RF Noise Powerstroke Diesel

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RFI measurements showed HF noise at a distance of 5 meters from my 2003 Power Stroke was 15-20dB less than my 1995 F-250. I never could get all the noise out of my 7.3L 1995 F-250 HD PSD. The measurements of noise field a small distance from the truck gave me great hope I could have a better HF mobile with minimal work. 

I  am delighted to report my 2003 F-250HD PowerStroke diesel has virtually no RFI with only very simple and fast corrections. Be aware this is for my antenna mounting location, and my style of truck. This is a new old-stock truck I found at a small rural dealer for a very good price!   

 

Antenna Mounting Location

I mounted the antenna in the stake hole of the bed that is located just behind the cab on the driver's side. I use this location because:

  1. I do not want to drill holes in a new truck
  2. I have no topper or camper shell to mount the antenna to
  3. I want maximum performance
  4. I would like to remove the mount if necessary

Antenna Mount

My antenna mount is home made. It is formed from a scrap sheet of stainless steel that looks to be about 7 gauge. In order to make the mount, I went though the following process:

  1. bent two angle-sticks of metal that match the rectangular dimensions of the stake hole
  2. TIG welded a 3/4inch nut inside one L to match the height of the existing Ford bolt hole
  3. TIG welded the L's into a rectangular tube
  4. TIG welded a few sharp points on the tube near the bolt hole to break the paint for a ground connection
  5. bent a wide long plate into an L that matched the size of a Tarheel mount 
  6. added holes to match the Tarheel plate
  7. TIG welded the bent plate to the end of the tube
  8. added gussets to brace the bend, since that was a weak area that allowed the antenna to flop around too much
  9. cut a 4" square load spreader out of 3/16 inch thick scrap stainless
  10. drilled a hole in the load spreader plate
  11. cut square of rubber as a buffer to prevent damage to the trick's bed rail top

 

You can see where I get my shield ground connection. I feed the antenna with the single wire to the lug normally used to mount the impedance correction coil. I mount my impedance correction coils on PL-259 plugs so I can change them easily. I screw them into the SO-239 connector normally used as a feed!  

 Noise Noise Noise

When the antenna was installed, I had S-7 injector noise on my IC-706. I cured it with the following:

I removed the Tarheel motor line choke bead because it was far too low an impedance. I was receiving on that lead, and it was bringing RF back into the cab, I use my own jumbo size 44 mix bead.

You will probably find most frame-type vehicles mount passenger compartments on rubber mounts to reduce noise and vibration. The cab is only grounded at the front. The bed is bolted solidly to the frame, and this means all RF currents must travel down the bed to the frame, follow the frame forward to the front of the cab, flow back along the cab towards the antenna. This actually makes a very short thick antenna out of the truck frame and cab of the truck! The common mode currents excite all the wiring under the vehicle, increase ground losses under the truck, and increase noise ingress from truck wiring into the antenna.

To cure the common mode current problems, I added a ground strap between the bed and the cab.

You can make the connection UNDER the chassis at the locations shown in the following picture:

   

Note that I heavily tin the edges of the 1" braid. I do this insure all of the braided wires stay fully in contact all of the time. The flex area must remain solder free.

I use stainless thread cutting screws, and stainless external tooth star washers under the braid in the tinned area to cut through the paint.

Adding this single strap made the following changes:

The base resistance of the antenna dropped from 45 ohms to 25 ohms on 7 MHz. This mean I greatly reduced radiation from the truck body or frame.

Noise level dropped 2-3 S units

RF in truck wiring dropped substantially  

Remaining Noise    

A small amount of noise remained audible after the grounding. I traced this noise to radiation from the exhaust system. The engine block was hot with RF, the injector control modules are mounted on the block and have leads that leave to other wiring in the truck. This lets the injector modules "push" against the wiring in the truck, making the poorly grounded engine block pump up and down with pulse voltages. Since the exhaust system bolts to the engine with a direct conductive path through the turbocharger, the tailpipe is actually excited just like an antenna.

I cured ALL of the remaining noise with the addition of one more ground strap between the exhaust and the frame just ahead of the muffler.

 

You can see the widening of the pipe as it expands into the muffler, with the fuel tank and transfer case in the background. The strap grounds under a nut used for the exhaust hanger. As before, I tin the leads and use stainless star washers to help maintain connections. Notice I clamped the strap under the factory exhaust joint clamp.

I now have absolutely no noise at all from the diesel engine on any band.

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