area using an RG8 Ground Strap described below. Be sure to file away a good clean area on the tailpipe and the chassis area you will tie it to. You may need to drill a pilot hole for the self tapping sheet metal screw on the car frame.

You still have to do the Bonding on the Hood, and observe good DC/RF Installation practices (DC cable for the radio is tied directly to the Battery) for all mobile Installations.

Here is how to build RG8 bonding straps. You need several sections of RG8 cable. Get the best you can, and stay away from Radio Shack grade of coax. The length you need is long enough to go from the rear hinge side of your hood to the quarter panel below it. A typical length is 12-16 inches, the same length works for the tailpipe. Look at your engine and see where you could attach at least 1 ground strap from it to either of the front quarter panels, that length may be as long as 24 inches. Look at any bolt/nut assembly that attaches a component to the engine. Details on what and how are later after building the Grounding Straps.

Building the Grounding Straps

Bonding the hood. Use two straps, one on each side of the hood near the hinge. Be sure to use an area that is double walled so the screw does not come up through the top of the hood. File this area clean and shinny about 1 sq inch. Do the same on the top rear of the quarter panel below it. Using a ¼ inch drive self/tapping/drilling sheet metal screws about ½ inch long, put it thru the ground strap and drill it into the metal. Repeat for both sides of the hood. Next ground the engine using the longer strap and be careful to use an existing bolt on the motor for the engine end of the strap and use the adjacent area you used for the hood on which ever side of the engine you connected to. You may see an existing braided connection to the engine, (it's one of those clips) but add one anyway.

An additional step would be to bond the rear firewall to both quarter panels. Remember to be careful where you do any drilling into the vehicle body.

Well that about wraps it up for reducing Vehicular Computer Hash. Most vehicles of late vintage have good spark plugs, plug wires and most only require a tune-up at 100,000 miles. I now have no ignition noise on my 98 Ford Explorer. When the time comes for a tune-up I will be replacing the plugs, wires, and rotor cap with original factory equipment as it works well now and I do not need to play around with something that is already working.

This article only covered the engine control computer. This was the only unit that I found was radiating noise, and then only when the engine was cranked and running. It does not run if the ignition is on and the engine is NOT running. Another check can be done to check for motion generated noise. That is done rolling down the road and coasting your vehicle and turning the engine off. Care must be taken when turning your engine off because some if not all steering wheels may lock in the ignition off position. This type of test must be done under very controlled areas where you would not be a hazard to anyone and your vehicle is still controllable in the event the engine is stopped for a short period of time. This is how I tested my before and after measurements of the various procedures. Good luck and safe driving while operating HF Mobile.