Re: How to build a Faraday Cage?

Glen,
Unfortunately, electromagnetic interference (EMI) can be pretty difficult 
to troubleshoot.  I'll try to cover some basic troubleshooting techniques 
first, then go into the Faraday Cage.

First off, the inverter chassis should have passed UL (USA) or CE 
(European, and more stringent than UL) certification, part of which is 
adherence to EMI specifications, especially in the radio frequency 
spectrum.  There should be a sticker on the device stating such 
compliance.  You may want to check this to be sure, but I assume that it 
is certified, although it's puzzling that the interference is this bad.  
In any case the MOST IMPORTANT issue is to make sure that you have a good 
ground for the shielding/chassis.  Is there a ground screw or stud on the 
chassis of the inverter?  It will be marked with a green color, or a 
symbol looking like three stacked horizontal bars with a vertical bar 
sticking out of the top, or a symbol like a garden rake with 3 tines.  If 
so, make sure it is securely fixed with a wire to the metal body of the 
truck.  If not, then attach a wire from one of the chassis screws (or 
drill and tap your own connection, making sure not to damage the internal 
componenets) and create your own ground.  Also make sure that your CB and 
other radios have a similar ground connection as well.  When making ground 
connections to the chassis, make sure that the location of the connection 
has electrical contact with as large a part of the truck chassis or frame 
as possible.  Use a crimp-on (or solder if possible) lug and screw for a 
solid connection, and scape away paint around the attachment point for 
better electrical contact.

Next you might want to try to isolate the noise source- is there noise 
with the inverter plugged in and turned off?  With it on but no external 
appliance connections?  Plug in the appliances one at a time, making sure 
that they are drawing power (i.e., compressor on, computer on, etc).  Do 
they all create interference, or jsut some?  If they all do, then chances 
are it's the inverter.  If not, then it's possible that the appliance is 
the problem.  Try moving the power cords and antenna cable and see if the 
noise changes.  Perhaps you can find a way to route the cables to minimize 
the noise.

Then, you can try to shield the cables/wires with a flexible metal braid 
which should share the chasis ground.  Make sure that the antenna wire for 
the CB is a good quality coaxial cable and that the outer shield (ground 
braid is not frayed and makes good contact with the connectors.

A Faraday Cage or shield is simply a conducting (i.e., metal) enclosure 
which is grounded.  It can be made from any sheet metal, solid or 
perforated (but see the section on cooling holes below in this case).  The 
purpose of a faraday cage is to contain electromagnetic radiation by 
forcing a ground plane all the way around the source.  This way, the 
ground plane forces there to be a consistent ground potential at the 
surface of the faraday shield, without fluctuations, so the EM rediation 
(ideally) is counteracted there.  Things to consider when you design the 
Faraday Cage:

Cooling holes.  The inverter is dissipating quite a bit of power at times, 
and likely has a cooling fan, and possibly even the chassis is made from 
aluminum with cooling fins on it to help cool componenets down inside.  
Whenever you put a hole in a Faraday Cage, you provide a place for the EM 
radiation to escape.  The critical measure of the hole is it's diameter.  
To effectively stop EM radiation, the hole diameter must be about 1/10 of 
the shortest wavelength you are trying to shield.  So, for a CB radio, the 
typical frequency is about 27MHz.  The frequency corresponds to a 
wavelength of 11.1 meters or about 34 feet.  FM radio is about 2.7-3.5 
meters.  However, you could be picking up harmonic frequencies, so make 
the cooling holes small (and plentiful if you need to), I'd say on the 
order of 1/8" or less from expereince.  You may want to provide two sets 
of cooling ports, and put a fan on the outside of one of them to force 
airflow around the inverter.  Just a thought.

Isolation.  Electrically isolate the Faraday Cage from the inverter 
chassis and provide a _separate_ ground (a very solid ground) connection 
to the truck chassis.

Another thought that has occurred to me is that since cars and trucks 
don't have a true ground, many rely on the tires to provide some ground 
connection to earth.  In some cars, for example, if you are always getting 
shocked by static electricity when getting in or out, changing the tires 
can solve the problem.  I think tires have some graphite in the rubber to 
make them semiconducting to keep the car from electrically floating.  Some 
tires are more or less conducting than others.  Is your EMI problem better 
in the rain?  Here's an experiment you can try:  If you don't solve the 
EMI problem with the Faraday Cage (or maybe try this first), on a dry day 
try the radio and note the ammount of interference.  Then hose down the 
front of the truck with water, making sure to throroughly wet the 
underside of the wheel wells, axles, tires, and ground around the truck, 
preferably near some grass so the water can seep into the ground.  Is 
there less interference?  If so, then you may have a ground problem with 
the wiring in the truck.  For this you may want to consult someone who 
knows what they are doing with automotive wiring.

Hope this helps...
Good luck!
Regards,
Drew Procyk