vertical over real ground
JosephKK wrote:
On Mon, 16 Feb 2009 10:21:56 -0800, Roy Lewallen wrote: Michael Coslo wrote: Roy Lewallen wrote: At HF, a vehicle isn't "ground" or a "counterpoise", but the bottom half of an asymmetric dipole. It radiates at least as much as the "antenna" due to currents flowing downward along the outside. Calling a vehicle "ground" or "counterpoise" doesn't impart magical properties -- it's a conductor carrying currents whose fields don't cancel. In other words, it's an integral, radiating portion of the antenna. You can't leave this significant part of the antenna out of a model and expect the model to give correct results. I'm assuming that there is a capacitor formed by the car body being some few inches away from the physical ground also? Yes. This alters the current distribution on the vehicle, and can make it an even more effective radiator than the "antenna". Maybe, maybe not. Roadway surfaces are rarely conductive. More like static dissipative materials. While the area is significant the opposite conductive pole plate is missing, computing the effective capacitance may be challenging. Much more than the roadway surface is involved. The skin depth in average soil varies from about 12.6 feet at 30 MHz to 15.9 feet at 3.5 MHz. So significant current flows to depths of several tens of feet, well below the road surface. Within this distance of the surface you'll usually find strata with various conductivities and permittivities, as well as possibly buried pipes, rebar, and who knows what else. Computing an "effective capacitance" is virtually impossible, and useless if the vehicle is to be moved even a short distance. So only generalizations are possible. But any reasonable assumption about the characteristics of the ground under the vehicle points to it having a significant impact on the current distribution. Roy Lewallen, W7EL |
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