[Milsurplus] Radio interference

[gl4d21a at juno.com]

Mark:

Step 1.  Find a copy of the CQ Mobile Radio handbook from the 50s and 
read it carefully.  It contains the basic pointers about what to do 
or not to do.

Step 2 Determine the culprit.  Ignition, generator or regulator.  
They may all three be noisy, but you need to fix them one at a time, 
because noise from one may mask the other when you fix it.  

Disconnect the output wire at the generators and run the engine.  Any 
noise will be due to ignition.  What people don't understand is that 
the noise is initially radiated by current loops in both the primary 
and secondary wiring.  

Step 3 - Ignition.  Place bypasses so the noise current path is 
shortest.  Use shielded wire from the points to the coil.  Make sure 
the distributor and coil are well grounded for RF.  The engine will 
run with a layer of paint under the coil mounting strap, for example, 
but the spark there will radiate lots of noise.  Make sure the bypass 
capacitor on the battery terminal of the coil is a good HF RF 
bypass.  The factory caps in passenger cars are optimized for MW, and 
are often poor at HF.  The shielded ignotion system may be making 
things worse if the HT insulation is not good, or the terminal joints 
are sparking.  Use GM 8 mm HT wire or Teflon coax center conductor if 
it will fit inside the braid shield.  Minimize the rotor gap in the 
distributor.  Make sure the distributor cap is clean inside and out.

Step 4. With the ignition cleaned up, check the generator.  Not the 
regulator.  In your case, one system at a time.  Connect the 
generator output, and disconnect the field.  Start the engine, and 
with no field excitation, the generator should put out no DC or 
noise.  If it does, repair/replace the generator.  There are two 
possibilities for generator design, and I forget which was used in 
the Jeep.  No matter anyway.  With the engine running, briefly jumper 
the field to A. Ground, and B. the armature terminal.  One connection 
will do nothing, the other will create full generator output, so 
don't do either for more than a few seconds to see if noise is 
generated.  Bypassing the armature terminal with one of the Sprague 
inline capacitors will cure generator noise.  It must be installed 
right at the generator terminal.  

Step 5. Regulator.  Reconnect everything as normal.  If the regulator 
is making noise, it will delay a very brief time after starting the 
engine.  Here, I again refer to the CQ Mobile handbook, as it has a 
very good illustration of what and how to install the necessary 
bypasses at the regulator(s).

Conclusion:  Keeping the noise bottled up at each source renders all 
the "tricks" like extra engine bonding, hood bonding, tailpipe 
bonding, etc. superfluous.  With an old Jeep, this approach is 
possible.  With modern automobiles and all the electronic sensors, 
computers and so forth, trying to get a handle on engine noise is 
much more elusive.

I will end with the following anecdote.  Back in the 60s, I resolved 
to fix my engine noise problems by making as much under the hood 
solid state as I could.  Ignition, alternator regulator, whatever.  I 
ended up with what I thought was ignition noise.  One pop per spark.  
After chasing that for a long while, I discovered the ignition noise 
went away when I disconnected the voltage regulator!  Each time the 
ignition put a puse of DC on the coil, the regulator responded 
instantly by turning on the alternator field.  I had used too fast 
transistors, and they were commutating the field current at the 
ignition rate with a high speed transient.  A few ferrites and a 
couple of bypass caps on the regulator fixed the "ignition noise".  
Moral?  Be sure of what you are fixing before you try to fix it.

HTH
73,
George
W5VPQ


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