ntsqd said:
Those with experience who say it'll be troublesome, but it seems to me that the wave length of most any ham or cb (of any country) is larger than the gap around the edges of the hood. So the trouble comes from radiation off the bottom side of the hood? Isn't this essentially a Faraday Cage? Or well grounded (i.e. not through just the hinges and the latch) is required for it to work as one?
With a mechanical diesel I can see a significant noise source entering the wiring under the hood and causing problems with anything electronic, though the IP's solenoid shouldn't care. The engine should stay running.
You need to be sensitive to EMI from both conduction and susceptibility. So The radio can be both a source of noise and be affected by it. In the case of the car engine, some things are quite good at being sources of noise and other are very good at being harmed by them.
The most obvious and common is the gasoline ignition system. High voltage, poorly shielded, highly impulsive energy. It's a textbook noise source and it does it's job well. Modern cars are pretty good about dealing with it, particularly with coils at the plugs. But it's still the main problem hams deal with. FWIW, your engine spark system is a tremendous emulation of the trans Atlantic spark gap transmitters that Marconi and Telsa were messing with back in the 19th century, just a lot lower energy. But an unshielded firing spark plug can make enough RF energy to be heard a fair distance away with a sensitive receiver. The jump to a very early radio is to pulse the spark with intelligence (i.e. Morse Code) and that's the extent of a radio.
Next is the alternator, this is present on all cars. Another great source of EMI, not badly affected by it. Rotating electromagnetic system with lots of energy. The problem here is a matter of how easily it gets into your radio. The noise is carried by the power wiring and you go and hook up your radio right to it, giving the EMI a very easy path into the radio.
The ECU, this can be a source for switching noise, but the power is so low that it by itself is not a major problem to deal with on radios. The things it switches might be a source for troublesome EMI because the signal is amplified to do something (other than ignition, primarily fuel injectors are noise sources). The main problem you are dealing with here is susceptibility. It does not take much energy to cause upsets in the ECU and in some cases to actually damage the ECU. But manufacturers know this and they are mechanically shielded and the inputs and outputs are protected reasonably well against EMI. Still, big enough source and you can overwhelm the protection, frying the ECU. Usually people see issues before permanent damage, like the engine does not run right, dies, etc. while you are transmitting on the radio.
Don't out think yourself, a grounded hood is kind of a Faraday Cage. But a cage is only good for frequencies that physically can flow through it. The gap around the hood is insulated with rubber bumpers and the latch and hinge are usually are painted and make poor conductors. So it's an non-ideal Faraday Cage and has waveguides along the edges to boot. Depending on the bandwidth, fundamentals, harmonics and directionality of the EMI, it's possible that you will never create a perfect cage. Plus, when you pass through the hood, the mount and coax are exposed inside the cage, so you have to be careful of routing and connectors inside the cage.
With a diesel engine, the main source of broadband noise is taken away, no matter if the engine has mechanical or electrical injectors. Ignition systems are huge problems. But electronic injectors are only slightly less of a problem, but mainly because they are much lower energy and generally not quite as noisy. But they are a periodic pulse and so they are a problem. Mechanical injection removes the pulse of the injectors, but will still have the alternator, so you have to work around that.
Also, don't forget about harmonics. Even at the 4th, 5th or higher order, harmonics can still carry significant energy and as the frequency goes up, the wavelength goes down. The 5th harmonic wavelength of a 440MHz fundamental is about 13cm and if you take the path of the hood flange around the hood, I'd be willing to bet it's could be a 5 inch folded guide, so a grounded hood and body could conceivably duct the RF energy quite well. In that case, a mesh or foil covering the gap would break up the guide, but adds to complexity of the shielding.
The reality of the situation is that EMI can enter your radio in many ways and in the case of a spark ignition engine, keeping EMI sensitive stuff away from the engine is best. But a well grounded body, hood and doors will cover the bulk of the RF EMI and keeping coax and antennas away is about all you need to do until you start trying to do AM DX, where background noise is a major issue. With FM and local AM, with reasonable care the local EMI will be low enough compared to the information that you can just squelch it out. With diesel engines you can significantly lower the background noise, but it's still there.
