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Related pages:
mobile antennas
Loading coils Field
Strength Measurements
Repairing Dash Gauges
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 new 2003 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.
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:
- I do not want to drill holes in a new truck
- I have no topper or camper shell to mount the antenna to
- I want maximum performance
- 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 looked to be about 7 gauge. In order to make the mount, I went though
the following process:
- bent two angle-sticks of metal that match the rectangular dimensions of
the stake hole
- TIG welded a 3/4inch nut inside one L to match the height of the existing
Ford bolt hole
- TIG welded the L’s into a rectangular tube
- TIG welded a few sharp points on the tube near the bolt hole to break the
paint for a ground connection
- bent a wide long plate into an L that matched the size of a Tarheel
mount
- added holes to match the Tarheel plate
- TIG welded the bent plate to the end of the tube
- added gussets to brace the bend, since that was a weak area that allowed
the antenna to flop around too much
- cut a 4″ square load spreader out of 3/16 inch thick scrap stainless
- drilled a hole in the load spreader plate
- 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
under the single
mounting bolt. I feed the antenna with
the single clear
insulated wire that
goes through a hole
in the mount. That
wire attaches to the lug normally used to mount the
little impedance correction coil
on the Tar Heel
antenna.
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
I always thought a diesel would be quiet for electrical noises. When the antenna was installed, I had S-7 injector noise on my IC-706.
I also had
interference into
the EEC (electronic
engine control). The fuel injection system has solenoids that, when they open,
create a very high voltage pulse. When the EEC (electronic engine control) opens
the solenoids, the spike is many hundreds of volts. This in normal back-pulse
that occurs in any inductor, relay, or solenoid when the holding or coil
operating current is removed. When the EEC opens the circuit for an injector
solenoid, all the energy stored in the magnetic field of that solenoid coil is
released at once. This makes a very sharp rise time high voltage pulse that is
directly coupled into the injector harness.
The Ford harness has a shield inside, but the shield is mostly useless. It is
grounded only at the EEC, so it really doesn’t shield anything very well. It
probably minimally serves some purpose, but is ineffective at suppressing radio
signals.
One might think we can eliminate the pulse. Normally we would clamp a spike
like that with diodes, or an R/C snubbing network. I was afraid to try that
because anything that clamps or snubs the pulse will also slow the release time
of the electromagnetic device. This would increase the on-time of the injector
control coils. A capacitor would also increase the turn-on pulse current of the
devices driving the injector control lines. I just didn’t want to experiment
with anything that could damage the EEC.
I did do a modification I used in my 1995 F250 7.3 L Powerstroke. I ran a
small tinned buss wire against the aluminum foil shield in the injector harness,
and wrapped it to hold it against the shield. I then grounded the shields at
multiple points. In my 1995, this was enough to cure radio noise.
I
cured all the RFI
problems in my 2003 with the above, and the following
actions:
I removed the Tar-heel motor line choke bead because
impedance was far too low.
The stock choke was
so small the motor
lead was receiving
signals and noise, and
the motor control
lead was bringing RF and noise back into the
cab. I used my own jumbo-size 44 mix bead
with lots of turns.
You can see it in
the picture. I also
ran well-grounded
shielded control
wire from the exit
point of the bed
into the cab.
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
one or two points. 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
or transmission line out of the truck frame and cab of the truck!
Antenna ground currents excite all the wiring under the vehicle, increase ground losses
in
the antenna system, and increase noise ingress from truck wiring
back into the radio system.
To cure common mode
RF 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
(self-tapping) fine
thread (1/4-20) screws.
To punch through the
paint I used stainless external tooth star
washers under the braid in the tinned area.
Stainless prevents
corrosion or
interaction with the
solder on the braid.
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 means the
grounding
substantially reduced
undesired radiation from the truck’s body
and frame.
Noise levels dropped 2-3 S units
RF in truck wiring dropped substantially,
I no longer saw
gauges move or had
rough idle when
transmitting (even
with over a
kilowatt!).
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
exit the engine and
connect
to other wiring. This lets
the injector module “push”
RF
against the wiring in the truck, making the poorly grounded engine block pump up
and down with
injector high
voltage pulses. 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.
The opposite is also
true, the tailpipe
can act like an
antenna and excite
the sensitive
Electronic Engine
Controls.
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
of the Power Stroke, the
fuel tank and transfer case
are in the background. The
new ground strap
attaches to the body under a nut
used for the exhaust hanger. As before, I tinned the
braid ends and used stainless star
washers to help maintain connections
and to prevent
direct contact
between the tinned
strap and the steel
body. I clamped the
exhaust end of the
flexible strap under the
factory exhaust joint clamp.
This is a stainless
steel exhaust system
with stainless U
bolts.
I now have absolutely no noise at all from the diesel engine on any band,
and transmitter RF
stays out of my
engine
controls…even at
the kilowatt level.
 since July 5 2004
©2004 W8JI
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